sulfur record of rising and falling marine oxygen and sulfate levels during the lomagundi event carbonates from approximately 2 3 2 1 billion years ago show markedly positive 13c values commonly reaching and sometimes exceeding 10 traditional interpretation of these positive 13c values favors greatly enhanced organic carbon burial on a global scale although other researchers have invoked widespread methanogenesis within the sediments to resolve between these competing models and more generally among the mechanisms behind earth s most dramatic carbon isotope event we obtained coupled stable isotope data for carbonate carbon and carbonate associated sulfate cas cas from the lomagundi interval shows a narrow range of 34s values and concentrations much like those of phanerozoic and modern marine carbonate rocks the 34s values are a close match to those of coeval sulfate evaporites and likely reflect seawater composition these observations are inconsistent with the idea of diagenetic carbonate formation in the methanic zone toward the end of the carbon isotope excursion there is an increase in the 34s values of cas we propose that these trends in c and s isotope values track the isotopic evolution of seawater sulfate and reflect an increase in pyrite burial and a crash in the marine sulfate reservoir during ocean deoxygenation in the waning stages of the positive carbon isotope excursion

experimental determination of carbonate associated sulfate δ34s in planktonic foraminifera shells understanding the coupling of oxygen carbon and sulfur cycles in the past is critical for reconstructing the history of biogeochemical cycles paleoclimatic variations and oceanic chemistry the abundance of sulfur isotopes 34 s in sulfate from ancient marine carbonates or carbonate associated sulfate cas is commonly used along with other archives mainly evaporites and barite to estimate the 34 s of seawater throughout earth history analyses of cas from hand picked foraminifera are potentially valuable because this group of organisms is used in numerous paleoceanographic studies they could provide coupled high resolution records of 13 c 18 o and 34 s isotopic changes directly linked to orbitally tuned records of climate change through the cenozoic such measurements have not previously been possible due to limitations of sensitivity in conventional irms based techniques however the recent development of cas analysis by multicollector inductively coupled plasma mass spectrometry mc icp ms now allows us to work on samples containing just a few nmol of sulfur with accuracy for 34 s values approaching 0 1 and consequently to analyze hand picked samples of foraminifera shells here we report the results of culture experiments with the planktonic species orbulina universa that establish a shell seawater 34 s calibration for future applications to the fossil record our new method uses 650 g of carbonate 15 shells per analysis the results show that s isotopes are fractionated consistently by 1 between seawater and o universa tests we also demonstrate that o universa faithfully records the so 2 4 ca 2 ratio of the seawater in which it grew

geochemical implications of sulfate in sedimentary carbonates abstract sulfate content reaches maximum partitioning in inorganically deposited mn calcite from aqueous solution when the carbonate contains 4 wt mno similar partitioning was also found for sr which rises to a maximum when calcite sediments contain 0 5 wt mno the behavior of the sulfate is explicable in terms of substitution of co 2 3 by so 2 4 this argument for the distribution of so 2 4 in inorganically formed carbonates is also consistent for sulfate in marine carbonates ir spectra of recent biogenic carbonates revealed no organic but only inorganic sulfate indicating that the sulfate occupies lattice sites in the carbonates total sulfate content of recent calcitic marine carbonates and pleistocene limestones increases with increasing mg content up to a maximum near 2 3 wt mg and decreases at larger mg contents this decrease in the sulfate content of these marine carbonates from that incorporated originally into the carbonate lattices is attributed to gradual exsolution during diagenesis in subaerial fresh water or on the sea floor in low so 2 4 waters the sulfate in coralline algae apparently increases the mg content above that predicted from observed shifts in the x ray diffraction peaks based upon so 2 4 free calcites

