sedimentary pyrite formation an update sedimentary pyrite formation during early diagenesis is a major process for controlling the oxygen level of the atmosphere and the sulfate concentration in seawater over geologic time the amount of pyrite that may form in a sediment is limited by the rates of supply of decomposable organic matter dissolved sulfate and reactive detrital iron minerals organic matter appears to be the major control on pyrite formation in normal non euxinic terrigenous marine sediments where dissolved sulfate and iron minerals are abundant by contrast pyrite formation in non marine freshwater sediments is severely limited by low concentrations of sulfate and this characteristic can be used to distinguish ancient organic rich fresh water shales from marine shales under marine euxinic conditions sufficient h2s is produced that the dominant control on pyrite formation is the availability of reactive iron minerals calculations based on a sulfur isotope model indicate that over phanerozoic time the worldwide average organic carbon to pyrite sulfur ratio of sedimentary rocks has varied considerably high cs ratios during permo carboniferous time can be explained by a shift of major organic deposition from the oceans to the land which resulted in the formation of vast coal swamps at that time low cs ratios compared to today during the early paleozoic can be explained in terms of a greater abundance of euxinic basins combined with deposition of a more reactive type of organic matter in the remaining oxygenated portions of the ocean the latter could have been due to lower oceanic oxygen levels and or a lack of transportation of refractory terrestrial organic matter to the marine environment due to the absence of vascular land plants at that time

influence of dissolved oxygen on secular patterns of marine microbial carbonate abundance during the past 490 myr abstract shallow marine benthic microbial carbonate sediments declined episodically during the phanerozoic they were relatively abundant during the cambrian mid ordovician late devonian mississippian mid permian mid late triassic and to a lesser degree in the late jurassic early cretaceous they were least abundant when invertebrate especially reef diversity was relatively high in the mid ordovician late devonian early mid permian late triassic mid late jurassic and early cretaceous to present day these intervals of microbial carbonate abundance and invertebrate diversity generally alternate transitionally with one another but lows and periods of overlap also occur lows when benthic microbial carbonate abundance and invertebrate diversity both declined are apparent during the pennsylvanian and early mid jurassic overlaps when microbial carbonates and invertebrates were both relatively common occur in the early cambrian and late jurassic overall these patterns broadly support suggestions that microbial carbonate abundance declined as calcified invertebrates increased to further explore these relationships we compared microbial carbonate abundance and invertebrate diversity with estimates of changes in sea surface temperature and marine dissolved oxygen for the past 490 myr this analysis suggests that invertebrate diversity varied directly with oxygen availability invertebrate diversity was low when dissolved oxygen was low in the early ordovician late devonian early triassic and early mid jurassic it appears that by reducing invertebrate diversity low oxygen levels favored microbial carbonates at the same time low oxygen levels likely stimulated anaerobic metabolisms favoring carbonate precipitation this study suggests that dissolved oxygen was a major control on secular patterns of marine microbial carbonate abundance during the phanerozoic together with marine temperature and carbonate saturation state

contemporaneous early diagenetic formation of organic and inorganic sulfur in estuarine sediments from st andrew bay florida usa estuarine sediment samples were collected from nine stations in st andrew bay near panama city florida usa contrasting grain size and varying concentration of organic carbon allowed a comparative study of the relationships among organic degradation processes sulfur cycling and diagenetic removal of dissolved sulfide produced by bacterial sulfate reduction abundance and sulfur isotopic composition were determined for dissolved sulfide dissolved sulfate pyrite humic acid sulfur fulvic acid sulfur and elemental sulfur presence of free dissolved sulfide and enrichment in 34s of porewater sulfate in samples from organic rich muddy sediments of st andrew bay indicate high rates of sulfate reduction and anoxic conditions below 5 mm depth such conditions are not reflected in samples from organic poor sandy near shore sediments collected from highly productive areas occupied by seagrass beds in this estuarine system pyrite and fulvic acid sulfur are the largest sinks for sulfide produced by bacterial sulfate reduction strong correlation between abundances of pyrite and fulvic acid sulfur suggests coincident formation in the uppermost 10 cm of sediment large differences in isotopic composition between these two species and consistent enrichment in 34s of the fulvic acid fraction however indicate two different reaction pathways abundance and sulfur isotopic composition of humic acid and fulvic acid sulfur suggest that organic sulfur is derived from mixing between 1 34s depleted sulfur with an unknown oxidation state that was recycled from bacterial hydrogen sulfide and 2 34s enriched sulfur interpreted as primary biosynthetic sulfur originally assimilated from dissolved sulfate samples taken from sandy sediment localities where aquatic vascular plants are rooted in the anoxic zone show a strong depletion in 34s in the fulvic acid sulfur fraction suggesting recycled hydrogen sulfide as the dominant source of organic sulfur

construction of the cambrian and ordovician seawater 87sr 86sr curve abstract the estimated path of seawater 87 sr 86 sr during the cambrian and ordovician is based on 172 analyses the sample suite is largely from locations in oklahoma new york tennessee and texas where rocks of this age are widely exposed and well studied the oldest reliable samples examined in this study are of middle cambrian age and yield an 87 sr 86 sr near 0 70888 sw 19 the estimated ratio rises to near 0 70915 sw 8 in the lower upper cambrian the highest value during the phanerozoic and probably for all geologic time the estimated path drifts lower to near 0 70903 sw 4 at the cambrian ordovician boundary this further declines to near 0 70875 sw 32 at the end of the ibexian and reaches a value near 0 70821 sw 86 at the whiterockian mohawkian boundary the decline in the seawater 87 sr 86 sr continues into later mohawkian time to flatten abruptly near 0 70782 sw 125 this ratio is essentially constant through the latest ordovician the curve rises near to a ordovician silurian boundary value poorly constrained from our data near 0 70784 sw 123 the early paleozoic seawater curve shows the alternating dominance of continental and oceanic sources of sr continental sr dominated seawater from the beginning of the cambrian into late cambrian from that time as the cambrian transgression proceeded and radiogenic crystalline rocks were covered oceanic sr began to dominate sr from low ratio oceanic sources dominated through early mohawkian time an abrupt balance is struck between oceanic and continental sources to end the decline in the late ordovician and maintained until near the end of the ordovician the general curve shape follows the pattern suggested by proposed sealevel changes the rise and fall of seawater 87 sr 86 sr during the cambrian and ordovician characterized seawater variation during all the paleozoic

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

sedimentology of rift climax deep water systems lower rudeis formation hammam faraun fault block suez rift egypt abstract in most marine rift basins subsidence outpaces sedimentation during rift climax times typically this results in sediment starved hangingwall depocentres dominated by deep marine mudstones with subordinate local development of coarser clastics in the immediate hangingwall derived from restricted catchments on the immediate footwall scarp to highlight the spatial variability of rift climax facies and the controls upon them we have investigated the detailed three dimensional geometry and facies relationships of the extremely well exposed miocene rift climax lower rudeis formation in the immediate hangingwall to the thal fault zone suez rift egypt detailed sedimentological analyses allows the lower rudeis formation to be divided into two contemporaneous depositional systems 1 a laterally continuous slope system comprising hangingwall restricted interpretation of individual facies facies relationships and their spatial variability indicate that deposition in the immediate hangingwall to the thal fault occurred via a range of submarine concentrated density flows surge like turbidity flows mass wasting and hemipelagic processes major controls on the spatial variability and stratigraphic architecture of the depositional systems identified reflect the influence of the steep footwall physiography accommodation and drainage evolution associated with the growth of the thal fault the under filled nature of the hangingwall depocentre combined with the steep footwall gradient result in a steep fault controlled basin margin characterised by either slope bypass or erosion with limited coastal plain or shelf area sediment supply to the slope apron deposits is controlled in part by the evolution and size of small footwall drainage catchments in contrast the localized submarine fan is interpreted to have been fed by a larger antecedent drainage network the structural style of the immediate footwall is also believed to exert a control on facies development and stratigraphic evolution in particular fault scarp degradation is enhanced by fault propagation folding which creates basinward dipping bedding planes in the pre rift footwall strata that large pre rift blocks slide on

