rapid releases of metal salts and nutrients following the deposition of volcanic ash into aqueous environments deposition of volcanic ash into aqueous environments leads to dissolution of adsorbed metal salts and aerosols increasing the bioavailability of key nutrients volcanogenic fertilization events could increase marine primary productivity leading to a drawdown of atmospheric co2 here we conduct flow through experiments on unhydrated volcanic ash samples from a variety of locations and sources measuring the concentrations and fluxes of elements into de ionized water and two contrasting ocean surface waters comparisons of element fluxes show that dissolution of adsorbed surface salts and aerosols dominates over glass dissolution even in sustained low ph conditions these surface ash leachates appear unstable decaying in situ even if kept unhydrated volcanic ash from recent eruptions is shown to have a large fertilization potential in both fresh and saline water fluorine concentrations are integral to bulk dissolution rates and samples with high f concentrations display elevated fluxes of some nutrients particularly fe si and p bio limiting micronutrients are released in large quantities suggesting that subsequent biological growth will be limited by macronutrient availability importantly acidification of surface waters and high fluxes of toxic elements highlights the potential of volcanic ash leachates to poison aqueous environments in particular large ph changes can cause undersaturation of caco3 polymorphs damaging populations of calcifying organisms deposition of volcanic ash can both fertilize and or poison aqueous environments causing significant changes to surface water chemistry and biogeochemical cycles

anatomy of a world class source rock distribution and depositional model of silurian organic rich shales in jordan and implications for hydrocarbon potential silurian organic rich hot shales were the source of large amounts of hydrocarbons in northern gondwana with supergiant and giant fields in the middle east and north africa a study of these black shales has been conducted in jordan where they represent the source for the risha gas field two organically enriched horizons occur in the silurian in jordan termed the lower hot shale and the upper hot shale deposition of the transgressive lower hot shale occurred during the early llandoverian and was restricted to the earliest silurian paleodepressions three lower hot shale depocenters have been identified in jordan they are located in the western risha eastern wadi sirhan and jafr areas the eastern risha area was part of a larger scale paleohigh covering northeast jordan most of syria and iraq and north central saudi arabia qusaiba area at least in jordan the high coincides with the depocenter of the latest ordovician glaciation sedimentation of the upper hot shale occurred around peak sea level during the late llandoverian and or early wenlockian and was limited to the distal parts of the shelf risha area in front of the prograding silty sandy deltaic front thermal maturity increases from immature in the southern desert outcrops to late or postmature in northern jordan organic richness and pyrolysis data deteriorate significantly with increasing thermal maturity because of hydrocarbon generation prior to maturation maximum organic richness is interpreted to have considerably exceeded 10 with good s2 remaining generative potential yields as reflected in the values of the immature lower hot shale in the shallow borehole bg 14 in the southern desert outcrop area and exploration well hunt jf 1 in the jafr area identification and isopach mapping of the lower hot shale is based on its characteristic uranium enrichment the gamma ray total organic carbon correlation coefficient increases with increasing maturity suggesting that uranium is less vulnerable to maturity effects than the organic matter itself the gamma ray log therefore can be used as a proxy for the prematuration organic richness representing a suitable basis for cross basin comparisons of original organic richness trends a two phase generation of hydrocarbons is interpreted for both the risha and wadi sirhan areas with the first as a pre hercynian phase in the carboniferous and the second as a less intense generation phase during the mesozoic to early tertiary in contrast gas generation of the silurian sourced giant fields in the central part of the arabian peninsula and iran was restricted to the post hercynian in jordan both structural and stratigraphic play concepts are envisaged including fault paleohigh and channel plays key elements that need attention in future exploration of the silurian plays in jordan are ordovician reservoir quality and timing of maturation poor permeabilities have been reported from wells in both the risha and wadi sirhan areas

carbon isotopes and event stratigraphy near the ordovician silurian boundary yichang south china abstract the organic carbon isotope data through the unweathered ordovician silurian boundary strata at the wangjiawan riverside section which is c 180 m southeast of the wangjiawan north section the gssp for the base of the hirnantian stage upper ordovician show that a positive 13 c org excursion begins just below the base of the hirnantian stage and peaks in the lower part of the n extraordinarius biozone this is followed by an interval of slightly reduced 13 c values and a second peak of 2 above pre hirnantian values which occurs in the lower part of the n persculptus biozone upper hirnantian the peaks in 13 c org values can be correlated well with episodes of glacial expansion described from africa and peri gondwanan europe evidence from sedimentological faunal and geochemical data from south china as well as the evidence of glacial sediments in north africa all suggests a short lived glaciation of 1 ma in the ordovician south pole region consisting of at least two major pulses the first phase of gradual glacial expansion began just before the hirnantian epoch and the second phase ended with rapid melting in the late hirnantian early n persculptus biozone

time specific black mudstones and global hyperwarming on the cambrian ordovician slope and shelf of the laurentia palaeocontinent abstract the early paleozoic featured nine intervals of strong expansion of an upper slope dysoxic anoxic d a water mass with eustatic rise or epeirogenic transgression strong expansion of this d a water mass led to deposition of time specific macroscale alternations of dark grey black mudstone within oxic green to red mudstone on the middle lower slope this d a facies even onlapped warm carbonate and cool water siliciclastic shelves as in the mesozoic d a muds were deposited in shallow water perhaps tens of metres deep with sea level rise these nine d a macroscale alternations correspond to intervals of global hyperwarming times of very intense greenhouse conditions that resulted from a feedback initiated by higher insolation and heat storage as shallow seas onlap tropical palaeocontinents warm epeiric seas heated the ocean and thermal expansion accelerated eustatic rise ever more extensive epeiric seas heightened oceanic and global temperature as heat storage capacity increased deep ocean circulation intensity fell below that of a greenhouse interval and lead to d a deposition low on the slope and on the platforms to provide the signature of global hyperwarming global hyperwarming differs from a hothouse interval as it does not require co 2 input from large igneous provinces to produce high temperatures and never shows deep sea anoxia late ordovician and late devonian black mudstones that cover much of laurentia record epeirogenic transgressions that led to global hyperwarming and suggest that cold water upwelling or plant terrestrialisation had nothing to do with epeiric sea anoxia global hyperwarming reduced oxygen solubility in these seas and erosion of orogens produced muddy water that limited light penetration and promoted shallow water anoxia the global hyperwarming hypothesis means that relative eustatic and epeirogenic sea levels complement the effect of global pco 2 on climate and sea level must also be regarded as a primary driver of phanerozoic climate

geochemical evidence for widespread euxinia in the later cambrian ocean it has been suggested that the cambrian ocean was oxygen deficient but physical evidence for widespread anoxia has been lacking gill et al present sulphur isotope data from cambrian rocks at six different locations around the world and find a positive sulphur isotope excursion in phase with a large excursion in the marine carbon isotope record which is thought to be indicative of a global carbon cycle perturbation at the time a prolonged period of anoxia during the cambrian may explain the previously enigmatic peculiarities seen in the fossil record it has been suggested that the cambrian ocean was oxygen deficient but physical evidence for widespread anoxia has been lacking now sulphur isotope data from cambrian rocks at six different locations around the world is presented with the finding of a positive sulphur isotope excursion in phase with a large excursion in the marine carbon isotope record which is thought to be indicative of a global carbon cycle perturbation at the time with the help of a box model these isotope shifts support the idea of large scale anoxic and sulphidic conditions in the later cambrian ocean widespread anoxia in the ocean is frequently invoked as a primary driver of mass extinction as well as a long term inhibitor of evolutionary radiation on early earth in recent biogeochemical studies it has been hypothesized that oxygen deficiency was widespread in subsurface water masses of later cambrian oceans1 2 possibly influencing evolutionary events during this time1 2 3 physical evidence of widespread anoxia in cambrian oceans has remained elusive and thus its potential relationship to the palaeontological record remains largely unexplored here we present sulphur isotope records from six globally distributed stratigraphic sections of later cambrian marine rocks about 499 million years old we find a positive sulphur isotope excursion in phase with the steptoean positive carbon isotope excursion spice a large and rapid excursion in the marine carbon isotope record which is thought to be indicative of a global carbon cycle perturbation4 5 numerical box modelling of the paired carbon sulphur isotope data indicates that these isotope shifts reflect transient increases in the burial of organic carbon and pyrite sulphur in sediments deposited under large scale anoxic and sulphidic euxinic conditions independently molybdenum abundances in a coeval black shale point convincingly to the transient spread of anoxia these results identify the spice interval as the best characterized ocean anoxic event in the pre mesozoic ocean and an extreme example of oxygen deficiency in the later cambrian ocean thus a redox structure similar to those in proterozoic oceans6 7 8 may have persisted or returned in the oceans of the early phanerozoic eon indeed the environmental challenges presented by widespread anoxia may have been a prevalent if not dominant influence on animal evolution in cambrian oceans