constraining pathways of microbial mediation for carbonate concretions of the miocene monterey formation using carbonate associated sulfate abstract carbonate concretions can form as a result of organic matter degradation within sediments however the ability to determine specific processes and timing relationships to particular concretions has remained elusive previously employed proxies e g carbon and oxygen isotopes cannot uniquely distinguish among diagenetic alkalinity sources generated by microbial oxidation of organic matter using oxygen nitrate metal oxides and sulfate as electron acceptors in addition to degradation by thermal decarboxylation here we employ concentrations of carbonate associated sulfate cas and 34scas along with more traditional approaches to determine the specific nature of concretion authigenesis within the miocene monterey formation integrated geochemical analyses reveal that at least three specific organo diagenetic reaction pathways can be tied to concretion formation and that these reactions are largely sample site specific one calcitic concretion from the phosphatic shale member at naples beach yields 34scas values near miocene seawater sulfate 22 vcdt abundant cas ca 1000 ppm depleted 13ccarb 11 vpdb and very low concentrations of fe ca 700 ppm and mn ca 15 ppm characteristics most consistent with shallow formation in association with organic matter degradation by nitrate iron oxides and or minor sulfate reduction cemented concretionary layers of the phosphatic shale member at shell beach display elevated 34scas up to 37 cas concentrations of 600 ppm mildly depleted 13ccarb 6 moderate amounts of mn ca 250 ppm and relatively low fe ca 1700 ppm indicative of formation in sediments dominated by sulfate reduction finally concretions within a siliceous host at montana de oro and naples beach show minimal cas concentrations positive 13c values and the highest concentrations of fe ca 11 300 ppm and mn ca 440 ppm consistent with formation in sediments experiencing methanogenesis in a highly reducing environment this study highlights the promise in combining cas analysis with more traditional techniques to differentiate among diagenetic reactions as preserved in the geologic record and shows potential for unraveling subsurface biospheric processes in ancient samples with a high degree of specificity

variability in sulfur isotope records of phanerozoic seawater sulfate the s of seawater sulfate reflects processes operating at the nexus of sulfur carbon and oxygen cycles however knowledge of past seawater sulfate s values must be derived from proxy materials that are impacted differently by depositional and post depositional processes we produced new timeseries estimates for the s value of seawater sulfate by combining 6710 published data from three sedimentary archives marine barite evaporites and carbonate associated sulfate with updated age constraints on the deposits robust features in multiple records capture temporal trends in the s value of seawater and its interplay with other phanerozoic geochemical and stratigraphic trends however high frequency discordances indicate that each record is differentially prone to depositional biases and diagenetic overprints the amount of noise quantified from the variograms of each record increases with age for all s proxies indicating that post depositional processes obscure detailed knowledge of seawater sulfate s s value deeper in time

40ar 39ar dating of hydrothermal biotite from high grade gold ore tangier gold deposit nova scotia further evidence for 370 ma gold metallogeny in the meguma terrane a hydrothermal biotite carbonate alteration zone adjacent to a high grade multi ounce gold vein in the tangier gold deposit meguma terrane nova scotia has given an 40 ar 39 ar plateau age of 374 ma the alteration halo is part of a vein package in the hinge zone of the tangier anticline which contains abundant auriferous quartz veins formed during flexural slip folding during a late stage reactivation of the regional fold belt the 374 ma age is similar to 40 ar 39 ar ages determined for hydrothermal vein minerals at seven other meguma gold districts thereby suggesting a terrane wide metallogenic event a similar age of vein formation and by inference gold mineralization and granite emplacement across the meguma terrane suggest that the cospatial and cotemporal nature of the two geological events may be related an integrated model including anatexis and devolitization of the lower crust possibly as a consequence of overriding a mantle plume in concert with late stage reactivation of the fold belt owing to continued transpression along a terrane bounding fault is proposed

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

phosphogenesis and organic carbon preservation in the miocene monterey formation at naples beach california the monterey hypothesis revisited the middle part of the miocene monterey formation at naples beach west of santa barbara california is predominantly composed of organic rich mudstone interstratified with phosphatic laminae minor lithologies include volcanic ash dolomite porcelanite and chert and condensed phosphatic beds sediments dated as 14 3 13 5 ma have average total organic carbon toc values around 8 5 wt and organic carbon oc accumulation rates are around 565 mg cm2 k y sediments dated as 13 5 13 ma are characterized by average toc values of 12 6 wt and oc accumulation rates of around 1130 mg cm2 k y the interval between 13 and 10 6 ma is marked by condensation average toc values are around 8 6 wt and oc accumulation rates diminished to around 55 mg cm2 k y the last interval studied is dated as 10 6 9 4 ma and average toc values are around 6 wt whereas oc accumulation rates rose again to 320 mg cm2 k y the presence of erosional surfaces angular unconformities and reworked clasts and nodules suggests that bottom current activity and gravity flow deposition have been instrumental in sediment accumulation the phosphatic laminae were precipitated at a very early stage of diagenesis during periods of nonsedimentation they formed less permeable sedimentary lids and may as such have contributed to enhanced oc preservation between 13 and 10 6 ma the thus formed phosphatic laminae were frequently subjected to subsequent sediment winnowing and reworking resulting in the formation of condensed phosphatic beds calculated p c molar ratios suggest that 1 the measured section is highly enriched in phosphorus p relative to oc 2 regeneration of organic p from organic matter decomposition was negligible and 3 the source of p was external likely upwelled bottom water rich in inorganic p in spite of good preservation conditions and correspondingly high toc contents the overall oc accumulation rates are moderate in comparison to those of actual high productivity areas which is mainly due to the episodic character of depositional processes and the intervening long periods of nondeposition and sediment reworking they preclude this section and by extrapolation the monterey formation in general from being an important oc sink during the middle miocene alternatively large oc sinks were probably created on the continent lignite deposits and in sedimentary depocenters which received increasing amounts of detrital sediments due to a combination of climate change spreading of grasslands and the increasing importance of mountain chains such as the himalaya the associated high nutrient fluxes may have been involved in the backstepping and drowning of carbonate platforms and in the generation of widespread phosphat rich deposits during the late early and early middle miocene