middle ordovician darriwilian conodonts from southern tibet the indian passive margin implications for the age and correlation of the roof of the world

carbon isotope δ13ccarb stratigraphy of the lower middle ordovician tremadocian darriwilian in the great basin western united states implications for global correlation abstract new stable carbon isotope data 13ccarb from lower middle ordovician tremadocian to darriwilian carbonate mudstone and wackestone rocks of the pogonip group are presented from two sections in the great basin region usa shingle pass east central nevada and the ibex area western utah the pogonip group is a succession of mixed carbonate and siliciclastic rocks that accumulated on a carbonate ramp under normal marine conditions during the late cambrian furongian to middle ordovician darriwilian the shingle pass and ibex area sections have been previously studied for their conodont biostratigraphy and contain a north american midcontinent conodont fauna that range from the cordylodus intermedius zone uppermost cambrian to the phragmodus polonicus zone darriwilian the 13c trend has four distinct characteristics recognized in both great basin sections 1 a drop in 13c from 1 at the base of the ordovician tremadocian to 0 7 2 a 1 to 2 positive 13c shift in the uppermost rossodus manitouensis zone during the late tremadocian 3 a gradual 13c increase from 2 to ca 0 during the end of the early ordovician floian and 4 a steady 13c decrease from 0 to 4 to 5 during middle ordovician dapingian darriwilian in the lower ordovician 13c trends reported here from the great basin are not consistent with a causal mechanism involving sea level change and the migration of isotopically distinct water bodies instead these lower ordovician isotope data most likely reflect primary seawater chemistry and changes in 13c on a global scale this interpretation is supported by the excellent correlation of 13c in the lower ordovician to other 13c trends reported from the sections in the argentine precordillera la silla and san juan formations and in western newfoundland st george and table head groups these correlations using 13c are consistent with published biostratigraphic data and provide an integrated and high resolution chemo biostratigraphic framework for the lower ordovician sedimentary record of the laurentian margin the middle ordovician portion of the 13c curves in the great basin represented by the kanosh and lehman formations shows significant isotopic depletion relative to the section in argentina thus although there is some indication that minima and maxima in the middle ordovician curves can be correlated the great basin sections show clear evidence of overprinting by local variables related to both diagenesis dolomitization and platform restriction

coordinated biotic and abiotic change during the great ordovician biodiversification event darriwilian assembly of early paleozoic building blocks abstract the ordovician period records an extraordinary biodiversity increase known as the great ordovician biodiversification event gobe which coincided with a series of environmental changes to the earth system notably a cooling global ocean increased oxygenation and increased nutrient supply from volcanism and continental weathering the co evolution of earth and its biota during this interval has been studied in various contexts on multiple paleocontinents emerging patterns depend on the lens of investigation here we summarize the current state of understanding by reviewing and synthesizing the fossil and sedimentary records recent paleontological studies mainly focused on rhynchonelliform articulated brachiopods bryozoa cephalopods trilobites graptolites echinoderms and reef organisms have documented details of diversification body size increase development of ecosystem complexity and intensification of inter continental dispersal from the late cambrian through late ordovician biomass increased markedly between the early and middle ordovician furthermore diversification rates increase statistically during the mid darriwilian age both globally and regionally coincident with these biotic changes geochemical proxies record significant changes to earth s physical system oceanic temperatures decreased and atmospheric oxygen levels increased to near modern levels in concert with the middle ordovician diversification of shelly fauna anoxic pulses ceased and evidence for deep ocean ventilation prevails in middle ordovician strata furthermore a major middle late ordovician change in oceanic strontium isotopic composition indicates increased weathering of juvenile volcanic rocks and delivery of nutrients to marine settings this multi proxy dataset records near simultaneous changes in fossil rich shallow marine environments during exactly the interval of greatest diversification by integrating biotic and geochemical datasets a clear picture of the co evolution of earth and its biota emerges indicating that the darriwilian was the critical interval facilitating increased capacity of ecosystems we therefore recommend restricting the term gobe to indicate this short interval of rapid diversification and ecosystem change and using ordovician radiation when referencing the sum of diversifications that occurred throughout the ordovician period

paired isotope records of carbonate and organic matter from the middle ordovician of argentina intrabasinal variation and effects of the marine chemocline abstract we investigate the expression of the middle darriwilian isotope carbon excursion mdice across marine shelf environments in the argentine precordillera previous work identified the mdice in the las chacritas formation in argentina but did not recognize the expression of the mdice in time equivalent strata of the deeper water las aguaditas formation albanesi et al 2013 palaeogeography palaeoclimatology palaeoecology v 398 p 48 66 recent biostratigraphic investigations of these units have for the first time provided the opportunity for high resolution correlation which suggest that the mdice or at least the initiation of the mdice should be observed in both the las chacritas and las aguaditas formations here we present new paired carbon isotope data of carbonate and organic carbon from the las chacritas and las aguaditas formations we identify a 2 positive shift in the isotopic composition of marine carbonate in the las chacritas formation whereas values abruptly fall to

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

paleo environmental changes during the middle late ordovician transition on the yangtze platform south china and their ecological implications abstract rocks of darriwilian and sandbian age middle late ordovician record the extraordinary peak of the great ordovician biodiversification event gobe and significant global environmental changes to track paleo environmental changes through this critical time we have analyzed iron speciation trace elements and total organic carbon in samples of the miaopo formation from an intra shelf basin in south china the results reveal moderate primary productivity and possible fluctuations in benthic redox upwelling during marine transgression is thought to have increased the availability of nutrients in surface waters sustaining the biological pump enhanced export of organic matter would have consumed dissolved oxygen at depth driving the system toward anoxia however it is also possible for deep water renewal events to transfer more dissolved oxygen into the bottom water those proxies suggests high primary productivity and varying sea level global and local not benthic anoxia as the driver of accumulated black shale thus the local marine ecosystem thrived in this oxic intra shelf basin challenging the preexisting paradox that the high diversity community flourished in an oxygen depleted environment

paired δ13ccarb and δ13corg records of upper ordovician sandbian katian carbonates in north america and china implications for paleoceanographic change late sandbian early katian marine carbonates from two sections in north america and one in south china were analyzed for paired 13ccarb and 13corg and revealed similar 13ccarb but varying trends in 13corg stratigraphy while all sections recorded a well known positive 13ccarb shift these 13corg records are the first through the guttenberg 13ccarb excursion gice in china oklahoma and west virginia in south china a positive excursion in 13corg is associated with the latter part of the positive 13ccarb excursion and closely resembles trends from iowa and pennsylvania that have been interpreted to reflect a lowering of atmospheric pco2 levels the 13corg trends from oklahoma and west virginia differ significantly from what is observed in china this likely indicates that local changes in nutrient cycling and phytoplankton growth rates were the dominant control on 12c fractionation in oklahoma and west virginia masking a possible global signal of lowered atmospheric pco2 levels observed in china