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

lithofacies and depositional mechanisms of the ordovician silurian wufeng longmaxi organic rich shales in the upper yangtze area southern china the ordovician silurian organic rich shales in the wufeng and longmaxi formations are widespread in southern china and are the most important shale gas producers in china the shales display varying total organic carbon content ranging from 0 2 to 6 and are composed of black siliceous mudstone shelly limestone siliceous clay rich mudstone gray silty shaly interlaminated mudstone gray argillaceous mudstone and calcareous or dolomitic mudstone detailed stratigraphic correlation of these formations was achieved and a high resolution sequence stratigraphy was constructed based on the integration of well log data mineralogical geochemical and petrophysical analyses the wufeng formation and the guanyinqiao bed were interpreted to be a condensed section with four cycles of sea level fluctuation the longmaxi formation consists of three members the long 1 member was interpreted to be a third order depositional sequence that can be subdivided into a transgressive systems tract an early highstand systems tract and a late highstand systems tract within the constructed isochronal stratigraphic framework the characteristics and genetic mechanism of different shale lithofacies were studied the deposition of black siliceous mudstone of high total organic carbon in the wufeng formation and the long 1 transgressive systems tract was likely influenced by volcanic activity the guanyinqiao shelly limestone or calcareous mudstone was associated with the ordovician silurian transition when glaciation and mass extinction took place the gray silty shaly interlaminated mudstone of moderate total organic carbon in the long 1 early highstand systems tract was the product of bottom flow intrusion the deposition of gray argillaceous mudstone and calcareous or dolomitic mudstone with low total organic carbon in the long 1 late highstand systems tract resulted from enhanced terrigenous input from peripheral uplifts the distribution pattern of different lithofacies varies greatly in the upper yangtze area and this difference is a result of dissimilar sediment provenances and variable hydraulic restriction levels that were controlled by basin geometry a better understanding of the distribution pattern and depositional history of the wufeng and longmaxi formations might provide guidance for future profitable shale gas exploration

carbon and sulfur isotope geochemistry of the hirnantian late ordovician wangjiawan riverside section south china global correlation and environmental event interpretation detailed geochemical analyses 13ccarb 13corg 34ssulfide and abundance of sulfide carbonate and organic carbon were performed on samples from the wangjiawan riverside section close to the gssp global stratotype section and point for the hirnantian stage of the ordovician new data show two increases in carbonate content coincident with two glacial pulses that reduced detrital input the new 34ssulfide data show distinct changes in this section with relatively high values in the kuanyinchiao formation a pattern observed throughout the yangtze platform however there is no consensus on the cause of these changes the new 13ccarb data show a sharp rise and peak in the extraordinarius zone below the previously published 13corg peak in the persculptus zone a compilation of the new results with other sections indicates the hirnantian carbon isotope excursion starts near the pacificus extraordinarius boundary and elevated values remain until the end of the excursion in the persculptus zone for both 13corg and 13ccarb the controversy over correlating hirnantian graptolite zones with chitinozoan zones can also be addressed the new 13ccarb data also allow direct comparison with hirnantian 13ccarb data from anticosti island and the baltic region which are zoned by chitinozoan fossils this comparison favors a correlation of the taugourdeaui and scabra chitinozoan zones with the extraordinarius graptolite zone

sulfate oxygen 17 anomaly in an oligocene ash bed in mid north america was it the dry fogs 1 the oxygen and sulfur isotope compositions of sulfate particularly the oxygen 17 anomaly provide clues to the origin of sulfate we report a variable oxygen 17 anomaly for water and acid leached sulfate from an oligocene volcanic ash bed in the northern high plains u s a we find no sulfate o 17 anomaly however in freshly collected ashes from recent eruptions around the world the oligocene paleoclimatic and sedimentary evidence argues against a hyperarid condition in the high plains thus a long term background atmospheric sulfate accumulation is less likely the origin combining sulfur isotope data we suggest that the anomalous sulfate was associated with volcanic eruptions in the west but was not directly carried by ash falls extreme dry fog sulfate haze events resulting from tropospheric oxidation of volcanic sulfur gases might provide a viable explanation for the sulfates in the ash bed that carry the oxygen 17 anomaly

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

multiple oxygen and sulfur isotopic analyses on water soluble sulfate in bulk atmospheric deposition from the southwestern united states 1 sulfate is a major component of bulk atmospheric deposition including dust aerosol fog and rain we analyzed sulfur and oxygen isotopic compositions of water soluble sulfate from 40 sites where year round dust traps collect bulk atmospheric deposition in the southwestern united states average sulfur and oxygen isotopic compositions 34s and 18o are 5 8 1 4 cdt and 11 2 1 9 smow n 47 respectively samples have an oxygen 17 anomaly 17o with an average value of 1 0 0 6 except for a weak positive correlation between 18o and 17o values r2 0 4 no correlation exists for 18o versus 34s 17o versus 34s or any of the three isotopic compositions versus elevation of the sample site exceptional positive 17o values up to 4 23 are found in samples from sites in the vicinity of large cities or major highways and near zero 17o values are found in samples close to dry lakes comparison of isotopic values of dust trap sulfate and desert varnish sulfate from the region reveals that varnish sulfate has average isotopic values that are 4 8 lower for 18o 2 1 higher for 34s and 0 3 lower for 17o than those of the present day bulk deposition sulfate although other factors could cause the disparity this observation suggests a possibility that varnish sulfate may have recorded a long term atmospheric sulfate deposition during the holocene or pleistocene as well as the differences between sulfur and oxygen isotopic compositions of the preindustrial bulk deposition sulfate and those of the industrial era

quantifying sulfate components and their variations in soils of the mcmurdo dry valleys antarctica 1 many soils of the mcmurdo dry valleys mdv antarctica being old hyperarid and frigid have accumulated abundant atmospheric salts over the last several million years this salt repository offers an opportunity to study atmospheric chemistry past and present the origin and transport of ions in soils the weathering activity of soils and postdepositional soil leaching processes within this unique environment in particular soil sulfate in the mdv is known to have multiple origins but the precise proportions of different sulfate components remain elusive here we test a hypothesis that soil sulfate in the mdv is a predictable mixture of three major components sea salt sulfate non sea salt sulfate and background sulfate derived from weathering and volcanic sources that to a large extent varies as a function of elevation and distance from the coast by measuring sulfate s three stable isotope parameters i e 17o 18o and 34s plus estimating independently the end member stable isotope parameters for the three components on the basis of published reports and our measured data we solve explicitly the mixing proportion for each component using a set of three simultaneous linear equations the results from four spatially representative soil profiles show that the sum of such calculated proportions for the three components is very close to unity in most samples suggesting that a simple three component mixing model is a good representation of the soil sulfate budget while significant uncertainties still exist in the isotope compositions of different sulfate end members these data provide important initial constraints on sulfate diversity as well as its spatial and vertical distribution in the mdv

carbon and sulfur isotopic anomalies across the ordovician silurian boundary on the yangtze platform south china abstract abundance of organic matter carbon isotopic compositions of organic matter 13corg and sulfur isotopic compositions of iron sulfide 34ssulfide across the ordovician silurian o s boundary was analysed from two sections wangjiawan western hubei nanbazi northern guizhou on the yangtze platform south china the organic abundance across the o s boundary at these two sections is generally high except for the hirnantian interval at wangjiawan section in western hubei the 13corg values vary from about 30 8 vpdb in the mid ashgill to 27 6 in the upper hirnantian interval which abruptly return to the pre hirnantian values at nanbazi section in northern guizhou 13corg values vary from 30 5 to 26 6 from the mid ashgill to the upper hirnantian horizons which shift negatively to the low spike at 29 2 in the lowermost rhuddanian then increase slightly to relatively persistent values around 28 similar variation patterns of 34ssulfide values are unravelled at the two sections showing a positive excursion from the mid ashgill to the upper hirnantian then a sharp negative shift in the lowermost rhuddanian although with different background values of 34ssulfide from these two sections these data together with those from other areas demonstrate large climatic fluctuations from warming to cooling then to warming oceanic changes from and anoxic to oxygenated to anoxic water columns during the o s transition climatic fluctuations together with multiple oceanic anoxia and sea level fluctuations were likely responsible for the stepwise massive demise of the o s biotic crisis

new evidence for compaction driven vertical fluid migration into the upper ordovician hirnantian guanyinqiao bed of south china abstract the late ordovician was a time of major climatic and sea level changes linked to the hirnantian glaciation marked by large positive excursions of 13ccarb and 13corg globally known as the hirnantian carbon isotopic excursion hice however some upper ordovician sections in south china exhibit negative 13ccarb shifts within the hirnantian age guanyinqiao bed we hypothesize that these unusual carbon isotope patterns were due to influx of 13c depleted dissolved inorganic carbon dic from the underlying wufeng and overlying longmaxi black shales as a result of greater compaction of these fine grained siliciclastic units compared to the coarse grained carbonate guanyinqiao bed the isotopically light dic was generated through oxidation of organic matter by either microbial sulfate reduction msr or methanogenesis with subsequent anaerobic oxidation of methane aom in the black shales this hypothesis is supported by petrographic and geochemical evidence of vertical fluid migration within the guanyinqiao bed and across its contacts in the shiqiao drillcore the guanyinqiao bed exhibits a v shaped depression at its top that is connected to vertical channels filled with secondary carbonate cements having low 13c values in the huanggexi section both the base and top of the guanyinqiao bed are characterized by strong u enrichments despite unambiguously oxic depositional conditions as inferred from other proxies fe speciation mo and v concentrations these anomalous u enrichments resulted from oxidative remobilization of u along the guanyinqiao black shale contacts and subsequent transport into the guanyinqiao bed through compactional flow this finding demonstrates the importance of recognizing remobilized trace metals in sedimentary successions in order to avoid potential errors in redox interpretations

oxygen and sulfur isotopic composition of volcanic sulfate aerosol at the point of emission volcanic sulfate aerosol is emitted from the vents of many active volcanoes but its origin has not yet been explained we report the first measurements of the isotopic compositions of near vent volcanic aerosol sulfate and use the combined sulfur and oxygen isotope systematics of the aerosol and magma at masaya volcano nicaragua to draw preliminary conclusions about the production mechanism of near source volcanic sulfate the sulfate aerosol does not display a significant mass independent oxygen or sulfur isotopic signature which suggests that primary volcanic aerosol does not contribute to the larger mass independent anomalies found in some ambient atmospheric aerosols and as preserved in ice cores the oxygen isotope composition of the sulfate appeared to become heavier on the particle filters with increasing amount of sample collected and suggests that the 18o value of the sulfate aerosol is 8 the sulfur isotopic composition of the sulfate did not vary with sample loading on the filters and had a mean 34s value of 7 7 0 8 similar to that of the magma 6 6 0 2 the low 17o value the 18o value of the magma 6 6 0 3 and the high 18o value of atmospheric oxygen 23 5 suggest that known atmospheric oxidation pathways at ambient temperatures are not the major routes of production for this sulfate instead the isotopic systematics of aerosol and magma are consistent with sulfate production either by high temperature equilibration of the magmatic gas mixture with small amounts of atmospheric oxygen or by direct emission of so4 2 from the magma