a neogene seawater sulfur isotope age curve from calcareous pelagic microfossils abstract until now our knowledge of the sulfur isotopic composition of seawater through geologic time has depended on stable isotopic analysis of sulfate from evaporites owing to the sporadic occurrence of evaporites through time the secular sulfur isotope age curve contains many gaps with little or no data in order to fill in some of these gaps particularly the neogene we have analyzed the sulfur isotopic composition of carbonate associated sulfate in carbonate tests of planktonic foraminifera other investigators have shown that sulfate may occur in biogenic calcites either lattice bound as micro fluid inclusions in adsorbed phases or as protein polysaccharides whatever the origin the sulfur isotopic composition of this sulfate appears to be representative of that of the water in which the organism lived as shown by results on recent calcareous foraminifera and macrofossils using this approach for study of miocene to recent pelagic marine sediments supplemented by new data for miocene marine evaporites from the gulf of suez we have found that the 34 s of seawater has decreased about 2 5 over the past 25 m y and that most of the decrease has occurred over the past 5 m y parallelling a decrease in the 13 c of dissolved oceanic bicarbonate from the same interval sedimentary redox models based on isotope records suggest that organic carbon and sulfide burial have both decreased over the past 5 m y alternatively an increase in weathering rates over the past 5 m y would not require a decrease in organic carbon or sulfide burial as long as the isotopic effect of the increased river input exceeds the isotopic effect of the burial of the reduced species in either case the net result would be a decrease in atmospheric p o 2

constraints on the late ediacaran sulfur cycle from carbonate associated sulfate we report new sulfur isotope compositions 34s in carbonate associated sulfate cas and pyrite from the lower nama group namibia 550 to 35 is highly unusual in earth history and in the terminal ediacaran is best explained by a high pyrite burial flux pyrite in the nama group is close in isotopic composition to coeval sulfate but the sulfur isotope fractionation between sulfate and pyrite varies widely among different studied basins suggesting highly heterogeneous redox and depositional conditions

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

isotopic evidence for an anomalously low oceanic sulfate concentration following end permian mass extinction abstract the cataclysmic end permian mass extinction was immediately followed by a global expansion of microbial ecosystems as demonstrated by widespread microbialite sequences disaster facies in shallow water settings here we present high resolution carbonate carbon 13 c carb and carbonate associated sulfate sulfur isotope 34 s cas records from the microbialite in the cili permian triassic p tr section in south china a stepwise decline in 13 c carb begins in the underlying skeletal limestone predating the main oceanic mass extinction and the first appearance of microbialite and reaches its nadir in the upper part of the microbialite layer the corresponding 34 s cas in the range of 17 4 to 27 4 is relatively stable in the underlying skeletal limestone and increases gradually from 2 m below the microbialite rising to a peak at the base of the microbialite two episodes of positive and negative shifts occurred within the microbialite layer and exhibit a remarkable co variance of sulfur and carbon isotope composition the large amplitude of the variation in 34 s cas as high as 7 per 100 kiloyears suggests a small oceanic sulfate reservoir size at this time furthermore the 13 c carb and 34 s cas records co vary without phase lag throughout the microbialite interval implying a marine driven c cycle in an anoxic ocean with anomalously low oceanic sulfate concentrations on the basis of a non steady state box model we argue that the oceanic sulfate concentration may have fallen to less than 15 perhaps as low as 3 of that in the modern oceans low oceanic sulfate concentration likely was the consequence of evaporite deposition and widespread anoxic sulfidic conditions prior to the main mass extinction by promoting methanogenesis and a build up of atmospheric ch 4 and co 2 low oceanic sulfate may have intensified global warming exacerbating the inimical environmental conditions of the latest permian