gradients in the carbon isotopic composition of ordovician shallow water carbonates a potential pitfall in estimates of ancient co2 and o2 abstract the carbon isotopic composition of the global dissolved inorganic carbon dic reservoir is best estimated from open ocean pelagic carbonate sediments 13 c carb however this is not practical for most of geologic time because seafloor subduction has removed the pre jurassic record and these time periods may have lacked planktonic calcifying organisms and therefore shallow water carbonate platform or periplatform sediments are utilized shallow water deposits are susceptible to a wide range of post depositional alteration processes and syn depositional controls on 13 c carb that include carbonate mineralogy water mass restriction and a host of related variables e g ph temperature organic decomposition evaporation and co 2 solubility that can produce local gradients in dic the degree to which shallow water 13 c curves diverge from open marine deposits may be critical to understanding how well global carbon cycle isotope mass balance models can predict organic carbon burial rates but documentation of such divergence is often hindered by factors that limit chronostratigraphic correlation in restricted water masses e g endemic faunas here we integrate strontium isotope 87 sr 86 sr stratigraphy and biostratigraphy to compare 13 c curves in a case study along a depth transect in middle late ordovician carbonate platform settings the restricted tidal flat and more open marine deposits are offset by a maximum of 2 during sea level drop and 0 during highstand flooding of the platform global carbon cycle models such as geocarbsulf use published 13 c carb curves to drive organic carbon burial rates under the assumption that 13 c carb reflects a global seawater signal we show here the potential pitfalls of using a published 13 c carb curve that violates this global assumption for the 460 million year middle late ordovician time bin in geocarbsulf improper usage of our locally depleted 13 c curve to drive global organic carbon burial would result in erroneous atmospheric o 2 10 8 o 2 equal to about a 30 reduction from what it should be at 15 2 o 2 using global 13 c and co 2 400 ppm too high equal to about a 13 increase from what it should be at 2570 ppm co 2 with detailed sedimentologic analysis it may be possible to identify and exclude 13 c samples that record the influences of local carbon cycling from global carbon cycle models such as geocarbsulf

strontium isotope 87sr 86sr stratigraphy of ordovician bulk carbonate implications for preservation of primary seawater values the present study on bulk carbonate 87 sr 86 sr stratigraphy represents a companion work to earlier research that presented a conodont apatite based ordovician seawater 87 sr 86 sr curve for the tremadocian katian stages 485 445 ma here we directly compare the curve based on conodont apatite including some new data not published in earlier work with a new curve based on 87 sr 86 sr results from bulk carbonate from the tremadocian sandbian stages we sampled eight lower to upper ordovician carbonate successions in north america to assess the reliability of bulk carbonate to preserve seawater 87 sr 86 sr and its utility for 87 sr 86 sr chemostratigraphy a high resolution 87 sr 86 sr curve based on 137 measurements of bulk conodont apatite is used as a proxy for seawater 87 sr 86 sr 87 sr 86 sr seawater in total 230 bulk carbonate samples that are paired to conodont samples were measured for 87 sr 86 sr in order to determine the conditions under which 87 sr 86 sr seawater is preserved in bulk carbonate results indicate that well preserved bulk carbonate can faithfully record the 87 sr 86 sr seawater trend but that its 87 sr 86 sr values are commonly more variable than those of conodont apatite on average bulk carbonate samples of the same age vary by 10 20 10 5 compared to 5 10 10 5 for conodont apatite the amount of isotopic alteration of bulk carbonate from seawater 87 sr 86 sr 87 sr 86 sr was determined by taking the difference between 87 sr 86 sr values of bulk carbonate and the approximated seawater trend based on the least radiogenic conodont 87 sr 86 sr values cross plots comparing 87 sr 86 sr values to bulk carbonate sr concentration sr and conodont color alteration indices cai an estimate of the thermal history of a rock body indicate that bulk carbonate is most likely to preserve 87 sr 86 sr seawater minimally altered when either 1 bulk carbonate sr is greater than 300 ppm or 2 carbonate rocks experienced minimal thermal alteration with burial temperatures less than 150 c carbonates with intermediate sr e g between 130 and 300 ppm can also yield 87 sr 86 sr seawater values but results are less predictable and local diagenetic conditions may play a greater role modeling results support the argument that seawater 87 sr 86 sr can be preserved in bulk carbonates with low sr if pore water rock ratios are low 87 sr 86 sr is similar to the seawater 87 sr 86 sr value preserved in limestone bulk carbonate samples that meet these criteria can be useful for high resolution measurements of 87 sr 86 sr seawater with a sample variation on par with fossil materials 5 particularly for successions where well preserved fossil material i e conodonts or brachiopods is not available such as precambrian strata sequences recording mass extinction events or otherwise fossil barren facies these criteria and model predictions based on bulk carbonate sr must be considered in the context of whether a limestone accumulated under calcite seas e g ordovician with relatively high seawater sr ca or aragonite seas in which case the diagenetic transformation of aragonite to calcite may result in incorporation of non seawater sr

darriwilian middle ordovician delta c 13 carb chemostratigraphy in the precordillera of argentina documentation of the middle darriwilian isotope carbon excursion mdice and its use for intercontinental correlation although documented from estonia latvia sweden eastern north america and china the middle ordovician darriwilian positive delta c 13 excursion known as the mdice has previously not been recognized with certainty in south america the most promising region in south america for detailed middle ordovician carbon isotope research is the precordillera of western argentina where there are many excellent exposures of biostratigraphically well dated carbonate successions spaning the tremadocian through sandbian stratigraphic interval for this project we collected numerous isotope and conodont samples from the middle darriwilian las chacritas and aguaditas formations at their type localities which yielded important biostratigraphic data as well as informative delta c 13 carb values in the e pseudoplanus zone in the upper half of the las chacritas formation there is a relatively modest but distinct delta c 13 carb excursion because its stratigraphic position and magnitude closely agree with the mdice in other parts of the world we recognize it as the first firm record of this excursion in south america the fact that the delta c 13 carb curve from the las aguaditas formation shows no such excursion is due to the existence of a stratigraphic gap between the lower and middle members of this formation that cuts out the excursion interval the precordilleran mdice is used for detailed long range correlations with successions in baltoscandia newfoundland and china illustrating the usefulness of also this delta c 13 carb excursion as a global stratigraphic tool a recent proposal of a greatly extended chronostratigraphic range of the table head group on newfoundland is rejected based on well established biostratigraphic evidence c 2013 elsevier b v all rights reserved less

contrasting long term global and short term local redox proxies during the great ordovician biodiversification event a case study from fossil mountain utah usa abstract the great ordovician biodiversification event gobe was a time of pronounced radiation within the paleozoic and modern evolutionary faunas despite increases in metazoan diversity during both the cambrian and the ordovician recent geochemical evidence suggests that the early paleozoic may have experienced recurring deep ocean euxinia the dynamic connections between deep ocean redox and fossil rich shelf environments remain unclear but one way to elucidate these relationships is to study rocks formed during the gobe using both long term global and short term local redox proxies here we report geochemical results from the middle ordovician juab formation and kanosh formation at fossil mountain utah the kanosh formation has been interpreted by different workers either as an anoxic restricted basin or as a well oxygenated normal marine basin based on sedimentological and paleontological characteristics here we employ the sulfur isotopic composition of carbonate associated sulfate cas as a proxy for long term whole ocean redox changes during the middle ordovician we use the abundances of organic carbon and total sulfur as proxies for short term localized changes in redox during the deposition of the fossil mountain sediments organic carbon abundances are low overall average 0 7 wt maximum 1 2 wt using the most conservative estimate although some units may have had considerably higher organic content before burial total sulfur is uniformly low with an average value of 0 02 wt and a maximum value of 0 07 wt the total sulfur and organic carbon abundance results suggest that anoxia was not prevalent in the depositional environment of the juab and kanosh formations similarly the sulfur isotopic results correlate well with those reported from time equivalent strata in newfoundland suggesting that the fossil mountain sediments were not deposited in a restricted basin even though the total sulfur and organic carbon data suggest that the local environment was not anoxic our sulfur isotopic results support the hypothesis that the global ocean was likely euxinic during the middle ordovician

association of orogenic activity with the ordovician radiation of marine life the ordovician radiation of marine life was among the most substantial pulses of diversification in earth history and coincided in time with a major increase in the global level of orogenic activity to investigate a possible causal link between these two patterns the geographic distributions of 6576 individual appearances of ordovician vician genera around the world were evaluated with respect to their proximity to probable centers of orogeny foreland basins results indicate that these genera which belonged to an array of higher taxa that diversified in the middle and late ordovician trilobites brachiopods bivalves gastropods monoplacophorans were far more diverse in and adjacent to foreland basins than they were in areas farther removed from orogenic activity carbonate platforms this suggests an association of orogeny with diversification at that time