u mo ratios and δ98 95mo as local and global redox proxies during mass extinction events abstract when used in conjunction with trace metal redox proxies the use of u mo ratios and 98 95 mo values can considerably enhance the interpretation of ocean oxygenation trends both locally and globally this is illustrated with studies from two biogenic chert dominated records of mass extinction the ordovician silurian boundary and the late permian in sections from the yangtze block in south china the latest ordovician mass extinction is well known to coincide with oxygenation episode in the hirnantian stage here revealed by trace metal proxies v v ni v cr ni co that show it sandwiched between anoxic euxinic black shales the predominantly anoxic euxinic ordovician silurian strata provide a record of seawater 98 95 mo values that are seen to gradually rise during the interval from 0 5 to 1 0 the exception to the 98 95 mo stability is the datum of the slight negatively 98 95 mo associated with decreasing of v cr and v v ni ratio suggesting the hirnantian oxygenation event the upper permian chert succession at shangsi reveals an overall better ventilated record in which the best oxygenated strata show the highest u mo values that is interpreted as relative enrichment of u in suboxic conditions at a few levels anoxic euxinic conditions show the 98 95 mo values to be 2 close to modern values implying that latest permian oceans were as well ventilated as those today the well known end permian oceanic anoxic event is therefore suggested to only develop at the end of this period

constraints on the accumulation of organic matter in upper ordovician lower silurian black shales from the lower yangtze region south china abstract the late ordovician to early silurian o s was an important period in which many significant geological events occurred during the o s transition organic rich black shales were widely deposited and became important unconventional energy sources paleoproductivity detrital input and redox conditions are the three major factors controlling organic matter om accumulation although the effect of each factor is relatively clear the complex influences of these factors remain poorly understood specifically the evolution of these factors and the corresponding influence on the accumulation of om during the o s transition has not yet been systematically investigated this paper investigates the constraints on om accumulation in o s transition black shales from the lower yangtze region based on graptolite fossil identification zircon u pb dating and geochemical analyses of a well sequenced drilling profile the results show that i the sy 1 core sequence was well constrained from graptolite zones wf2 447 62 ma to lm5 441 57 ma across the o s boundary with decreasing sedimentation rates srs in the upper katian wf2 to wf3 and increasing srs in the lower rhuddanian ii complementary volcanic inputs katian and rhuddanian and upwelling events hirnantian exerted strong control over paleoproductivity iii orogenic processes in south china controlled detrital inputs and redox conditions iv om accumulation in the lower yangtze area across the o s transition occurred in three stages in stage 1 wf2 wf3 strong volcanism boosted productivity whereas om preservation was not favored in the oxic environment in stage 2 wf4 lm3 upwelling and volcanism resulted in high productivity and provided the optimal configurations for om enrichment in stage 3 lm4 and later rare om was preserved due to terrestrial dilution and unfavorable preservation conditions eventually wf4 lm3 became the prominent om rich biozones due to the effective coordination of the controlling factors namely upwelling volcanism and redox conditions

sulfate oxygen 17 anomalies in desert varnishes rock varnishes are ubiquitous in arid regions on earth and are believed to be commonly present on mars here we report high water soluble sulfate and nitrate contents in desert varnishes from the death valley region of southwestern u s a and that sulfate in varnishes possess 17o 18o ratios that do not fall on terrestrial mass dependent fractionation line sulfate from wet and dry atmospheric deposition is probably the source of the 17o anomalies the anomalies are only moderately lower than that of aerosol sulfates collected from the greater los angeles area indicating probably more than half of the sulfate in desert varnish is supplied by atmospheric deposition this finding suggests that earth surface environments are constantly accumulating 17o anomalous sulfate from the atmosphere arid and stable conditions facilitate the preservation of these atmospheric signatures this finding also indicates that different 17o 18o ratios found in different components in the martian meteorites may result from atmospheric chemical processes

sizes and shapes of 10 ma distal fall pyroclasts in the ogallala group nebraska abstract size distributions of distal ashfall particles from correlated 10 ma layers in nebraska measured using laser diffraction methods are lognormal with mode diameters of 90 m this ashfall is 100 bubble wall shards of rhyolite glass and apparently represents a distal ashfall from an eruption 1400 km away measured terminal velocities of these ash particles are 0 2 18 cm s consistent with stokes law settling of spherical particles with diameters of 9 50 m surface area of the ash particles measured with gas adsorption is 20 30 times the surface area of equivalent stokes spheres these results highlight the effects of shape and atmospheric drag in distal ashfalls they also highlight atmospheric transport and fallout of distal ashfall particles because these deposits resemble many other ashfalls preserved in the great plains of north america throughout the tertiary and quaternary because the ashfalls preserve major mammalian death assemblages they demonstrate that deposits with modes of op

phanerozoic organic carbon rich marine sediments overview and future research challenges one of the major obsessions of many early workers to the mid 1900s was the application of uniformitarian principles to deposi tional models for black shales arthur and sageman 1994 the purpose of this overview of organic carbon oc rich marinesediments is to provide a brief but current summary of the historicaldevelopments principle concepts and remaining challenges in inte grated sapropel and black shale research as such it provides asubstantive introduction to the special issue of palaeogeography palaeoclimatology palaeoecology on organic carbon rich sedimentsthrough the phanerozoic processes progress and perspectives giventhis focused scope the overview does not aim to be comprehensive orcomplete but to provide a solid setting for the fourteen individualresearch papers that constitute this special issue and two previousspecial issues meyers and negri 2003 negri et al 2006 thatcoverresearch aspects complementary to this one like the individualcontributions this introduction and overview is organized intocenozoic mesozoic and palaeozoic units the cenozoic and palaeozoicunitsaredeliberatelylargerthanthemesozoicunit acknowledgingthatmuch ground on mesozoic black shale is already covered in the twoprevious special issues because the late cenozoic sapropels of themediterraneanbasinhavebeenextensivelystudied understandinghowthese near modern analogs of ancient black shales were depositedprovidesagoodfoundationforunderstandinghowtheoldersequencesmayhaveevolved incontrast becausefarlessisgenerallyknownabouttheblackshalesofthepalaeozoicthancomparableoc richsequencesofeither the cenozoic or the mesozoic a more comprehensive summaryand comparison of the more ancient sequences is particularly appro priate to the theme of this special issue 1 introductionsediments rich in organic carbon are restricted to small areas alongcontinental margins in the modern ocean and have rarely accumulatedduringthecenozoic yetorganic rich sediments werewidely depositedduring multiple intervals of mesozoic palaeozoic time and even earlierduringtheproterozoic archean globalmarinedepositsdocumentthatepisodes of accumulation of oc rich sediments occurred in differentregions and at different times these episodes were linked to climaticandpalaeoceanographicperturbationsthat resulted inmassive uctua tions in hydrologic and nutrient cycles and in oceanchemistryand thatrecurred throughout geologic time however the paucity of survivingpalaeozoic and earlier black shale sections makes it dif cult toimpossible to recognize the internal structure of global events that arecommon inyounger oc rich sedimentary sequences asunderlinedbelow themarinesectionsinmanylocalitiesconsistof intercalated carbonates and shales that may grade into purecarbonate or black shale sedimentation often re ecting shallowing

origin sequence stratigraphy and depositional environment of an upper ordovician hirnantian deglacial black shale jordan the upper ordovician succession of jordan was located 60 s less than 100 km from the hirnantian ice sheet margin new graptolite dates indicate glaciation ended in jordan in the late hirnantian persculptus biozone the succession records two glacial advances within the ammar formation and the subsequent deglaciations organic rich black shales batra formation form part of the final deglacial transgressive succession that in filled an existing low stand glacial continental shelf topography the base of the black shale is coincident with the maximum flooding surface during transgression interfluves and sub basin margins were breached and black shale deposition expanded rapidly across the region the top of the black shales coincides with peak highstand the expanding puddle model sensu wignall for black shale deposition adapted for the peri glacial setting provides the best explanation for this sequence of events we propose a hypothesis in which anoxic conditions were initiated beneath the halocline in a salinity stratified water column a fresher surface layer resulted from ice meltwater generated during early deglaciation during the initial stages of marine incursion nutrients in the monimolimnion were isolated from the euphotic zone by the halocline increasing total organic carbon toc and 13corg up section indicates the organic carbon content of the shales was controlled mainly by increasing bioproductivity in the mixolimnion the strakhov model mixolimnion nutrient levels were sustained by a continual and increasing supply of meltwater derived nutrients modulated by obliquity changes in high latitude insolation anoxia was sustained over tens to hundreds of thousands of years the formation of black shales on the north gondwana shelf was little different to those observed in modern black shale environments suggesting that it was the nature of the ordovician seas that pre disposed them to anoxia

high resolution carbon isotopic records from the ordovician of south china links to climatic cooling and the great ordovician biodiversification event gobe abstract the great ordovician biodiversification event gobe represented the largest increase in diversity in the marine biosphere during the phanerozoic eon yet its causes and consequences remain poorly understood patterns of isotopic variation in high resolution 13ccarb and 13corg records from a well exposed section at honghuayuan in south china may provide important insights regarding the gobe the honghuayuan isotopic profiles which can be correlated with c isotopic records from contemporaneous sections globally reveal large perturbations to the global carbon cycle during the ordovician a 8 increase in 13corg values in the floian implies a large albeit transient increase in the burial rate of organic matter during the mid early ordovician that may have contributed to climatic cooling and played an important role in triggering the gobe a 4 increase in 13ccarb and high frequency variation in 13corg in the darriwilian to sandbian suggest a second episode of elevated organic carbon burial rates accompanied by substantial instability in the global carbon cycle during the late middle and early late ordovician this pattern may mark the onset of climate changes culminating in the end ordovician hirnantian glaciation and mass extinction event that terminated the gobe

ecological ranking of phanerozoic biodiversity crises the serpukhovian early carboniferous crisis had a greater ecological impact than the end ordovician we propose a new ecological ranking of the major phanerozoic biodiversity crises in which the serpukhovian biodiversity crisis is ranked fifth in ecological impact lesser than the late devonian but greater than the end ordovician and the end ordovician mass extinction is ranked sixth it is interesting that both the end ordovician mass extinction and the serpukhovian biodiversity crisis were triggered by glaciations other than that common trigger the two events were very different glaciation in the ordovician triggered an enormous jump in the extinction rate of marine organisms and was taxonomically very severe yet the ecological impact of those extinctions was minimal glaciation in the serpukhovian triggered a precipitous drop in the speciation rate but only moderate diversity losses yet the ecological impact of those diversity losses and ecosystem restructuring was an ecological level of magnitude larger than that seen in the end ordovician mass extinction