carbon sulfur and oxygen isotope evidence for a strong depth gradient and oceanic oxidation after the ediacaran hankalchough glaciation in order to understand spatial variations of stable isotope geochemistry in the quruqtagh basin northwestern china in the aftermath of an ediacaran glaciation we analyzed carbonate carbon isotopes 13ccarb carbonate oxygen isotopes 18ocarb carbonate associated sulfate sulfur 34scas and oxygen isotopes 18ocas and pyrite sulfur isotopes 34spy of a cap dolostone atop the ediacaran hankalchough glacial diamictite at four sections the four studied sections ykg mk h and zbs represent an onshore offshore transect in the quruqtagh basin our data show a strong paleobathymetry dependent isotopic gradient from the onshore to offshore sections 13ccarb values decrease from 2 to 16 vpdb whereas 18ocarb values increase from 4 to 1 vpdb both 34scas and 34spy show stratigraphic variations in the two onshore sections mk and ykg but are more stable in the two offshore sections h and zbs 18ocas values of onshore samples are consistent with terrestrial oxidative weathering of pyrite we propose that following the hankalchough glaciation seawater in the quruqtagh basin was characterized by a strong isotopic gradient the isotopic data may be interpreted using a three component mixing model that involves three reservoirs deep basin water surface water and terrestrial weathering input in this model the negative 13ccarb values in the offshore sections are related to the upwelling of deep basin water where anaerobic oxidation of dissolved organic carbon resulted in 13c depleted dic whereas sulfur isotope variations are strongly controlled by surface water sulfate and terrestrial weathering input derived from oxidative weathering of pyrite the new data provide evidence for the oceanic oxidation following the hankalchough glaciation

paired δ34s data from carbonate associated sulfate and chromium reducible sulfur across the traditional lower middle cambrian boundary of w gondwana abstract in this study we present the first high resolution data from coupled 34 s analyses of carbonate associated sulfate cas and chromium reducible sulfur crs from three lower middle cambrian sections in western gondwana cas and crs were extracted and analyzed from marine dolostone limestone and nodular limestone from spanish and french successions in parallel carbonate samples were also analyzed for 13 c carb 18 o carb and major trace element concentrations ca fe mg mn sr 34 s cas values vary between 17 6 and 33 2 with a maximum cas concentration of ca 900 ppm 34 s crs data show a similar broad range between 5 1 and 29 7 with maximal crs contents up to ca 3700 ppm notably there is little stratigraphic variation in the 34 s cas data in each of the sections confounding inter basinal chemostratigraphic correlations nonetheless the absolute differences in 34 s cas between sections as well as variations in cas and crs concentrations are attributed to paleoenvironmental differences between proximal and distal parts of the carbonate ramp as well as effects of subaerial exposure and riverine input thus the generated 34 s cas data deliver not only valuable paleoecological and paleoenvironmental information they also illustrate a heterogeneity in the seawater sulfate sulfur isotopic composition of the western gondwanan ocean consequently the lack of correlation between our gondwanan 34 s cas data and time equivalent sections of laurentia and siberia is probably not only caused by the absence of an internationally accepted biostratigraphic correlation but rather supports the view that sulfate was non conservative anion in seawater during the cambrian period

refining the extraction methodology of carbonate associated sulfate evidence from synthetic and natural carbonate samples sulfur and oxygen isotope analyses of trace and whole mineral sulfate are valuable in investigating diagenetic processes and the microbial communities that produced them seawater anoxia and paleoclimate oxygen isotopes are particularly useful in that they may also record alterations to the original isotope ratio be it from post depositional processes or oxidation of sulfide minerals during the chemical extraction procedure here we rigorously test several common methodological procedures of extracting carbonate associated sulfate cas for sulfur and oxygen isotope analyses in order to generate a method that will extract only the cas while preserving associated organic reduced sulfur and sulfides for analysis the results of these experiments on synthetically generated carbonates demonstrate that our proposed protocol sufficiently removes all non cas sulfate and does not result in oxidation of included sulfides analytical reproducibility standard deviation of synthetic carbonates is 0 1 1 for 34s and 0 3 1 for 18o extractions of low pyrite high organic matter geologic samples from the monterey formation across a range of facies types demonstrate a reproducibility 1 of 0 4 0 7 for 34s and 0 8 1 3 for 18o resulting from sample heterogeneity 34s and 18o from monterey formation samples do not demonstrate oxidation of organic matter suggesting our proposed protocol will preserve organic sulfur a high pyrite concentration jet rock concretion demonstrates that additional sulfate can be produced during the non cas leaching processes from oxidation of pyrite we show that pyrite from the jet rock concretion ceases to oxidize when the sample is leached under an anoxic environment