stable carbon isotope stratigraphy in the ordovician of south china abstract about 500 samples from 11 ordovician sections in south china have been analysed for their 13 c carb composition four sections are located in the yangtze platform region the remaining seven sections in the jiangnan slope region the sections span more or less the entire ordovician but the sample resolution in its lower part tremadocian to dapingian is lower than in its upper part darriwilian to hirnantian overall the data show a good correlation with the published global isotope curve at the cambrian ordovician boundary positive values are observed in the succeeding tremadocian the values decrease and remain negative into the early floian in the late floian the values increase to slightly positive the correlation of dapingian rocks between the yangtze and the jiangnan regions is somewhat problematic which might be due to high siliciclastic input in the yangtze section in the darriwilian a small positive excursion is observed in the yangtze platform area however it is missing in the jiangnan slope area this excursion probably correlates with the mid darriwilian excursion mdice reported from baltoscandia both chemostratigraphy and conodont biostratigraphy indicate a possible gap between the shihtzupu formation and the pagoda formation i e most of the sandbian is probably not represented in the upper yangtze region the earliest katian is characterised by a pronounced positive excursion correlating with the well known global guttenberg excursion gice in the late katian at least one additional positive excursion was determined the precise biostratigraphy however is not clear and therefore it is not known which of the known four to five remaining katian 13 c excursions identified in baltica correlate s with the one s observed in the present study and further biostratigraphic study is required the lower part of the hirnantian excursion hice is probably not preserved with only small remnants of the hice observed near the base of the kuanyinchiao bed

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

a major sulphur isotope event at c 510 ma a possible anoxia extinction volcanism connection during the early middle cambrian transition a new approach to constraining seawater 34s and sulphate concentration using francolite bound sulphate reveals an abrupt increase in 34s to 50 around the early middle cambrian boundary such high 34s values are best explained by increased rates of pyrite burial due to ocean anoxia coupled with an increased sensitivity of the ocean sulphate reservoir to perturbations due to low sulphate concentrations of 500 700 gl 1 we argue that the spread of anoxic waters at this time was partly the result of greenhouse warming related to the eruption of the kalkarindji large igneous province of northern australia and that it triggered the collapse of early metazoan reef ecosystems during the latest early cambrian mass extinctions of the last 260 myr have all coincided with enhanced volcanic activity while several are also associated with positive shifts in seawater 34s extending this correlation back in time further implicates volcanically induced climate change as a major determining factor in biosphere evolution

detection of magnetically susceptible dyke swarms in a fresh coastal aquifer groundwater constitutes the main source of freshwater in shalatein on the western coast of the red sea in egypt the fresh aquifer of shalatein is intensively dissected by shallow and deep faults associated with the occurrence of dykes and or dyke swarms in this context synthesis of electrical resistivity ground magnetics and borehole data was implemented to investigate the freshwater aquifer condition locate the intrusive dykes and or dyke swarms and demarcate the potential freshwater zones nine schlumberger ves s with maximum current electrode half spacing ab 2 of 682 m were conducted the subsurface was successfully delineated by general four layers the fresh aquifer of the quaternary and pre quaternary alluvium sediments was effectively demarcated with true resistivities ranged from 30 to 105 m and thickness ranged between 20 and 60 m a ground magnetic survey comprised 35 magnetic profiles each 7 km in length magnetic data interpretation of the vertical derivatives first and second order downward continuation 100 m apparent susceptibility depth of 100 m and wavelength filters butterworth high pass of wavelengths 300 m clearly reflected the deep seated structure the computed depth by the 3d euler deconvolution for geological contacts and faults si 0 ranged from 14 to 545 m whereas for dyke and sill si 1 it ranged from 10 to 1 095 m the western part of the study area is recommended as a potential freshwater zone as it is characterized by depths 100 m to the top of the dykes higher thickness of the fresh aquifer 45 60 m depths to the top of the fresh aquifer ranging from 25 to 40 m and higher resistivities reflecting better freshwater quality 70 105 m

geochemical evidence for expansion of marine euxinia during an early silurian llandovery wenlock boundary mass extinction abstract repeated biotic crises have become the hallmark for the silurian with the three most significant marine turnover events being related to dramatic global environmental perturbations causal mechanisms linking these marine extinction events with positive carbon isotope 13 c excursions paleoceanographic change and climate remain poorly constrained here we examine the positive 13 c excursion across the llandovery wenlock boundary and the associated ireviken extinction event this positive 13 c excursion has been interpreted to reflect a major change in global oceanographic climate state and enhanced organic carbon burial new geochemical data from two paleocean basins have been analyzed to determine local and global redox conditions using carbon and sulfur isotopes 34 s and iodine i ca mg proxies the high resolution 13 c and 34 s data from both sections show positive excursions indicative of global perturbations to each of these elemental cycles with minimal temporal offsets between the two systems numerical box modeling of 13 c and 34 s data indicates that these isotopic shifts can be generated by significant increases in the burial of organic carbon and pyrite which are most likely due to enhanced burial under euxinic anoxic and sulfidic conditions independently i ca mg values point to locally anoxic bottom waters in the distal and deeper basinal setting in nevada before during and after the ireviken positive 13 c excursion i ca mg values in the proximal shelf setting in tennessee show relatively oxic waters during the onset of peak 13 c values after which bottom water oxygen concentrations dropped throughout the remainder of the excursion this multiproxy paleoredox dataset provides the first direct evidence for local and global expansion of reducing marine conditions coincident with the silurian biotic event and positive 13 c excursion integration of these geochemical data for local and global scale changes in marine redox conditions with the paleontological data and evidence for eustatic sea level rise points toward a shoaling of anoxic and or euxinic waters onto the shelf as a driver for the ireviken extinction event

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

paired carbon isotopic analysis of ordovician bulk carbonate δ 13 c carb and organic matter δ 13 c org spanning the great ordovician biodiversification event abstract paired measurements of bulk carbonate 13ccarb organic matter 13corg and their difference 13c can be used to estimate changes in isotopic fractionation through time as a function of o2 co2 in the atmosphere however because local scale processes can also affect 13c it is essential to compare sections from widely separated water masses here we present new 13corg data from ordovician carbonate rocks from the great basin oklahoma and appalachian basin and compare with published 13ccarb records from these sections and paired 13c values from other carbonate successions around north america these new data complement previous studies that focused on upper ordovician 13ccarb excursions and now provide a composite ordovician 13corg record new lower ordovician tremadocian stage 13corg data range from ca 26 to 28 decreasing throughout the lower middle ordovician floian dapingian stages to ca 29 to 31 13corg values remain at their lowest throughout the sandbian and are similar to other published upper ordovician sandbian katian 13corg data from north america 13c values from well preserved intervals generally vary between 26 to 28 throughout the lower to middle ordovician tremadocian to early darriwilian but increase to 31 during the mid late darriwilian and mid sandbian similar to published data from younger late ordovician positive 13c excursions known as the guttenberg gice and hirnantian hice events the overall 13c trend shows a 3 increase throughout the early middle ordovician and coincides with a previously interpreted period of ocean cooling and some of the earliest pulses of global biodiversity of marine invertebrates and planktonic organisms modeling studies predict that pco2 decreased during this time suggesting that the effect of pco2 on 13c may have been overwhelmed by other controls such as an in increase in po2 or a higher o2 co2 ratio during this biodiversification event

asteroid breakup linked to the great ordovician biodiversification event the great ordovician biodiversification event coincides 470 million years ago with the break up of a large asteroid and the resultant frequent bombardment of earth with asteroid fragments