deformation structures and an alteration zone linked to deposition of volcanogenic sulphate in an ancient playa oligocene of nebraska usa historic sulphur rich volcanic eruptions have altered global climate for as much as five years and much larger events are known from the geologic record at scotts bluff nebraska early oligocene strata of the lower arikaree group contain a tephra bed with abundant calcite pseudomorphs after gypsum previous work has shown sulphate from the pseudomorphs in this tephra bears a high 17 o anomaly indicative of oxidation of sulphur gases by ozone or hydrogen peroxide in the atmosphere possible sources of the tephra were caldera eruptions at about 28 ma in the san juan volcanic field of southwestern colorado 500 km sw of the study site and the eastern great basin 1000 km wsw the present sedimentological study shows that tephra and volcanogenic sulphate were deposited and preserved within a small surfacedischarging playa that developed on the irregular upper surface of aeolian siltstones of the subjacent white river group sulphate solutions including perhaps sulphuric acid percolated downward within the vadose zone dissolving early formed smectite cement within underlying volcaniclastic sandstones reddening these rocks along an irregular alteration front preserved fine scale stratification within the sandstones precludes the possibility that reddening took place during pedogenesis displacive growth of gypsum at the playa centre folded tephra beds and forced tephra into underlying sandstones forming elongate cones the large mass fraction of gypsum now replaced by calcite in the playa sediments suggests a huge long distance delivery of sulphate aerosols some of the sulphate and tephra may have come from the same eruption or the fine grained tephra may simply have aided preservation of dry fog sulphate derived from an unrelated effusive eruption of lava

anomalous 17o compositions in massive sulphate deposits on the earth the variation of 18o that results from nearly all physical biological and chemical processes on the earth is approximately twice as large as the variation of 17o this so called mass dependent fractionation is well documented in terrestrial minerals1 2 evidence for mass independent fractionation 17o 17o 0 52 18o where deviation from this tight relationship occurs has so far been found only in meteoritic material and a few terrestrial atmospheric substances3 in the rock record it is thought that oxygen isotopes have followed a mass dependent relationship for at least the past 3 7 billion years ref 4 and no exception to this has been encountered for terrestrial solids5 here however we report oxygen isotope values of two massive sulphate mineral deposits which formed in surface environments on the earth but show large isotopic anomalies 17o up to 4 6 these massive sulphate deposits are gypcretes from the central namib desert and the sulphate bearing miocene volcanic ash beds in north america the source of this isotope anomaly might be related to sulphur oxidation reactions in the atmosphere and therefore enable tracing of such oxidation these findings also support the possibility of a chemical origin of variable isotope anomalies on other planets such as mars6

sources of iron for pyrite formation in marine sediments more than two hundred aerobic continental margin aerobic deep sea dysaerobic and anaerobic euxinic sediments have been examined for their variations in different operationally defined iron fractions each of which represents a different reactivity towards dissolved sulfide aerobic continental margin deep sea and dysaerobic sediments contain similar contents of highly reactive iron dithionite soluble iron plus pyrite iron poorly reactive iron iron soluble in hcl less that soluble in dithionite and unreactive iron total iron less that soluble in hcl by contrast non turbidite euxinic samples from the black sea as well as euxinic samples from the cariaco basin and framvaren are enriched in highly reactive iron these sediments contain a small lithogenous fraction and a large biogenous organic c rich fraction which decays by sulphate reduction in an iron rich water column to form pyrite rich sediment other anaerobic euxinic samples from the black sea orca basin and kau bay contain lower concentrations of biogenous sediment and are not therefore enriched in highly reactive iron degrees of pyritization dop for all the aerobic dysaerobic and anaerobic euxinix samples except those low in biogenous material are consistent with analogous ancient sediments and indicate that most pyrite formation occurs form the highly reactive iron fraction

a late amazonian alteration layer related to local volcanism on mars hydrated minerals on mars are most commonly found in ancient terrains dating to the first billion years of the planet s evolution here we discuss the identification of a hydrated light toned rock unit present in one chasma of the noctis labyrinthus region stratigraphy and topography show that this alteration layer is part of a thin unit that drapes pre existing bedrock crism spectral data show that the unit contains hydrated minerals indicative of aqueous alteration potential minerals include sulfates such as bassanite caso4 1 2h2o or possibly hydrated chloride salts the proximity of a smooth volcanic plain and the similar crater model age late amazonian 100 myr of this plain and the draping deposits suggest that the alteration layer may be formed by the interaction of water with ash layers deposited during this geologically recent volcanic activity the alteration phases may have formed due to the presence of snow in contact with hot ash or eventually solid gas interactions due to the volcanic activity the relatively young age of the volcanic plain implies that recent alteration processes have occurred on mars in relation with volcanic activity but such local processes do not require conditions different than the current climate

lower silurian hot shales in north africa and arabia regional distribution and depositional model abstract lowermost silurian organic rich hot shales are the origin of 80 90 of palaeozoic sourced hydrocarbons in north africa and also played a major role in petroleum generation on the arabian peninsula in most cases the shales were deposited directly above upper ordovician peri glacial sandstones during the initial early silurian transgression that was a result of the melting of the late ordovician icecap deposition of the main organic rich shale unit in the north african arabian region was restricted to the earliest silurian rhuddanian stage acuminatus atavus and probably early cyphus graptolite biozones during this short period 1 2 m y a favourable combination of factors existed which led to the development of exceptionally strong oxygen deficiency in the area in most countries of the study area the post rhuddanian silurian shales are organically lean and have not contributed to petroleum generation the distribution and thickness of the basal silurian hot shales have been mapped in detail for the whole north african region using logs from some 300 exploration wells in libya tunisia algeria and morocco in addition all relevant accessible published and unpublished surface and subsurface data of the lower silurian shales in north africa and arabia have been reviewed including sedimentological biostratigraphic and organic geochemical data the lowermost silurian hot shales of northern gondwana are laterally discontinuous and their distribution and thickness were controlled by the early silurian palaeorelief which was shaped mainly by glacial processes of the late ordovician ice age and by pan african and infracambrian compressional and extensional tectonism the thickest and areally most extensive basal silurian organic rich shales in north africa occur in algeria tunisia and western libya while on the arabian peninsula they are most prolific in saudi arabia oman jordan and iraq the hot shales were not deposited in egypt which was a large palaeohigh at that time the depositional model presented may help in better understanding the source potential of the basal silurian shales in less explored regions of north africa and arabia including morocco northern niger and the kufra basin in southeast libya

changes in marine productivity and redox conditions during the late ordovician hirnantian glaciation abstract changes in marine productivity and redox conditions during the end ordovician hirnantian glaciation and ordovician silurian transition were investigated through mo isotope and major and trace element analyses of the wangjiawan hubei province and nanbazi guizhou province sections from the yangtze platform of south china katian shales of the wufeng formation which yield the graptolites dicellograptus complanatus dicellograptus complexus and paraorthograptus pacificus were deposited under euxinic conditions at both localities as shown by high mo ef u ef and 98 mo values a major sea level regression during the hirnantian glaciation resulted in shallowing and a shift toward better oxygenated conditions within the yangtze sea as well as deposition of thin bedded siliceous sediments calcirudite debris flows and limestone turbidites of the kuanyinchiao formation the base of which correlates with the first phase of the end ordovician mass extinction the termination of the hirnantian glaciation at the top of the kuanyinchiao formation was associated with a major sea level transgression a rapid expansion of euxinia in the yangtze sea as documented by a return of high mo ef u ef and 98 mo values and the second phase of the mass extinction during which the cool adapted hirnantian fauna went extinct the long term cooling trend of the middle and late ordovician which culminated in the hirnantian glaciation was driven by enhanced burial of organic carbon as documented by 13 c carb 13 c org and 34 s py records increased organic carbon burial was linked to high rates of marine productivity as shown by high toc and biogenic ba concentrations especially at the deeper less restricted wangjiawan locale producing the hice hirnantian isotopic curve excursion 13 c carb excursion the locus of organic carbon burial during the hirnantian crisis shifted to deeper water environments that were located outside the study region the relatively rapid onset and termination of the hirnantian glaciation were probably due to crossing of tipping points in the late ordovician climatic oceanic system

tectonic evolution of the west junggar region xinjiang china the west junggar region of northwestern china consists of a hercynian age fold belt occupying a late paleozoic zone of convergence between three major eurasian plates siberian tarim and kazakhstan the northern part of the west junggar area includes part of the irtysh zaysan fold belt that extends to the northwest into the ussr where it marks a broad boundary between the siberian and kazakhstan plates the central part of the west junggar area includes the southeastern extension of the chingiz tarbgatay early paleozoic island arc sequence the southern part of the west junggar area is truncated by the deep fundamental junggar alakol fault that has brought the yili microplate into juxtaposiition with the west junggar region in the vicinity of ebin nur lake to the east the junggar area is covered by sediments of the junggar basin paleogeographic reconstructions and geologic evidence indicates that these plates began converging in the middle carboniferous and by late carboniferous alkali granites began invading the imbricated leading edges of the coalescing plates paleomagnetic data reveals that the plates continued to move along strike slip faults bringing the tarim plate and yili microplate into their present positions sometime in the early mesozoic six separate ophiolite occurrences in the west junggar region are strongly tectonized and dismembered the tangbale ophiolite is considered to be late cambrian 508 20 ma and represents the earliest known oceanic crust in western china petrologic and chemical data indicate that the tangbale ophiolite represents a possible back arc or forearc tectonic setting situated close to a developing island arc the darbut ophiolite has been tectonized to a melange and is overlain by late devonian flysch ages of the radiolaria in the cherts associated with the pillow lavas indicate that it formed in the middle devonian other occurrences of ophiolite melange in the northern part of the west junggar region may be as young as late devonian these ophiolites record ocean crust formation in the early to middle paleozoic within the junggarian ocean and their present tectonic setting indicates continental accretion and imbrication shortly after their formation associated blueschist blocks in the ophiolite melange further supports the concept of imbrication brought about by b type subduction and associated obduction postcollisional alkali granites intrude the oceanic sequences of the west junggar region and are considered to represent melted lower crustal material lead isotopic systematics on feldspars from six separate granite plutons indicate ratios that could only be considered oceanic as far as the magmas are concerned the implications of this finding is that the junggar basin basement rocks in the north and west are probably oceanic evidence presented in this study shows a complex history for the development of the west junggar region major movements of consolidating plates in the late paleozoic followed by rejuvenation of these same zones of closure during the himalayan tectonic event of the late tertiary demonstrates that convergence zones and fold belts may respond to vertical and horizontal stress even after many million years of inactivity