the relationship of diagenesis with a complex microbial ecosystem in the phosphatic interval of the miocene monterey formation evidence from stable isotopes and mineralogy abstract a combination of evidence from mineralogy sulfur and oxygen stable isotope ratios from early diagenetic carbonate associated sulfate cas and structurally substituted sulfate in phosphate both termed cas here and carbon and oxygen stable isotope ratios from carbonate describe a complex redox system and complex microbial ecosystem within the phosphate rich interval of the monterey formation 34s and 18o of cas are lower than miocene seawater sulfate as reconstructed from marine evaporite and barite minerals combined with mineralogical speciation modeling and mineralogical observations isotopically low cas isotope values suggest mixing from three isotopically characterized sulfate pools two isotopically higher sulfate pools one with the composition of miocene seawater sulfate and a second isotopically evolved porewater modified by sulfate reducing microbes srm and a third isotopically lower porewater sulfate pool resulting from the oxidation of h2s produced by srm by sulfide oxidizing bacteria sob high carbonate 13c and diagenetic dolomite validates prior claims of methanogenesis differences in mineralogy are consistent with differences in isotope ratios e g higher 13c in dolomites suggesting products from a vertical distribution of oxic to anoxic redox environments and associated ecosystems are preserved these mineralogical and isotopic fluctuations repeat in various facies changes suggesting that redox environments fluctuated over time this interpretation is supported by prior sedimentologic analysis which interpreted that deposition of the phosphatic interval of the monterey formation was influenced by gravity deposition processes promoted by tectonic activity gravity deposition would serve to transport more oxygenated waters to the deep water depositional environment and support microbially mediated sulfide oxidation at depth whereby h2s generated by srm in the absence of sufficient iron could diffuse upward to a more oxygenated zone near the suboxic oxic boundary h2s oxidation would decrease porewater ph slightly and inhibit the preservation of carbonate minerals and promote precipitation of phosphate increasing depth in sediments generates successively more reducing conditions promoting sulfate reduction which may have been coupled with the anaerobic oxidation of methane and carbonate mineral precipitation the most reducing conditions would support methanogenesis and dolomite production after consumption of the majority of sulfate

low marine sulfate concentrations and the isolation of the european epicontinental sea during the early jurassic two records of seawater sulfate isotope composition from the early jurassic demonstrate that large isotopic gradients existed between the european epicontinental sea and the open tethys ocean these differences can be explained by the modification of open ocean sulfate isotopic compositions by water mass isolation sea level rise and the effects of changing regional weathering and pyrite burial fluxes during a time of rapid environmental change both records contain large positive isotopic excursions in the section from europe yorkshire uk a 6 excursion begins in the early exaratum subzone of the toarcian in the middle of the organic rich shale representing a well known oceanic anoxic event an open tethyan margin record from tibet records a much larger 19 excursion but the section is less well dated two age interpretations are possible sparse biostratigraphic evidence places this excursion in the aalenian but we suggest that it may correlate with the positive excursion in yorkshire hence these records may document both a toarcian event and an aalenian sulfur isotope event or the early toarcian anoxic event alone conservative estimates of the rate of isotopic change with time based on the tibetan section suggest that early jurassic seawater sulfate concentrations were between 1 and 5 mm much lower than previously thought