evaluating the relationship between the carbon and sulfur cycles in the later cambrian ocean an example from the port au port group western newfoundland canada abstract we present a high resolution 34 s sulfate and pyrite and 13 c carbonate record from the middle upper cambrian port au port group a mixed carbonate siliciclastic succession exposed in western newfoundland canada the results illustrate systematic 34 s sulfate shifts of 15 over relatively short stratigraphic intervals 10 m likely 34 s sulfate pyrite ca 23 and a generally positive coupling between changes in 13 c carbonate and 34 s sulfate together these results indicate that middle to late cambrian sulfate concentrations were low and that the sulfate reservoir was more sensitive to change than it was in either terminal neoproterozoic or cenozoic oceans however a simple carbon c and sulfur s isotope box model of the late cambrian ocean illustrates that low sulfate concentrations alone fail to account for the 15 34 s sulfate shifts recognized in port au port strata such large shifts can be generated only if fluctuating oceanic redox is invoked marine anoxia forces reduced c s burial and elevated 34 s driving larger 34 s changes per mole of organic carbon buried the conclusion that later cambrian oceans featured both low sulfate levels and widespread subsurface anoxia supports hypotheses that link fluctuating marine redox conditions in the delayed recovery of skeletal animals and metazoan reefs from late early cambrian extinction

strontium isotopic variations in jurassic and cretaceous seawater a high resolution seawater strontium isotope curve has been generated through the analysis of well dated and well preserved belemnites and oysters from the middle and upper jurassic and the lower cretaceous of great britain analysis of fe and mn concentrations in these fossils has yielded criteria for eliminating samples that are diagenetically altered the strontium isotope curve remains relatively flat through the aalenian and early bajocian rapidly descends through the late bajocian and bathonian and reaches a minimum in the callovian and oxfordian it then begins a rapid increase in the kimmeridgian and portlandian that continues through much of the early cretaceous the curve levels off in the barremian suddenly dips downwards in the aptian and recovers gradually through the albian the strontium isotopic variations are sufficiently large and the data are presented with sufficient stratigraphic detail to allow precise correlation to the classic ammonite zones and lithologic sections of great britain using the techniques of strontium isotope stratigraphy model results indicate that much of the variation in seawater 87sr86sr between 120 and 40 ma can be explained by changing the intensity of mid ocean ridge hydrothermal fluxes proportionally to estimated mid ocean ridge crustal generation rates it is also possible that the variations during the rest of the mesozoic and the permian are primarily reflections of changing hydrothermal inputs the model results have several important implications first they provide an example in which the variations in the strontium isotope curve are not necessarily driven by changes in fluvial inputs second they suggest that from at least the aptian through the eocene variations in continental weathering were minimal this heightens the importance of the rapid rise in seawater 87sr86sr beginning 40 ma as a significant transition to an extended period of increasing fluvial 87sr fluxes continuing to the present finally the results suggest that several documented short term excursions towards lower 87sr86sr in the latest triassic pliensbachian toarcian callovian oxfordian aptian albian and cenomanian turonian are interpretable as pulses of seafloor hydrothermal activity if so the strontium isotope record offers a means of constraining the timing duration and magnitude of known or proposed hydrothermal events in the geological record

δ34scas and δ18ocas records during the frasnian famennian late devonian transition and their bearing on mass extinction models abstract many proposed extinction mechanisms for the late devonian frasnian famennian boundary ffb mass extinction event include the spread of anoxia in bottom waters during the two kellwasser anoxic events which have in turn been linked to changes in continental weathering volcanic hydrothermal fluxes sea level and climate change although there is strong evidence for bottom water anoxia in epicontinental seas at tropical equatorial palaeolatitudes the global extent of these changes is poorly understood largely due to the lack of an oceanic and high latitude record in this study marine 34 s sulphate and 18 o sulphate records derived from the analysis of carbonate associated sulphate cas were constructed from ffb sections in belgium and poland in order to examine the effect of the postulated changes in ocean redox conditions on the global sulphur cycle the results reveal no overall changes in seawater 34 s sulphate and 18 o sulphate over the ffb interval suggesting that any changes in sulphur cycling namely relating to rates of pyrite burial and weathering were not significant on a global scale this supports the idea that the kellwasser events were not oceanic phenomena and that they were restricted to epicontinental seas at low latitudes the exception to the stability of the ffb 34 s sulphate record is a brief negative excursion during the mass extinction interval in the linguiformis zone albeit based on one datapoint at this level in two different sections this may be attributable to short sharp marine regression and an increased flux of light sulphur from weathered pyrite

the great ordovician biodiversification event gobe the palaeoecological dimension abstract the great ordovician biodiversification event gobe saw a spectacular increase in marine biodiversity at all taxonomic levels largely within the phyla established much earlier during the so called cambrian explosion the diversification was probably the result of a combination of several geological and biological processes and the positive feedbacks resulting from them the present paper reviews the palaeoecological dimension of the gobe it involved major increases in s and biodiversity largely associated with the rise of the paleozoic evolutionary fauna dominated by suspension feeders and involving a greater occupation of ecospace and more complex ecological structures in the ecological evolutionary units p1 and p2 in the benthos these include more complex food webs than those of the cambrian greater tiering especially above the sediment water interface and the development of guild structures indicating increased competition between taxa for particular resources the ordovician is characterized by a profound change in reef composition with a switch from microbial dominated reefs in the early and middle ordovician to metazoan dominated reefs in the late ordovician increases in complexity of deep water trace fossil assemblages began in the early ordovician and mark the increasing exploitation in that environment and the development of hardgrounds permitted bioerosion and encrusting strategies together with the appearance of cryptic communities within the water column the gobe involved a significant increase in the diversity of the phytoplankton and the development of a diverse zooplankton including planktotrophic larvae from a range of invertebrate clades this revolution in the plankton enabled the establishment of a diverse fauna of pelagic vertebrates molluscs and arthropods and promoted the rise of suspension feeders in the benthos an escalation amongst predators and thus community evolution may also have been a major driver of biodiversification

carbon isotope record of late cambrian to early ordovician carbonates of the argentine precordillera a more than 2000 m thick cambrian ordovician carbonate platform succession developed on the exotic argentine precordillera terrane during rifting from laurentia and drifting towards gondwana on base of these carbonates a carbon isotope curve could be developed for the cambrian ordovician we measured 13ccarb and 13corg values on bulk rocks selected components and diagenetic cements whereas the carbon isotope signals of intertidal and supratidal rocks are altered by diagenesis most subtidal carbonates exhibit primary marine values we can report reliable curves for the middle late cambrian transition and for the latest cambrian to earliest middle ordovician the 13c curve matches well with the published data from global cambrian ordovician boundary sections excursions and shifts in the carbon isotope curve correspond to events in sequence stratigraphy this indicates the interdependence on sea level which rules the productivity and or preservation of organic carbon and therefore the partition between corg and ccarb burial

heavy sedimentary sulfur isotopes as indicators of super anoxic bottom water conditions iron monosulfides and pyrite total cr reducible s trs in the bottom sediments of the exceptionally sulfide rich euxinic fjord framvaren south norway are unusually enriched in 34 s most of these trss probably form in bottom waters where reactive detrital iron phases encounter isotopically heavy dissolved sulfide enrichment in 34 s in framvaren is a result of intense bacterial reduction of a sulfate reservoir whose exchange with the open sea is limited by shallow sills and a low salinity surface water layer 34 s enriched trs in the deposits of ancient euxinic environments is commonly interpreted as evidence for early diagenetic sulfide formation where residual less reactive iron phases were in contact with isotopically heavy dissolved sulfide sulfate depleted pore waters for prolonged periods during burial however the framvaren data indicate that isotopically heavy pyrite may form in relatively shallow 80 180 m super anoxic bottom water pools in contrast the presence of isotopically light pyrite does not indicate an absence of restricted conditions but only that most iron sulfide formation was completed before 34 s enriched sulfide could be encountered in the water column