a critical look at iron paleoredox proxies new insights from modern euxinic marine basins enrichments in reactive iron occur under euxinic marine conditions that is where dissolved sulfide is present in the water column these enrichments result primarily from the export of remobilized iron from the oxic shelf which is scavenged from the euxinic water column during syngenetic pyrite formation and deposited in the underlying sediments strongly elevated ratios of highly reactive iron to total iron fehr fet and total iron to aluminum fet al and high degrees of pyritization dop are each products of this enrichment process these paleoredox proxies are among the most faithful recorders of ancient euxinia contrary to previous arguments iron enrichment is decoupled from biogenic sediment inputs but it does appear to be a uniquely euxinic phenomenon in other words we can rule out a major contribution from preferential physical transport of fehr rich detrital sediment to the deep basin which could also operate under oxic conditions furthermore enrichment via the shuttling of iron remobilized from oxic shelves appears to be limited by inefficient transport and trapping processes in deep oxic basins elevated fet al ratios in the euxinic sediments also cannot be a product of internal enhancement of the reactivity of the detrital iron pool without net fehr addition these conclusions are supported by observations in the modern black sea orca basin and effingham inlet fet al ratios are unambiguous recorders of paleoredox even in sediments that have experienced high degrees of metamorphic alteration however this study suggests that high siliciclastic accumulation rates can swamp the enrichment mechanism resulting in only intermediate dop values for euxinic sediments and fet al ratios that mimic the oxic shelf such dilution effects are well expressed in black sea basinal turbidites and rapidly accumulating muds on euxinic basin margins under conditions of persistent euxinia varying extents of fehr enrichment can illuminate spatial and temporal gradients in siliciclastic sedimentation the magnitude of enrichment is a function of the source shelf to sink ocean basin areal ratio suggesting that iron proxies can also record ocean scale paleoenvironmental properties through muted enrichments at times of very widespread euxinia for the first time manganese data are interpreted in light of the redox shuttle model as for the iron data the black sea orca basin and effingham inlet show enrichments in total manganese in the deep euxinic basin suggesting export from the suboxic porewaters of the oxic shelf and scavenging and burial in the basin the black sea data reveal iron and manganese enrichment across the broad deep euxinic basin suggesting efficient lateral transport and deep water mixing tied to the physical properties of the water column

high resolution stable isotope stratigraphy of upper ordovician sequences constraints on the timing of bioevents and environmental changes associated with mass extinction and glaciation the two phases of the late ordovician mass extinction are approximately coeval with the periods of rapid climate change associated with the onset and demise of the gondwanan glaciation in this paper we argue that the distinctive late ordovician carbon isotope profile provides a chronostratigraphic ruler against which a sequence of environmental and biotic events may be located the ruler also allows regional and global high resolution correlation of successions representing very different environments cores from the upper ordovician succession of estonia and latvia record a large 1 3 c c a r b o n a t e excursion up to 6 with a similar profile shape the consistent relationship between the chemostratigraphy and biostratigraphy in the baltic region suggests that the isotope profile has a regional chronostratigraphic value the presence of similar profiles in nevada united states suggests that the excursion is a global chronostratigraphic signal this interpretation enables a detailed correlation to be made between upper ordovician shallow marine and basinal sequences that have wholly different faunas successions in the baltic area and in canada that do not display the model profile are interpreted as incomplete reinter pretation of these important successions significantly modifies the global database used to assess the pattern of diversity change during the mass extinction key levels of environmental change have been located against the carbon isotope profile new oxygen isotope data from brachiopod and ostracode calcite set tight limits on the start of the glacial events cooling and sea level fall started at the same stratigraphic level as the start of the carbon isotope excursion the later rise in sea level and fall in oxygen isotope values record the end of the glaciation these restrict the duration of the main glaciation to only 1 5 graptolite zones we propose that models of the carbon cycle should be adapted to be consistent with the temporal relationships between carbon cycling sea level fall and temperature change documented here the chronostratigraphic ruler provided by the carbon isotope profiles is used as a scale to determine the sequence of biotic changes and to allow high resolution correlation of biotic events at different locations this approach identifies regional similarities and differences in the patterns of extinction the main phase of graptolite extinction in the monitor range nevada for example is synchronous with the chitinozoan extinction in the baltic region but chitinozoan taxa survive to higher levels in nevada the benthic faunas in the baltic region demonstrate that the main extinction event corresponded with the beginning of the isotope excursion at the start of the hirnantian the level at which the marine environment started to change rapidly but that there were further extinctions of species within the early hirnantian the cold adapted hirnantia fauna did not appear immediately after the extinction in this area the relationship between the second phase of extinction and the carbon isotope excursion is less clear but the available data suggest that the extinction coincides with a time of rapid environmental change but not at the inception of environmental change as happened in the first phase

global and regional controls on marine redox changes across the ordovician silurian boundary in south china abstract the ordovician silurian o s transition coincided with significant environmental and biological changes in south china the yangtze platform experienced both global and regional events at this time including sea level fluctuations tectonic movements volcanic eruptions mass extinction and widespread anoxia the o s transitional strata of the yangtze platform comprise organic rich black shales that are an important oil source rock to explore the evolution of watermass chemistry and its relationship to organic matter accumulation we conducted an integrated fe s c geochemical study i e fe speciation 13corg 34spy pyrite s and toc of the o s boundary sections at datianba chongqing municipality and shuanghe sichuan province these sections were located in the restricted inner yangtze sea in contrast to the previously studied wangjiawan section which was deposited in a more open setting of the outer yangtze sea in contrast to the well oxygenated conditions at wangjiawan the datianba and shuanghe sections record persistently anoxic and episodically euxinic conditions during the hirnantian which can be explained by the unique paleogeography and restricted hydrography of the inner yangtze sea we propose that the regional kwangsian orogeny which was driven by collision of the yangtze and cathaysia blocks played a key role in basin development watermass chemistry changes and accumulation of wufeng formation black shales within the yangtze sea the more widespread black shales of the lower silurian lungmachi formation were linked to the post hirnantian global marine transgression

sulfate in modern playa settings and in ash beds in hyperarid deserts implication for the origin of 17o anomalous sulfate in an oligocene ash bed abstract a large amount of calcite pseudomorphs after gypsum were discovered in a volcanic ash bed and its underlying sand siltstone in the gering formation late oligocene northwestern ne usa the highly positive 17o anomalies in the residual sulfate indicate that the sulfate could only have come from the oxidation of sulfur gases by ozone in the atmosphere we previously proposed that it was most likely the result of an extreme tropospheric sulfate haze dry fog event that was associated with volcanic eruptions in the west to test the dry fog hypothesis we examine here two alternative scenarios that also involve atmospheric sulfate deposition but do not imply a dramatic volcanic and dry fog source 1 is it possible that some playas in arid or semiarid regions accumulate the type of atmospheric sulfate deposits as we see in the mid gering bed by the inflow of surface water or groundwater 2 could a period of hyperaridity and stability during the late oligocene have caused a significant accumulation of atmospheric background sulfate onto the mid gering ash bed surface sulfates from a variety of playa settings were collected and measured for their 17o anomalies the sulfate isotope both o and s and concentration profiles of two volcanic ash beds that are currently exposed in the central atacama desert chile and in taylor valley of the antarctica dry valleys were studied along with their sedimentary features these two ash beds were chosen for comparison because of their significant levels of atmospheric sulfate accumulated during prolonged and hyperarid climatic conditions results showed no or minimal 17o anomalies in any of the playa sulfate samples from around the world and none of the parameters in the mid gering ash profile matches those in either of the two volcanic ash profiles in modern hyperarid settings conclusion the observed 17o anomalous gypsum deposit in the mid gering ash bed is most likely the result of a series of extreme dry fogs that resulted in an intense pulse of atmospheric sulfate deposition to a semiarid playa setting

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

large perturbations of the carbon and sulfur cycle associated with the late ordovician mass extinction in south china high resolution 13 c data of organic carbon from a continuous section of the late ordovician early silurian reveal two positive 13 c excursions that are associated with the mass extinction in south china the fi rst stratigraphic 34 s measurements on pyrite tied to well established biostratigraphy indicate a large perturbation of the sulfur cycle consistent with major sea level changes related to the glaciation the elevated 34 s values of pyrites and a large short lived negative 34 s excursion of 20 associated with the decay of the glaciation suggest deep water anoxia during the hirnantian stage in contrast to the conventional view that the global oceans were oxygenated we suggest that deep water anoxia may have contributed to the late ordovician mass extinction in south china and possibly elsewhere

marine redox conditions in the middle proterozoic ocean and isotopic constraints on authigenic carbonate formation insights from the chuanlinggou formation yanshan basin north china abstract to improve our understanding of ocean chemistry and biogeochemical cycling following the termination of large scale paleoproterozoic iron formation if deposition 1 85 billion years ago ga we conducted a fe s c mo geochemical study of the 1 65 ga chuanlinggou formation yanshan basin north china despite the cessation of if deposition our results suggest the presence of anoxic but non euxinic ferruginous conditions persisted below the surface mixed layer for the deepest portion of the continental rifting basin and that this pattern is apparently independent of the local organic carbon content however our paired s isotope data of carbonate associated sulfate and pyrite suggest presence of sulfate in pore fluids which is not consistent with insufficient sulfate for bacterial sulfate reduction in the water column despite evidence for deposition under anoxic conditions sedimentary molybdenum mo concentrations are mostly not enriched relative to average continental crust this relationship is consistent with the notion that sulfide dominated conditions in the water column and or the sediments are required for mo enrichment and validates past assertions that mo enrichment patterns in ancient shales track both the local presence and global distribution of euxinia specifically in addition we identified extensive diagenetic carbonate precipitation in the upper chuanlinggou formation with only moderately negative 13 c values 3 4 1 4 we propose with support from a numerical model that these diagenetic carbon isotope values were most likely derived from precipitation of carbonates dominantly in the methanic zone within the sediments diagenetic carbonate precipitation in the methanic zone is likely to have been more extensive in the proterozoic than the phanerozoic due to porewater oxidant limitation