integrated chemostratigraphy of the doushantuo formation at the northern xiaofenghe section yangtze gorges south china and its implication for ediacaran stratigraphic correlation and ocean redox models abstract the ediacaran doushantuo formation in the yangtze gorges of south china plays an important role in our understanding of biological evolution global correlation and ocean redox conditions because of the availability of high resolution paleontological and geochemical data and numerous radiometric dates however integrated study has been focused on the jiulongwan section that was largely deposited below wave base in a restricted shelf lagoon jiang et al 2011 zhu et al 2011 studies of shallower water successions are lacking and this presents a challenge to test ediacaran stratigraphic correlation and ocean redox models to fill this knowledge gap we conducted a high resolution integrated study of the doushantuo formation at the northern xiaofenghe nxf section approximately 35 km to the northeast and paleogeographically updip of the jiulongwan section with the exception of the basal 20 m nxf sediments were deposited above normal wave base integrated biostratigraphic and chemostratigraphic data indicate that the 140 m thick nxf section correlates with the lower doushantuo formation member i and much of member ii i e the lower ca 70 m of the formation at jiulongwan geochemical data from nxf and other doushantuo sections indicate that euxinic conditions may have been limited to a shelf lagoon represented by the jiulongwan section that was restricted between the proximal inner shelf and a distal shelf margin shoal complex at least during early doushantuo time following the deposition of the doushantuo cap dolostone further integrated studies are necessary to test whether euxinic conditions existed in open marine shelves in south china and elsewhere during the ediacaran period

the sulphur isotopic composition of trace sulphates in carboniferous brachiopods implications for coeval seawater correlation with other geochemical cycles and isotope stratigraphy the carboniferous 34s record based on the analysis of structurally substituted sulphate sss in brachiopod shells is characterized by a significant decline from values around 21 1 in the tournaisian to 12 1 in the gzelian superimposed on this broad evolution is a higher order structure with a relative minimum in the late visean followed by a maximum in the bashkirian on the 107 year time scale 34s is positively correlated with 13ccarb in contrast to the long term negative linkage between sulphur and carbon observed for the entire phanerozoic this indicates that mechanisms other than the redox balance control the carbon and sulphur cycles on this time scale the 87sr 86sr and 34s records display parallel trends during the tournaisian to moscovian this positive correlation suggests that tectonics may be of importance for the evolution of 34s on time scales of 107 years a possible scenario to explain the co evolution of the isotope systems and their linkage to tectonism and palaeoclimate invokes an increased nutrient supply to the oceans during periods of enhanced uplift and continental weathering this nutrient flux may have resulted in increased burial of organic matter and enhanced bacterial sulphate reduction

putative martian microbes called microscopy artifacts life on marsin the heated debate over the putative microfossils in martian meteorite alh84001 most scientists have focused on arcane arguments about mineral isotopic compositions and formation temperatures but for many they simply look lifelike looks can be deceptive however and three meteoriticists now argue that there s nothing lifelike about the martian bugs rather they re simply a trick of the eye abetted by the peculiarities of the powerful microscopes used to image them in a short paper in this week s nature this group presents its own nanometer scale images of alh84001 they argue that most of the microfossils are nothing more than narrow ledges of mineral protruding from the underlying rock that under certain viewing conditions can masquerade as fossil bacteria

the preservation of δsso434 and δoso418 in carbonate associated sulfate during marine diagenesis a 25 myr test case using marine sediments a 25 million year record of the sulfur and oxygen isotope composition of marine carbonate associated sulfate cas was constructed from 55 nannofossil ooze samples at three different locations we tested the impact of early marine diagenesis on cas by comparing the cas extractions to the sulfur and oxygen isotope composition of the associated pore fluid sulfate to determine the degree of pore fluid incorporation during carbonate recrystallization neither the sulfur nor the oxygen isotope composition of cas are completely overprinted by incorporation of pore fluid sulfate we compare our record to the sulfur and oxygen isotope records of coeval barite the sulfur isotope record is in agreement with the barite record within 2 2 except where very young 2 ma sediments are in the presence of highly evolved pore fluid sulfate view the mathml source s34 70 a simple recrystallization model is used to illustrate the sensitivity of cas view the mathml source sso434 to sedimentation rate and to emphasize that careful sample selection along with an analysis of early diagenetic environmental conditions is crucial when interpreting cas sulfur isotopes oxygen isotopes in cas are more complex and do not reproduce similar values to those of coeval barite we conclude that oxygen isotopes in sulfate may remain a useful proxy but merit closer attention in the future

sulfur isotope records around livello bonarelli northern apennines italy black shale at the cenomanian turonian boundary sulfur isotope ratios for carbonate hosted sulfate show a rapid increase 6 to 9 m y across the livello bonarelli black shale marche umbrian apennines of italy which was deposited at the cenomanian turonian boundary in the middle cretaceous 93 5 ma the increase suggests that rates of pyrite burial increased substantially over the event resulting in a decrease of 9 27 in the oceanic reservoir of so 4 2 the enhanced pyrite burial may have significantly increased the flux of oxygen to the atmosphere 9 27 10 18 mol m y furthermore removal of the large amount of iron associated with this process might have had a profound effect on iron cycle