sulfur cycling in the aftermath of a 635 ma snowball glaciation evidence for a syn glacial sulfidic deep ocean we have analyzed 34 s sulfate and pyrite 18 ocarbonate and 13 ccarbonate and major and trace elemental concentrations including extractable fe and mn phases in four sections of the maieberg formation the cap carbonate sequence to the marinoan glaciation in northern namibia 34 ssulfate profiles and other geochemical characteristics in the basal transgressive cap dolostone keilberg member are nearly identical in all sections and indicate deposition from a water mass with very low sulfate concentrations in the overlying interval that consists of rhythmites deposited during the transgressive high stand large geochemical disparities occur between sections that we interpret to have been deposited in open ocean versus restricted settings in the former a large negative shift in 34 ssulfate of 20 above the cap dolostone accompanies a change in mineralogy from dolomite to limestone a sharp decrease in 18 o and 13 c and a positive spike in fe and mn concentrations in the latter dolomite persists above the cap dolostone 34 ssulfate increases abruptly 18 o and 13 c are invariant and a spike in mn and fe concentrations is present but subtle these contrasting geochemical signatures in coeval sections can be explained by strong lateral chemical gradients that developed as cold euxinic deep water and a cap of warm oxic brackish water flooded the continental shelf during the post glacial transgression the geochemical di e rences between open ocean and restricted sections diminishes upsection presumably recording the gradual mixing and homogenization of the waters along the otavi platform

dynamic sulfur and carbon cycling through the end ordovician extinction revealed by paired sulfate pyrite δ34s geochemical records of the end ordovician hirnantian stage show parallel positive excursions in the stable isotope compositions of sedimentary pyrite sulfur 34spyr organic carbon 13corg and carbonate carbon 13ccarb these isotope excursions coincide with the end ordovician glaciation and mass extinction a relative increase in pyrite burial fpyr attributed to marine anoxia has been invoked to explain the sulfur isotope excursion and link oceanic redox conditions to the extinction of marine fauna an increase in fpyr would necessarily generate a parallel excursion of equal magnitude in the isotopic composition of coeval marine sulfate 34sso4 34sso4 here we present new high resolution paired sulfur isotope data from carbonate associated sulfate 34scas and pyrite from the hirnantian stage of western anticosti island quebec canada these data document a positive 20 enrichment in 34spyr comparable in magnitude to previous reports but no parallel excursion in 34scas this pattern provides new constraints on the origin of the 34spyr excursion and the nature of carbon sulfur coupling through hirnantian time specifically these observations preclude enhanced pyrite burial as the cause of the hirnantian 34spyr excursion and suggest the possible role of anoxia in the mass extinction may need to be reevaluated rather the global 34spyr excursion is best explained by a transient reduction in the isotopic fractionations expressed during microbial sulfur cycling epyr the epyr record shows a strong inverse correlation with 13c suggesting a mechanistic link between carbon cycling and processes controlling epyr during the hirnantian changes in sea level or marine redox state associated with glaciation could further impact the expression of the biological fractionation e g through syndepositional sediment reworking and or chemocline migration and resultant restricted exchange of porewater sulfate the magnitude of isotopic fractionation during microbial sulfate reduction is partially controlled by metabolic rates which are sensitive to the abundance type and lability of metabolically relevant substrates environmental change associated with the end ordovician glaciation may have elevated the flux of organic material to marine sediments or caused an increase in physical reworking of sediments leading to increased microbial sulfate reduction rates and reduced epyr as such the hirnantian 34spyr excursion may be viewed as a dynamic biological response to global climate change highlighting the connections between the carbon and sulfur biogeochemical cycles

the strontium isotopic composition of ordovician and silurian brachiopods and conodonts relationships to geological events and implications for coeval seawater utilized for strontium isotope measurements the results document a gradual decrease in 87 sr 86 sr from 0 7091 to 0 7087 from tremadoc to llandeilo a sharp decline to 0 7078 during the late llandeilo early caradoc little change during caradoc and the ashgil and a steady rise to 0 7087 through the silurian these long term 10 7 yr variations with magnitudes in the range of 10 23 are interpreted to be controlled primarily by continental collisional tectonics and its associated erosion and weathering the gradual decrease in 87 sr 86 sr ratio during the early ordovician may record the reduction in uplift and weathering rates due to waning of the pan african orogenies the rapid decline near the llandeilo caradoc boundary suggests a strong hydrothermal flux likely due to increased sea floor spreading and a possible superplume event the latter may have caused the prominent transgressive phase the largest in the phanerozoic which would have muted continental flux input although the caradoc was the main interval for the taconic orogeny its impact on the sr continental flux may have been delayed until the early llandovery this effect complemented by reworking of glacial deposits near the ordovician silurian boundary and enhanced by phases of the silurian salinic orogeny may have combined to give the progressive increase in the strontium isotope ratio through the silurian the scale and directionality of these changes makes the strontium isotope curve valuable for dating and correlation purposes copyright 1998 elsevier science ltd

persistent oceanic anoxia and elevated extinction rates separate the cambrian and ordovician radiations recurrent mass extinction events at biomere a biostratigraphic unit boundaries characterize the middle cambrian to early ordovician tremadocian time interval that is between the major cambrian and ordovician radiations of animal life a role for anoxia in maintaining elevated extinction rates in the late cambrian has been proposed based on coincidence of an extinction with positive excursions in 13ccarb and 34scas cas carbonate associated sulfate here we examine an early ordovician extinction event at the base of the north american stairsian stage upper tremadocian and demonstrate concurrent onset of positive excursions in 13c and 34s inferred to reflect enhanced organic matter burial under anoxic waters sea level rise may have brought anoxic waters onto the shelf to initiate extinctions the evidence for 13c excursions and elevated extinction rates appears to wane in the tremadocian consistent with progressive oxygenation of the oceans reaching a threshold that helped facilitate initial stages of the great ordovician biodiversification event

sulfur isotope fractionation in modern euxinic systems implications for paleoenvironmental reconstructions of paired sulfate sulfide isotope records abstract the sulfur s isotope difference between sulfates and sulfides preserved in sedimentary rocks 34 s has been utilized to reconstruct ancient marine sulfate levels with implications for oxygenation of the earth surface and biogeochemical cycling s isotope data from modern low sulfate euxinic systems illustrate that preserved 34 s values are positively correlated with sulfate concentration however absolute constraints on the range of low sulfate levels over which preserved 34 s values vary with sulfate concentration remain poorly constrained here we present a compilation of s isotope data for modern euxinic systems demonstrating that preserved 34 s values increase with sulfate concentration at low sulfate levels and approach values that are similar to in situ s isotope fractionation values from microbial sulfate reduction at high sulfate levels we compare these results to a closed system model of s isotope cycling in a euxinic ocean in order to evaluate when the size of the sulfate reservoir is sufficiently small that rayleigh fractionation affects the preservation of s isotope signatures we conclude that the reservoir effect places constraints on 34 s values deposited in euxinic settings at sulfate concentrations 34 s values can be used to evaluate ancient sulfate levels at higher sulfate levels 10 mm 34 s values are similar to the kinetic isotope fractionation due to microbial sulfate reduction and therefore provide information about biological and environmental controls on sulfate reduction rates and location of pyrite formation the results of this compilation provide an improved model for the use of 34 s records to evaluate paleoenvironmental conditions in euxinic depositional environments