global carbon isotopic events associated with mass extinction and glaciation in the late ordovician mass extinctions and glacioeustatic sea level changes in the lower part of the hirnantian final stage of the ashgill are accompanied by shifts in marine stable isotope compositions previously published stable isotope changes have been used to identify the onset and demise of the gondwana glaciation and to suggest relationships between biotic changes and carbon cycling within the oceans however the existing isotopic data set had limitations because it was derived from ordovician low latitude settings and from carbonates or organic carbon in separate areas we report new data from ordovician high latitude carbonates and demonstrate parallel shifts in organic and carbonate 13c from baltica brachiopod shells from a high palaeolatitude periglacial setting in argentina have elevated 13c values similar to those described previously from low latitude sites the new data demonstrate that the positive hirnantian 13c excursion previously only recognised from low palaeolatitude areas was widespread and probably global in extent the poor preservation state of the brachiopods unfortunately prevented the determination of a reliable oxygen isotopic value from the same material preliminary carbon isotopic data from thermally immature organic matter from estonia provide the first indication of a synchronous shift in organic and inorganic 13c in sediments from the same basin this work provides new data of critical importance for constraining models of end ordovician palaeoceanography and climate change

anoxia vs productivity what controls the formation of organic carbon rich sediments and sedimentary rocks discussion the prevailing explanation for the origin of organic rich sediments and rocks invokes deposition under conditions of anoxia however recent research suggests that high primary production and not water column anoxia provides the first order control on the accumulation of organic rich facies in the modern oceans oxygen minima do not appear to have any direct effect on carbon accumulation in continental margin or marginal sea environments sediments accumulating in the modern black sea the type euxinic basin are not particularly enriched in organic matter despite the presence of an anoxic water column although a sapropel containing extremely high carbon concentrations was deposited during the holocene at a time when the basin was oxic results of a recently published co pled ocean atmosphere model indicate that during the cretaceous thermohaline and surface circulation in the oceans was similar to or more intense than modern conditions despite the overall equable climate such conditions confound the idea that circulation in the cretaceous atlantic for example was punctuated by oceanic anoxic events brought about by more sluggish circulation sporadic temporal and spatial increases in primary production reflecting changes in the behavior and or state of the ocean atmosphere system constitute a more tenable explanation for the occurrence of modern and quaternary carbon rich sediments and cretaceous black shales consequently the fundamental control on the accumulation of carbon rich facies in the oceans and marginal seas is not the presence or absence of anoxia

caliche as a geological repository for atmospheric sulfate 1 recent studies demonstrate that important information on sulfur source oxidation pathway transport pattern and reaction kinetics of atmospheric oxidation of sulfur gases can be uncovered by measuring multiple isotope compositions of the oxidation product secondary atmospheric sulfate sas sas from earth s distant past however is rarely preserved in the rock records because of its high solubility and small quantity we report here from the measurement of 47 modern and fossil caliche samples that caliches contain variable amounts of carbonate associated sulfate cas and the 17o values of the cas range from 0 12 to 2 32 with most values higher than the background value of 0 05 for seawater sulfate indicating the presence of an sas component derived from the oxidation of sulfur gases by atmospheric ozone or related oxidants this study demonstrates that caliche a pedogenic carbonate common in the geologic record could serve as a repository for ancient sas

facies patterns and geography of the yangtze region south china through the ordovician and silurian transition abstract lithofacies and biofacies changes through the ordovician and silurian transition on the yangtze platform are coincident with stepwise mass extinctions recovery events global sea level changes and regional palaeogeographic configurations facies patterns through three intervals the late mid ashgill hirnantian and early rhuddanian indicate that black shale occupied most of the yangtze platform region during the late mid ashgill and early rhuddanian while the hirnantian possesses more diverse facies types the black shale was replaced by carbonate facies during the hirnantian coincident with a global sea level low stand biofacies changes from the late mid ashgill graptolitic to the hirnantian shelly facies were apparently signalled by gradual and diachronous existence of a mixed facies the manosia facies in the yangtze region however the biofacies change was abrupt from the hirnantian to the early rhuddanian when graptolitic facies occupied most areas on the yangtze platform leaving only a very narrow limited belt of shelly facies along the southern margin of the platform in northern guizhou the early rhuddanian unified biofacies pattern is a result of a global sea level rise which may be synchronized with a recovery bioevent of graptolites after hirnantian mass extinction

glacial interglacial variations in marine phosphorus cycling implications for ocean productivity 1 using a box model of organic carbon c and phosphorus p cycling in the global ocean we assess the effects of changes in the continental supply of reactive p oceanic mixing and sea level on the marine p cycle on glacial interglacial timescales our model results suggest that mixing is a dominant forcing during early glaciation causing retention of p in the deep ocean thereby lowering primary production and associated organic c and p burial sea level fall is the dominant forcing during late glaciation when reduced coastal trapping of reactive p enhances its transfer to the open ocean restoring primary production and p burial during postglacial periods changes in circulation and weathering dominate open ocean processes and oceanic primary production peaks as primary production is reduced upon glaciation changes in the marine p cycle are unlikely to drive enhanced primary production and co2 drawdown during glaciation

late ordovician mass extinction a new perspective from stratigraphic sections in central nevada integrated sequence stratigraphic biostratigraphic and chemostratigraphic analyses of three stratigraphic sections in central nevada indicate that late ordovician glaciation induced sea level fall produced diachronous stepwise faunal turnover in graptolites conodonts chitinozoans and radiolarians and also triggered a strong but transient positive 13c excursion this pattern is very different from that described for most mass extinction events

paleo environmental cyclicity in the early silurian yangtze sea south china tectonic or glacio eustatic control abstract the yangtze sea of the south china craton experienced strong environmental perturbations during the first 5 6 myr of the silurian the lower silurian lungmachi formation in the jiaoye 1 drillcore chongqing china records three sea level cycles stratigraphic sequences characterized by rapid deepening at the base of each cycle followed by slow shallowing each deepening event was associated with shifts toward higher marine productivity more intensely reducing bottomwater conditions reduced watermass restriction lower detrital input and enhanced siliciclastic weathering intensity we infer control by glacio eustasy in which deglacial sea level rises and concurrent climatic warming triggered the observed environmental changes shallowing was associated with renewed glaciation and characterized by the opposite set of conditions the results of this study thus provide evidence in support of continued continental glaciation during the early silurian i e following the termination of the major end ordovician hirnantian glaciation the study core also exhibits a long term shallowing trend through the entire lower silurian the origin of which may have been tectono epeirogenic uplift of the south china craton during the kwangsian orogeny

the oxygen cycle of the terrestrial planets insights into the processing and history of oxygen in surface environments physical and chemical processes that operate at low temperature in earth s surface environments and in other planetary settings have left an indelible record of the evolution and interactions of planetary surface and near surface reservoirs this chapter reviews the chemistry of oxygen elemental and isotopic in lunar planetary and earth surface reservoirs the discussion begins with a brief review of the relative abundances of oxygen isotopes in planetary materials including lunar samples the snc martian meteorites and components that are inferred to come from martian surface pools this is followed by a brief largely historical account of the development of oxygen isotope geochemistry in earth surface materials the discussion then transitions using an earth systems perspective into an overview of developments in oxygen isotope geochemistry of atmospheric compounds this treatment includes recent studies that 1 use high precision measurements of 17 o and 18 o to characterize atmospheric oxygen 2 apply information about isotopically substituted species isotopologs for characterizing the position dependent isotopic fractionations of some oxygen bearing atmospheric species and 3 investigate non mass dependent isotopic fractionations in ozone and other atmospheric species the discussion then examines the evidence and models that point to change in the oxygen cycle and oxidation chemistry in earth surface environments over the course of geologic time we use the subdivisions of the earth systems the hydrosphere geosphere and biosphere as a basis for examining these changes this record is explored in the context of hypotheses and data that relate to the nature of the changes and transformations in the oxidation state of these different components of the earth s surface environments we conclude with a brief accounting of areas of research where scientifi c inquiry is presently very active

geochemistry of ash leachates from the 1993 lascar eruption northern chile implication for recycling of ancient evaporites abstract volcanic formations in the central andes overlie extensive amounts of ancient evaporites of cenozoic age recycling of buried salts in present closed basins through the hydrological cycle is a well known process another salt recycling process through volcanic activity has been detected in a recent eruption of the lascar volcano in northern chile about 10 9 metric tons of air fall tephra was deposited over the surrounding closed basins leaching experiments were carried out on ashes deposited near the summit and in a lower neighbouring valley summit ash leachates are almost pure caso 4 solutions whereas the valley ashes leachates are enriched in minor components leachates of the summit ash are near neutral whereas those of the valley ash have ph as high as 10 5 sulfur isotope composition of the leached sulfate is consistent with the volcanic recycling of tertiary gypsum present in the sedimentary section beneath the volcanic cordillera as a rough estimate about 700 000 tons of easily soluble caso 4 were added in only three days to the surrounding closed basins the high ph of the valley ash leachates is due to the hydration of alkali and alkaline earth metal oxides formed during pyrolysis of vegetation by hot ash efflorescent salts encrusted on plants and dissolution of volcanic glass provided some of the minor components the almost pure caso 4 composition of the leach waters is compositionally distinct from any present inflow waters in the central andes addition of caso 4 may explain some of the differences between the observed brine compositions and those predicted by simulating the evaporation of regular inflows the recycling of sedimentary gypsum is probably not restricted to lascar volcano the recycling of ancient salts and possibly also the pyrolysis of vegetation may have been crucial processes in the salt balance of closed basins during the intense volcanic activity of the tertiary

thermal barriers and the fate of perched faunas thermal barriers provide an explanation for the geologically sudden extinction of benthic faunas of epeiric seas when these seas disappeared by contracting to the open ocean biotic interactions could not have caused the sudden extinctions and neither could reduction of the regional area of seafloor because substantial areas of shallow seafloor remained along neighboring continental shelves when epeiric seas drained instead temperature contrasts must have been responsible epeiric seas had strongly seasonal climates and when some receded to continental margins many of their species would have encountered waters that failed to provide the maximum or minimum temperature required for reproduction when epeiric seas receded poleward equatorward or into coriolis driven currents many species faced lethal temperatures the history of the jurassic sundance sea provides a striking example of the fate of a warm adapted fauna driven westward into an area dominated by a cool coriolis driven current