paleoceanographic and tectonic controls on deposition of the monterey formation and related siliceous rocks in california abstract the timing of paleoceanographic and tectonic events that shaped the deposition of the monterey formation of california and related siliceous rocks has been determined by application of a refined biochronology the base of the monterey at 17 5 ma coincides with rising global sea level and a switch in biogenous silica deposition from the caribbean and low latitude north atlantic to the north pacific major polar cooling which began at 15 ma postdates the base of the monterey by more than 2 ma and cannot be invoked to cause the deposition of diatomaceous sediments occurring in the lowermost monterey later polar cooling in the early late miocene however apparently caused increased upwelling and deposition of purer diatomites in the upper monterey the top of the monterey at about 6 ma coincides with a major sea level drop and is commonly marked by an unconformity equivalent unconformities are widespread around the rim of the north pacific and typically separate more pelagic sediments from overlying sediments with a greater terrigenous component above the monterey diatoms persist in california sediments to 4 5 4 0 m y where their decline coincides with increased deposition of diatoms in the antarctic carbon isotope records in the pacific and indian oceans record storage of 12 c in the monterey formation and equivalent organic rich sediments around the rim of the north pacific a 1 0 excursion in 13 c beginning at 17 5 ma coincides with rising sea level and probably reflects storage of organic material in monterey like marginal reservoirs a reverse 1 0 shift at 6 2 ma closely approximates the top of the monterey and may represent erosion of these marginal reservoirs and reintroduction of stored organic carbon into the ocean atmosphere system initiation of transform faulting and extension in the california margin in the latest oligocene and early miocene caused the subsidence of basins which later received monterey sediments a major tectonic event centered at 10 9 ma which included rotation of the santa barbara basin and a change in pacific plate motion is recorded by an unconformity and or interval of compressed sediments in many monterey sections another change in plate motion at about 6 ma may be expressed by the angular discordance observed in many sections across the unconformity at the top of the monterey

an isotopic c o sr study of vein gold deposits in the meguma terrane nova scotia implication for source reservoirs vein quartz carbonate and tourmaline from 19 meguma gold deposits in the meguma terrane of nova scotia have been analyzed for stable delta 18 o delta 13 c and radiogenic 87 sr 86 sr isotopes in order to assess the nature and origin of the vein forming fluids as with other mesothermal gold provinces the meguma gold deposits are well suited to such a study because carbonate is the next most abundant phase after quartz in these mesothermal lode gold deposits all vein types have been sampled for quartz and carbonate and in addition all compositional and textural varieties of carbonate have been sampled vein quartz is of uniform isotopic composition with most 18 o values between 10 2 to 17 6 per mil avg ca 14 per mil except for one deposit west gore sb au where values go to 19 4 per mil there is no systematic variation for quartz within a deposit or position within the meguma group stratigraphy vein carbonate 18 o values range from 11 8 to 27 5 per mil with most in the 13 to 16 per mil range the relatively 18 o enriched carbonates reflect exchange with low temperature fluids based on analyses of quartz carbonate pairs that indicate disequilibrium fractionation delta sub quartz carbonate or 0 this is best illustrated by the strong negative correlation between delta 18 o carbonate and delta sub quartz carbonate the 18 o water is estimated at 10 or 3 per mil for a temperature of vein formation of 350 degrees to 400 degrees c and using the appropriate mineral water fractionation equations whereas delta 18 o water values partly overlap the field for magmatic fluids the values are wholly consistent with a metamorphic fluid and it is considered unlikely that a primary magmatic fluid signature has been substantially modified due to wall rock influences given that mesothermal gold systems are sites of high fluid rock ratios vein carbonate has delta 13 c values of 13 1 to 25 9 per mil but a slight negative correlation between delta 13 c carbonate and delta sub quartz carbonate suggests that the primary values lie in the range 20 to 25 per mil thus the delta 13 c values indicate a reduced biogenic source for the carbon oxidation of the reduced carbon as indicated by co 2 in fluid inclusions may have occurred via hydrolysis of graphite or dissolution of carbonate minerals both of which occur in the wall rock lithologies of the meguma group the latter of which has the appropriate isotopic composition the initial 87 sr 86 sr of the vein fluid estimated from the analyses of 52 vein carbonates 17 deposits and four tourmalines three deposits ranges from 0 70118 to 0 72284 and within deposits considerable variation is observed there is insufficient data to quantify the extent of the low temperature overprint which has modified the c and o isotope data although it is likely that some influence is present nevertheless the data cannot be reconciled by a source confined exclusively to the meguma group which suggests therefore involvement of another reservoir s the isotopic heterogeneity can be explained by variable amounts of contamination of a primary fluid with radiogenic sr derived from meguma group lithologies by interaction along the fluid path or at the site of vein formation concomitant with wall rock alteration as discussed above a dominantly magmatic source is not considered feasible this fluid source is suggested to be within the structural basement to the meguma group and the liscomb gneisses are the favored source based on the combined results of the sr isotope data presented herein and previously published pb isotope data collectively the data indicate that a primary fluid of metamorphic origin has had its isotopic signature variably modified due to interaction with different reservoirs the most affected isotopic systems are c and s based on earlier work on delta 34 s values which are abundant as graphite carbonate and sulfides in the meguma wall rock respectively the range in 87 sr 86 sr values of the fluid also reflects contamination but this was quite variable the uniform delta 18 o water value for the fluid indicates that this was the least affected isotopic system except for the later exchange of carbonate at low temperatures