evidence for an active fault below the northwestern alpine foreland of switzerland summary this study is devoted to the analysis of a prominent concentration of earthquakes whose epicenters delineate an active 20 30 km long n s trending tectonic feature near the town of fribourg in the molasse basin of western switzerland this feature coincides with the possible southward continuation of the nne ssw trending rhine graben located approximately 80 km further north in addition these epicenters are located in the vicinity of the fribourg syncline and the alterswil culmination whose structural axes are oriented n s in this area instead of being aligned with the predominant regional ne sw structural trend most of the earthquakes belong to one of three series of events that occurred over a time span of 2 4 months in 1987 1995 and 1999 they include four events with magnitudes between 3 and 4 and one with a magnitude of 4 3 focal depths constrained by modelling spmp pmp traveltime differences with synthetic seismograms are around 2 km which places these events in the sedimentary cover fault plane solutions correspond to almost pure strike slip mechanisms with nearly n s and e w oriented nodal planes high precision relative locations of individual events within the different earthquake clusters as well as of the relative locations of the clusters to each other show that these earthquakes are associated with left lateral motion along a n s trending fault system deep reaching large scale flower structures in the mesozoic and tertiary overburden are observed on interpreted seismic profiles close to the hypocenters the unusual n s trend of the fribourg syncline can be attributed to movements along these faults during oligocene and miocene times also magnetic data support the assumption of a n s striking fault system in the fribourg area possibly related to a permo carboniferous trough though the direct link between the fault traces in the overburden and the active fault system at depth could not be established in this study their similar deformational style and their vicinity suggest that they are related the total length of the inferred fault carries the potential of a magnitude 6 earthquake and thus constitutes a significant source of seismic hazard

sr c and o isotope geochemistry of ordovician brachiopods a major isotopic event around the middle late ordovician transition here we present sr c and o isotope curves for ordovician marine calcite based on analyses of 206 calcitic brachiopods from 10 localities worldwide these are the first ordovician wide isotope curves that can be placed within the newly emerging global biostratigraphic framework a total of 182 brachiopods were selected for c and o isotope analysis and 122 were selected for sr isotope analysis seawater 87sr 86sr decreased from 0 7090 to 0 7078 during the ordovician with a major quite rapid fall around the middle late ordovician transition most probably caused by a combination of low continental erosion rates and increased submarine hydrothermal exchange rates mean 18o values increase from 10 to 3 through the ordovician with an additional short lived increase of 2 to 3 during the latest ordovician due to glaciation although diagenetic alteration may have lowered 18o in some samples particularly those from the lower ordovician maximum 18o values which are less likely to be altered increase by more than 3 through the ordovician in both our data and literature data we consider that this long term rise in calcite 18o records the effect of decreasing tropical seawater temperatures across the middle late ordovician transition superimposed on seawater 18o that was steadily increasing from 3 standard mean ocean water smow by contrast 13c variation seems to have been relatively modest during most of the ordovician with the exception of the globally documented but short lived latest ordovician 13c excursion up to 7 nevertheless an underlying trend in mean 13c can be discerned changing from moderately negative values in the early ordovician to moderately positive values by the latest ordovician these new isotopic data confirm a major reorganization of ocean chemistry and the surface environment around 465 to 455 ma the juxtaposition of the greatest recorded swings in phanerozoic seawater 87sr 86sr and 18o at the same time as one of the largest marine transgressions in phanerozoic earth history suggests a causal link between tectonic and climatic change and emphasizes an endogenic control on the o isotope budget during the early paleozoic better isotopic and biostratigraphic constraints are still required if we are to understand the true significance of these changes we recommend that future work on ordovician isotope stratigraphy focus on this outstanding middle late ordovician event

tracking euxinia in the ancient ocean a multiproxy perspective and proterozoic case study the evolution and extinction of life are tied intimately to the oxygen state of the ocean and particularly to the presence of anoxic and h2s containing euxinic water on a global scale anoxia and euxinia were more common in the past relative to today s 0 5 euxinic seafloor we are able to constrain the distributions of these conditions through a combination of indirect numerical modeling methods and more direct geochemical proxies with particular emphasis on fe s mo analysis of fine grained siliciclastic rocks for the latter establishing the spatiotemporal pattern of oceanic redox is more difficult with very old rocks because of the limited availability of well dated well preserved materials that span shallow and deep environments across time lines despite these difficulties the multiple approaches synthesized in our case study point to global oxygen deficiency in the deep ocean and perhaps euxinia during most if not all of the proterozoic and likely extending into the early paleozoic

behavior of carbonate associated sulfate during meteoric diagenesis and implications for the sulfur isotope paleoproxy abstract although carbonate associated sulfate cas is used widely as a proxy for the sulfur isotope composition of ancient seawater little is known about the effects of diagenesis on retention of primary 34 s signals our case study of the key largo limestone pleistocene florida is the first systematic assessment of the impact of meteoric diagenesis on cas properties geochemical and petrographic data show that meteoric diagenesis has affected the exposed coralline facies to varying degrees yielding differences now expressed as sharp reaction fronts between primary and secondary carbonate minerals within individual coral heads specifically analyses across high resolution transects in the key largo limestone show that concentrations of strontium and sodium decrease across the recrystallization front from original aragonite to meteoric low magnesium calcite by factors of roughly 5 and 10 respectively predictably 18 o values decrease across these same fronts the 13 c relationships are more complex with the most depleted values observed in the latest formed calcite such trends likely reflect carbon isotope buffering capacity that decreased as reaction progressed as well as protracted development of soil profiles and the associated terrestrial biomass and thus depleted 13 c during sea level lowstand conversely 34 s values of cas vary within a narrow buffered range from 20 6 to 22 6 compared to 20 8 22 0 of coeval pleistocene seawater across the same mineralogical transition despite sulfate concentrations that drop in the diagenetic calcite by an average factor of 12 collectively these data point to robust preservation of primary 34 s for carbonates that have experienced intense meteoric diagenesis which is encouraging news for those using the isotopic composition of cas as a paleoceanographic proxy at the same time the vulnerability of cas concentrations to diagenetic resetting is clear

modeling atmospheric o2 over phanerozoic time a carbon and sulfur isotope mass balance model has been constructed for calculating the variation of atmospheric o2 over phanerozoic time in order to obtain realistic o2 levels rapid sediment recycling and o2 dependent isotope fractionation have been employed by the modelling the dependence of isotope fractionation on o2 is based for carbon on the results of laboratory photosynthesis experiments and for sulfur on the observed relation between oxidation reduction recycling and s isotope fractionation during early diagenetic pyrite formation the range of fractionations used in the modeling agree with measurements of phanerozoic sediments by others results derived from extensive sensitivity analysis suggest that there was a positive excursion of o2 to levels as high as 35 during the permo carboniferous high o2 at this time agrees with independent modeling based on the abundances of organic matter and pyrite in sediments and with the occurrence of giant insects during this period the cause of the excursion is believed to be the rise of vascular land plants and the consequent increased production of o2 by the burial in sediments of lignin rich organic matter that was resistant to biological decomposition

environmental and diagenetic variations in carbonate associated sulfate an investigation of cas in the lower triassic of the western usa an integrated stable isotope elemental and petrographic analysis of early triassic spathian carbonates and evaporites along a proximal to deep environmental transect reveals significant variations in 34s composition of carbonate associated sulfate cas the variations in the 34s of cas are strongly correlated with the ca mg composition of carbonates suggesting that the variations are driven by the degree of dolomitization the 34s of dolostones and evaporites are similar to one another and exhibit lower 34s values than limestones from all localities three hypotheses may explain the differences in 34s between proximal dolostones evaporites and inner middle shelf limestones 1 limestones experienced anaerobic sulfate reduction and subsequent incorporation of 34s enriched sulfate into cas during diagenesis while dolostones did not this is unlikely because of the lack of correlation between 34scas and toc as well as other indicators of diagenesis 2 dolomitization controlled the 34scas in proximal paleoenvironments where the source of the 34s depleted fluids was either continentally derived or the result of rayleigh distillation during evaporite formation and 3 a 34s depth gradient existed during the early triassic such that limestones formed in distal waters are more enriched in 34s versus evaporites and dolostones formed in proximal settings we do not favor this hypothesis because the strong correlation between ca mg and 34scas implies that dolomitization controls the 34scas in these samples results from subtidal well preserved non dolomitized limestones suggest that the 34s of spathian seawater sulfate may have been heavier than previously suggested from analyses of evaporite deposits alone