shale microfacies eagle ford group cenomanian turonian north central texas outcrops and subsurface equivalents abstract the eagle ford group is a mixed siliciclastic carbonate unit that records a late cretaceous cenomanian to turonian transgression a major condensed interval cycle 2 5 of the uza 2 supercycle occurs within the eagle ford shale eagle ford strata are overlain disconformably by the austin chalk the austin eagle ford contact represents the turonian coniacian boundary 89 ma the cenomanian turonian 92 ma boundary occurs within the eagle ford group regionally eagle ford strata consist of two major depositional units a lower transgressive unit dominated by dark well laminated shales exhibiting only minor evidence of bioturbation and an upper regressive unit consisting of thinly interstratified high frequency cycles shales limestones and carbonaceous quartzose siltstones petrographic characterization of shales from eagle ford outcrops near austin waco and dallas texas and subsurface equivalents in a core getty 1 j t wilson from lasalle county texas reveals six microfacies 1 pyritic shales 2 phosphatic shales 3 bentonitic shales 4 fossiliferous shales 5 silty quartzose shales and 6 bituminous claystone and shales transgressive eagle ford shales consist mainly of microfacies 1 4 and 6 the eagle ford condensed interval consists of microfacies 1 2 and 3 and highstand eagle ford shales are comprised of microfacies 4 and 5 eagle ford microfacies exhibit distinctive sedimentological aspects and have source rock characteristics that vary systematically within a sequence stratigraphic framework transgressive eagle ford shales occurring below the condensed interval have optimum oil prone source potential whereas overlying regressive shales are gas prone transgressive and condensed eagle ford strata represent poorly oxygenated low energy below storm wave base marine paleoenvironments of deposition in contrast overlying regressive eagle ford lithofacies accumulated in higher energy above storm wave base well oxygenated shallow marine paleoenvironments

different sulfate sources and their post depositional migration in atacama soils abstract sulfate is one of the major salts accumulated in the hyperarid atacama desert chile its origin has been attributed to local weathering volcanic source sea salt and atmosphere deposition previous studies concluded that atmospheric sulfate is present in the desert but this term was poorly defined in terms of chemical origin and its contribution to the total sulfate budget in soil is debated the contribution of each potential sulfate source is expected to vary geographically due to different source locations efforts to quantify the contribution from each source have been hampered by the lack of well defined isotopic compositions for its end members in this study we discard the vague term of atmospheric sulfate by recognizing two major sulfate sources on the basis of their formation pathways primary and secondary sulfate primary sulfate including those from rock weathering sea salt and volcanic ash leachates do not have an 17o anomaly secondary sulfate derived from the oxidation of reduced sulfur gases biological or volcanic emissions by atmospheric ozone or hydrogen peroxide however has an 17o anomaly we developed a method to extract water soluble anions sequentially or entirely from bulk soil samples and analyzed the sulfate concentrations and 17o anomalies for four geographically representative soil profiles in the central atacama desert 23 s to 25 5 s we found that sequential sulfate extractions on the same soil sample produce sulfate with decreasing 17o values except for a positive bulge at 20 to 50 cm in depth there is a general increase in the 17o value for total sulfate with soil depth for all the four soil profiles that we examined these findings are consistent with the hypothesis that secondary sulfates 17o anomalous sulfates are more soluble and thus preferentially leached in liquid water than primary sulfate the finding also warns that incomplete sulfate extraction from sulfate rich soils yields unpredictable results we infer a 17o value of 4 6 for secondary sulfate based on data from partial sulfate extraction experiments total contribution of secondary sulfate in the central atacama desert is estimated to range from 9 to 24 with the highest percentage in the central depression

large scale climatic fluctuations in the latest ordovician on the yangtze block south china likely under warm and humid conditions low cia values however indicate the near absence of chemical weathering thereby refl ecting cool and or arid conditions in this paper the paleo climatic changes during the ordovician silurian transition were reconstructed in the light of cia values derived from the chemical composition of fi ne grained siliciclastic rocks in the mid upper yangtze river region china the infl u ences of paleoclimatic changes on biota are also discussed abstract the ordovician silurian transition was a critical interval in earth s history marked by dramatic climatic oceanic and biological turnovers here we present the chemical index of alteration cia as a proxy of changes in intensity of chemical weathering and its varia tions across the ordovician silurian boundaries wufeng through guanyinqiao to longmaxi formations from wangjiawan and nanbazi on the yangtze block south china in order to explore the climatic changes our data show that the cia values of sediments commonly range from 75 to 90 in the wufeng and longmaxi formations indicating a high degree of chemical weathering and thus a hot and humid climate during deposition in contrast a sharp drop in cia values most 60 70 in the guanyinqiao formation or hirnantian suggests an overall cold and arid climate interrupted by several intervals of warm climate when deposited the temporal coincidence of two phases of massive biotic extinctions with the beginning and end of the cold climate epoch respectively suggests that the large climatic changes could be one of the main controls on the mass extinctions although other factors may also have played a role

a sulfidic driver for the end ordovician mass extinction the end ordovician extinction consisted of two discrete pulses both linked in various ways to glaciation at the south pole the first phase starting just below the normalograptus extraordinarius zone particularly affected nektonic and planktonic species while the second pulse associated with the normalograptus persculptus zone was less selective glacially induced cooling and oxygenation are two of many suggested kill mechanisms for the end ordovician extinction but a general consensus is lacking we have used geochemical redox indicators such as iron speciation molybdenum concentrations pyrite framboid size distribution and sulfur isotopes to analyze the geochemistry in three key hirnantian sections these indicators reveal that reducing conditions were occasionally present at all three sites before the first pulse of the end ordovician extinction and that these conditions expanded towards the second pulse even though the n extraordinarius zone appears to have been a time of oxygenated deposition pyrite is significantly enriched in 34s in our sections as well as in sections reported from south china this suggests a widespread reduction in marine sulfate concentrations which we attribute to an increase in pyrite burial during the early hirnantian the s isotope excursion coincides with a major positive carbon isotope excursion indicating elevated rates of organic carbon burial as well we argue that euxinic conditions prevailed and intensified in the early hirnantian oceans and that a concomitant global sea level lowering pushed the chemocline deeper than the depositional setting of our sites in the n persculptus zone an interval associated with a major sea level rise our redox indicators suggests that euxinic conditions and ferruginous in some places encroached onto the continental shelves in our model the expansion of euxinic conditions during the n extraordinarius zone was generated by a reorganization of nutrient cycling during sea level fall and we argue overall that these dynamics in ocean chemistry played an important role for the end ordovician mass extinction during the first pulse of the extinction euxinia and a steepened oxygen gradient in the water column caused habitat loss for deep water benthic and nektonic organisms during the second pulse the transgression of anoxic water onto the continental shelves caused extinction in shallower habitats

predominance of stratified anoxic yangtze sea interrupted by short term oxygenation during the ordo silurian transition abstract the ordo silurian transition was a critical interval in geological history during which profound biotic climatic and oceanic changes occurred in order to explore the oceanic palaeoredox changes multiple geochemical proxies including ratios of s c fehr fet fep fehr and dop values are presented here from three sections wangjiawan western hubei sanjiaguan northern hunan and nanbazi northern guizhou across the ordo silurian boundary on the yangtze platform these palaeoredox data indicate a predominance of stratified anoxic ferruginous ocean on the yangtze block during this transition which was interrupted by a brief episode of oceanic oxygenation in the early hirnantian this oxygenation temporally coinciding with the end ordovician glaciation and global glacio eustatic sea level fall likely resulted from enhanced circulation of polar cold dense oxygen rich water onto the low latitude shelf the prior and subsequent longer term episodes of anoxic ocean particularly the later one which started in the late hirnantian occurred in parallel with the rapid climatic warming and sea level rise which could have slowed down oceanic circulation thereby enhancing oceanic stratification anoxia and organic preservation oceanic redox changes together with rapid climatic and sea level fluctuations were likely responsible for the stepwise massive demise of the ordo silurian biotic crisis

organic geochemical characteristics mineralogy petrophysical properties and shale gas prospects of the wufeng longmaxi shales in sanquan town of the nanchuan district chongqing thick marine shales occur in the wufeng formation and longmaxi formation of sanquan town of the nanchuan district chongqing municipality which is located on the southeast margin of the sichuan basin however few details of the characteristics of the wufeng longmaxi shales in this area have been reported in this study a well approximately 100 m 328 ft deep was drilled a high quality shale total organic carbon toc 2 0 wt clay 40 interval that was approximately 24 m 79 ft thick with an average toc value of 3 0 wt mainly occurs in the ordovician wufeng formation katian and hirnantian and base of the silurian longmaxi formation rhuddanian shales with higher toc values commonly have a higher porosity and specific surface area tectonic movements may also have been very important factors that influenced the petrophysical properties of the shales for example a detachment layer that resulted from complex tectonic movements is extensive in the wufeng formation the cracks and microcracks in the detachment layer can result in good pore connectivity consequently the detachment layer can be an effective migration pathway the longmaxi wufeng shales of sanquan town are also compared with those of the famous jiaoye 1 well in the jiaoshiba shale gas field in the eastern sichuan basin although the shales in sanquan town have considerable shale gas generation potential the shale gas resource potential in sanquan town is probably poor because the escape of shale gas may be accelerated by the detachment layer in destroyed anticlines and synclines