a global perturbation to the sulfur cycle during the toarcian oceanic anoxic event abstract the mesozoic era was punctuated by intervals of widespread anoxia within the ocean termed oceanic anoxic events or oaes the chemostratigraphy of these intervals also contains evidence of transient perturbations to many biogeochemically important elemental cycles here we present high resolution sulfur isotope data from three stratigraphic sections spanning the toarcian oceanic anoxic event t oae of the early jurassic all sections show a similar increase in the sulfur isotope ratio of sulfate parallel to an overall positive excursion in carbon isotopes during the oae interval based on forward box modeling the sulfate s isotope excursion can be generated by transiently increasing the burial rate of pyrite in marine sediments likely deposited under euxinic i e anoxic and sulfidic conditions in the water column in addition modeling shows that prolonged recovery of the 34 s of seawater sulfate at least 8 ma after the initial rise associated with the oae was due to the relatively long residence time of sulfate in the jurassic ocean estimates from our modeling put the toarcian marine sulfate concentrations at 4 to 8 mm the similarity of the sulfur isotope records from the north european epicontinental or epeiric sea and tethyan continental margin suggests that local modification of the marine sulfur isotope signal was minimal a point explored with isotope mixing models importantly our results indicate that the sulfur isotope excursion reflects a globally significant perturbation in the sulfur cycle and that pyrite burial in the north european epeiric seaway alone cannot account for the excursion this study along with recent work from other phanerozoic intervals of widespread marine oxygen deficiency confirms that the sulfur cycle can be perturbed significantly by enhanced pyrite burial during periods of prolonged oceanic anoxia euxinia

oxygen isotope fractionation between calcite and fluid as a function of growth rate and temperature an in situ study the 18o content of carbonates expressed as 18o is widely used in paleothermometry yet it is clear from comparison of experimental results and natural carbonate samples that chemical isotopic equilibrium is not always achieved in nature this observation underscores the importance of exploring possible effects of growth rate on isotopic fractionation which is the focus of this study in situ secondary ion mass spectrometry sims analyses of 18o were performed on single crystals of experimentally grown calcite the change in growth rate v over time within each crystal was monitored by addition of multiple rare earth element ree spikes into the fluid from which the calcite grew the values of 18o in bulk calcites and experimental fluids were measured by stable isotope ratio mass spectrometers the average sims 18o overlaps with those determined by conventional mass spectrometry within analytical uncertainty the fractionation factor expressed as 18o approaches its equilibrium value in slowly grown calcites v 0 02 nm s and decreases by 1 5 with increasing growth rate from 10 3 to 0 34 nm s here 18o 103 ln 18o and 18o 18o 16o calcite 18o 16o fluid our results provide the first in situ evidence that under equilibrium conditions 18o may be depleted in the near surface region of calcite relative to the bulk crystal lattice and that this 18o depleted zone can be captured during rapid crystal growth crystal growth rate is therefore a potentially important consideration when using 18o in natural carbonates as a proxy for ocean and terrestrial temperature