rare sulfur and triple oxygen isotope geochemistry of volcanogenic sulfate aerosols we present analyses of stable isotopic ratios 17 o 16 o 18 o 16 o 34 s 32 s and 33 s 32 s 36 s 32 s in sulfate leached from volcanic ash of a series of well known large and small volcanic eruptions we consider eruptions of mt st helens washington 1980 1 km 3 mt spurr alaska 1953 1 km3 gjalp iceland 1996 1998 1 km 3 pinatubo phillipines 1991 10 km 3 bishop tuff long valley california 0 76 ma 750 km 3 lower bandelier tuff toledo caldera new mexico 1 61 ma 600 km 3 and lava creek and huckleberry ridge tuffs yellowstone wyoming 0 64 ma 1000 km 3 and 2 04 ma 2500 km 3 respectively this list covers much of the diversity of sizes and the character of silicic volcanic eruptions particular emphasis is paid to the lava creek tuff for which we present wide geographic sample coverage this global dataset spans a significant range in 34 s 18 o and 17 o of sulfate 29 30 and 3 3 respectively with oxygen isotopes recording mass independent 17 o 0 2 and sulfur isotopes exhibiting mass dependent behavior products of large eruptions account for most of these isotopic ranges sulfate with 17 o 0 2 is present as 1 10 m gypsum crystals on distal ash particles and records the isotopic signature of stratospheric photochemical reactions sediments that embed ash layers do not contain sulfate or contain little sulfate with 17 o near 0 suggesting that the observed sulfate in ash is of volcanic origin mass dependent fractionation of sulfur isotopic ratios suggests that sulfate forming reactions did not involve photolysis of so2 like that inferred for pre 2 3 ga sulfates from archean sediments or antarctic ice core sulfate associated with few dated eruptions even though the sulfate sulfur isotopic compositions reflect mass dependent processes the products of caldera forming eruptions display a large 34 s range and exhibit fractionation relationships that do not follow the expected equilibrium slopes of 0 515 and 1 90 for 33 s 32 s vs 34 s 32 s and 36 s 32 s vs 34 s 32 s respectively the data presented here are consistent with modification of a chemical mass dependent fractionation of sulfur isotopes in the volcanic plume by either a kinetic gas phase reaction of volcanic so 2 with oh and or a rayleigh processes involving a residual rayleigh reactant volcanic so 2 gas rather than a rayleigh product these results may also imply at least two removal pathways for so 2 in volcanic plumes above zero 17 o values and their positive correlation with 18 o in sulfate can be explained by oxidation by high 18 o and high 17 o compounds such as ozone and radicals such as oh that result from ozone break down large caldera forming eruptions have the highest 17 o values and the largest range of 18 o which can be explained by stratospheric reaction with ozone derived oh radicals these results suggest that massive eruptions are capable of causing a temporary depletion of the ozone layer such depletion may be many times that of the measured 3 8 depletion following 1991 pinatubo eruption if the amount of sulfur dioxide released scales with the amount of ozone depletion

stretching the envelope of past surface environments neoproterozoic glacial lakes from svalbard the oxygen isotope composition of terrestrial sulfate is affected measurably by many earth surface processes during the neoproterozoic severe snowball glaciations would have had an extreme impact on the biosphere and the atmosphere here we report that sulfate extracted from carbonate lenses within a neoproterozoic glacial diamictite suite from svalbard with an age of similar to 635 million years ago falls well outside the currently known natural range of triple oxygen isotope compositions and indicates that the atmosphere had either an exceptionally high atmospheric carbon dioxide concentration or an utterly unfamiliar oxygen cycle during deposition of the diamictites

late ordovician to earliest silurian graptolite and brachiopod biozonation from the yangtze region south china with a global correlation late ordovician to earliest silurian is an important geological period marked by large geological and biological events however the strata and fossils of this interval are not complete in many parts of the world based on studies of 43 sites in south china in particular the continuous sections on the yangtze platform we recognize a complete succession including seven graptolite zones and two shelly faunas in ascending order the graptolite zones are the dicellograptus complanatus dicellograptus complexus paraorthograptus pacificus including lower subzone tangyagraptus typicus subzone and diceratograptus mirus subzone normalograptus extraordinarius normalograptus ojsuensis normalograptus persculptus akidograptus ascensus and parakidograptus acuminatus zones the shelly faunas are the foliomena nankinolithus and hirnantia faunas which may be correlated with d complanatus zone and n extraordinarius n ojsuensis to part of n persculptus zones respectively the biozonation through this interval from the yangtze region can be correlated with that of other parts of the world such as dob s linn in scotland spain and portugal thuringia saxonia bavaria bohemia poland kazakhstan kolyma malaya peninsula yukon canadian arctic islands nevada argentina niger and victoria australia the hirnantian substage which has been proposed by us recently includes the n extraordinarius n ojsuensis zone hirnantia fauna and n persculptus zone the base of the hirnantian substage is marked by the first appearance data fads of n extraordinarius and n ojsuensis which have been determined to be synchronous on a global scale

oxygen 17 excesses of the central namib gypcretes spatial distribution abstract we present here sulfate oxygen isotopic data 72 samples with both 18o and 17o systematically collected from the central namib desert surface soils from two shore inland west east transects exhibit a gradual increase in the sulfate oxygen 17 excess 17o 17o 0 52 18o until at ca 70 km inland where no continuous gypcrete deposit is observed further east inland the oxygen isotopic compositions for water soluble sulfates extracted from soils and gypcretes range from 8 3 to 13 3 and 0 06 to 1 11 for 18o and 17o respectively the lateral pattern is similar to what has been seen in the cold deserts of the antarctic dry valleys however unlike the dry valleys no discernible correlation is found between 18o and 17o or between the depth of soil horizon and 17o in the namib possible explanations include a relatively smaller component of dimethylsulfide dms derived sulfate in the total gypsum deposits and or more active surface processes e g flooding and leaching in the central namib desert than in the antarctic cold deserts although current state of knowledge is insufficient to delineate quantitatively the sulfate contributions from different sources and reactions the measurement of sulfate 17o does identify an unmistakable atmospheric sulfate component and provides additional independent information regarding sources and reactions

long lived glaciation in the late ordovician isotopic and sequence stratigraphic evidence from western laurentia the timing and causes of the transition to an icehouse climate in the late ordovician are controversial results of an integrated 13c and sequence stratigraphic analysis in nevada show that in the late ordovician chatfieldian stage mid caradoc a positive 13c excursion in the upper part of the copenhagen formation was closely followed by a regressive event evidenced within the prominent eureka quartzite the chatfieldian 13c excursion is known globally and interpreted to record enhanced organic carbon burial which lowered atmospheric p co2 to levels near the threshold for ice buildup in the ordovician greenhouse climate the subsequent regressive event in central nevada previously interpreted as part of a regional tectonic adjustment is here attributed in part to sea level drawdown from the initiation of continental glaciation on gondwana this drop in sea level which may have contributed to further cooling through a reduction in poleward heat transport and a lowering of p co2 by suppressing shelf carbonate production signals the transition to a late ordovician icehouse climate 10 m y before the widespread hirnantian glacial maximum at the end of the ordovician

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

end ordovician extinctions a coincidence of causes the end ordovician hirnantian extinction was the first of the five big phanerozoic extinction events and the first that involved metazoan based communities it comprised two discrete pulses both linked in different ways to an intense but short lived glaciation at the south pole the first occurring at or just below the normalograptus extraordinarius graptolite biozone mainly affected nektonic and planktonic species together with those living on the shallow shelf and in deeper water whereas the second within the n persculptus graptolite biozone was less focused eradicating faunas across a range of water depths in all about 85 of marine species were removed proposed kill mechanisms for the first phase have included glacially induced cooling falling sea level and chemical recycling in the oceans but a general consensus is lacking the second phase is more clearly linked to near global anoxia associated with a marked transgression during the late hirnantian most recently however new drivers for the extinctions have been proposed including widespread euxinia together with habitat destruction caused by plate tectonic movements suggesting that the end ordovician mass extinctions were a product of the coincidence of a number of contributing factors moreover when the deteriorating climate intensified causing widespread glaciation a tipping point was reached resulting in catastrophe

major oceanic redox condition change correlated with the rebound of marine animal diversity during the late ordovician to understand probable influences of environmental changes on the decrease and rebound of biodiversity during the late ordovician a detailed analysis of total organic carbon toc pyrite v v ni ratios and s isotopic compositions of pyrite in the late ordovician sediments from poland was performed the significant enrichments of toc and pyrite as well as high v v ni ratios 0 6 for the jeleniow formation of the latest darriwilian to middle katian stages provide evidence for predominant anoxic bottom water conditions in contrast low toc and pyrite as well as low v v ni ratios for the wolka formation of late katian age indicate oxic depositional conditions the s isotopic data are consistent with the interpretations of oceanic redox conditions based on multiple proxies for paleoredox the results from the baltica area reveal a major oceanic redox condition change during the late katian and suggest that oceanic oxygenation may have contributed to the rebound of marine animal diversity during the late ordovician

mass independent isotopic signatures of volcanic sulfate from three supereruption ash deposits in lake tecopa california abstract hundreds to thousands of megatons of sulfur dioxide released by supereruptions can change chemical and physical properties of the atmosphere and thus induce climate perturbations we present oxygen and sulfur isotope analyses of sulfate in 48 volcanic ash samples and 26 sediment samples from dry lake beds in the tecopa basin california usa these ash layers represent three supereruptions including the 0 64 ma lava creek tuff 2 04 ma huckleberry ridge tuff and 0 76 ma bishop tuff mass independent oxygen signatures 17o up to 2 26 that are present in these ash units and not in associated sediments indicate oxidation of volcanic so2 by mass independent ozone and its products in this study we consider the formation deposition preservation and dilution of mass independent volcanic sulfate mivs using the isotopic compositions of the sulfates we construct a mixing model that demonstrates that the main source of sulfate in lake tecopa is mass dependent sediment derived sulfate mdsds 77 however ash beds still preserve up to 23 of mivs that initially had undiluted 17o value around 8 and 33s as low as 0 35 and 36s up to 1 08 therefore despite potential dilution by mdsds the mivs signatures can be preserved in the geologic record for few million years if deposited as gypsum in arid environments alkaline or saline lake the oxygen and sulfur mass independent signatures of the volcanic sulfates indicate that photolysis and oxidation of volcanic so2 has been achieved in the upper atmosphere since only supervolcanic eruptions were shown to generate massive amount of mass independent sulfate it requires that up to 20 60 of the global ozone layer is consumed as a result of supervolcanic so2 released this may occur as a result of a strong physical and chemical degradation of the tropopause we speculate that the distinction between the high troposphere and the low stratosphere at least locally could be erased by supereruptions and recorded by mivs