strontium isotopic composition of mid cretaceous seawater the 87sr 86sr ratio in fish teeth separated from mid cretaceous marl and black shale from the northeastern apennines and venetian alps italy define three periods of low 87sr 86sr ratio at 121 to 124 million years ago ma 110 to 115 ma and 89 to 91 ma the 87sr 86sr excursions correspond to oceanic anoxic events represented by the livello selli livello 113 livello urbino and livello bonarelli black shale marker beds and probably reflect an increase in the low 87sr 86sr hydrothermal strontium flux associated with the emplacement of the ontong java and kerguelen plateaus 120 to 110 ma and the caribbean plateau 89 to 91 ma the modeled flux is consistent with the volumes and eruption rates of the oceanic plateaus but is far smaller than expected from the proposed cretaceous crustal production rates of 50 to 100 percent greater than modern

burial of organic carbon and pyrite sulfur in sediments over phanerozoic time a new theory in present day marine sediments almost all of which are deposited in normal oxygenated seawater rates of burial of organic carbon c and pyrite sulfur s correlate positively and bear a constant ratio to one another c s 3 on a weight basis by contrast calculations based on the isotopic model of garrels and lerman 1981 indicate that at various times during the phanerozoic the worldwide burial ratio must have been considerably different than the present day value this ratio change is caused by the requirement that increases in the worldwide mass of organic carbon must be accompanied by equivalent decreases in the mass of sedimentary pyrite sulfur in order to maintain a roughly constant level of o2 in the atmosphere such apparently contradictory behavior can be explained if the locus of major organic carbon burial has shifted over time from normal marine environments as at present to non marine freshwater or to euxinic environments in the geologic past a shift to predominantly freshwater burial can help explain predicted high c s ratios in permo carboniferous sediments and a shift to euxinic environments can help explain predicted low c s ratios during the early paleozoic it is demonstrated that the three environments today exhibit distinguishably different average c s ratios

climatic and oceanic changes during the middle late ordovician transition in the tarim basin nw china and implications for the great ordovician biodiversification event abstract the middle late ordovician transition darriwilian to sandbian age witnessed a major pulse of the great ordovician biodiversification event gobe and distinctive oceanic geochemical fluctuations such as coeval negative c and sr isotope excursions in this study investigations into geochemical variations notably the hg abundance or hg toc have been carried upon the organic rich black shale of the middle upper ordovician saergan formation to unravel the causes of this pulse based on these data three phases were identified phase 1 0 to 3 m is characterized by rising hg toc up to 138 ppb wt and ti al values as well as high ciacorr corrected chemical index of alteration values 68 9 72 3 with negligible enrichment of redox sensitive elements rse and nutrient elements e g u 5 2 ppm v 153 ppm mo 1 8 ppm p2o5 0 2 suggesting intensified volcanism which could have emitted significant amounts of greenhouse gases thereby leading to climate warming in contrast phase 2 3 to 11 m is characterized by decreasing hg toc and ti al ratios relatively low though slightly fluctuating ciacorr values generally depleted in rse except moderately enriched u up to 14 6 ppm and increased p al and ba al ratios implying weakening volcanic activity and subsequent climate cooling and the potential for improved seawater ventilation as a result of oceanic upwelling phase 3 sandbian age 11 13 m witnessed continuous decrease in hg toc ratio an increase in ti al and ciacorr values fairly low values of rse enrichment and p al and ba al ratios indicating recurrent climate warming and the potential for slowed oceanic circulation and attenuated upwelling of nutrient rich deep waters onto the shallow shelf these changes could have diminished bioproductivity and organic output onto the seafloor this study offers insights into volcanic climatic oceanic interactions during a major pulse of the gobe around the middle late ordovician transition while black shales were extensively deposited

comparative sedimentology of shelf carbonates of the middle ordovician st paul group central appalachians abstract observations of present day environments are commonly used to aid in the interpretation of ancient sedimentary sequences this approach is used to determine the paleoclimate and depositional processes operating during formation of the limestones and dolostones of the lower middle ordovician st paul group row park and new market formations of maryland and southern pennsylvania a 200 m thick carbonate platform deposit within the group six facies defined by suites of sedimentary structures faunal assemblages and early diagenetic features are arranged in a large scale regressive transgressive sequence these consist of facies i lower burrowed peloidal facies facies ii burrowed skeletal facies facies iii lower laminated peloidal facies facies iv stromatolitic peloidal facies facies v upper laminated peloidal facies and facies vi upper burrowed peloidal facies a comparison of the st paul group deposits with those of the modern peritidal carbonate environments of shark bay western australia the abu dhabi and trucial coast areas of the persian gulf and andros island and the great bahama bank reveals 1 an overall similarity in features between the st paul group and the sediments of andros island and the great bahama bank and 2 a dissimilarity between those two deposits and the sediments of shark bay western australia and shoreline areas of the persian gulf for example the supratidal zones of the latter two typically contain evaporite minerals e g gypsum and anhydrite whereas the supratidal deposits of andros island and the st paul group are freshwater algal marsh and lake sediments the subtidal deposits of shark bay and the persian gulf generally are mechanically layered skeletal sand the subtidal deposits of the great bahama bank and the st paul group are bioturbated peloidal micrites the st paul group and the great bahama bank deposits are similar on a facies scale down to fine details observed microscopically bahamian modern analogs of the facies of the st paul group are for facies i shallow restricted bank and subtidal pond environments for facies ii a normal marine subtidal environment similar to the shoals which rim parts of the great bahama bank for facies iii and v tidal channel levee pond and marsh subenvironments of the channeled belt environment hardie and garrett 1977 for facies iv levee marsh and freshwater lake subenvironments of the interior of andros island and for the facies vi a semirestricted subtidal bank environment based on the features of the st paul group and by analogy with modern deposits the st paul group was deposited in a low energy rainy environment where deposition was controlled by storm processes many facies display penecontemporaneous marine cements and dissolution features

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

hydro engineering geophysical parameters and design response spectrum for sustainable development in ras muhammed national park sinai the egyptian government is preparing a sustainable development master plan for the ras muhammed national park rmnp south sinai noteworthy the scarcity of the freshwater resources and close proximity to the active seismic zones of the gulf of aqaba implicate geophysical investigations for the fresh groundwater aquifers and construct a design response spectrum respectively accordingly 14 vess hydro engineering geophysical analysis pumping tests downhole seismic test a design response spectrum for buildings and borehole data were carried out in the study area the unconfined freshwater aquifer was effectively depicted with true resistivities thickness and ec ranged from 56 to 135 m 11 to 112 m and 1 4 to 7 1 ms m respectively the northeastern part was characterized by higher aquifer potentiality where coarser grains size highest thickness 112 m high true resistivity 135 m groundwater flow 0 074 m3 day tortuosity 1 293 1 312 formation resistivity factor 4 1 4 6 and storativity 0 281 0 276 an increase in pumping rate was accompanied by an increase in well loss increase in aquifer losses decrease in well specific capacity and decrease in well efficiency design response spectrum prognosticated the short buildings 7 floors in rmnp to be suffering from a high peak horizontal acceleration and shear forces for acceleration between 0 25 and 0 35 g therefore appropriate detailing of shear reinforcement is indispensable to reduce the risk of structural damages at rmnp

testing carbonate associated sulfate cas extraction methods for sulfur isotope stratigraphy a case study of a lower middle ordovician carbonate succession shingle pass nevada usa abstract sulfur isotopes measured from sedimentary rocks are used to reconstruct the global sulfur cycle and chemostratigraphic correlation relative and absolute changes in the sulfur isotopic composition of the ocean 34sseawater are essential for estimating changes in redox conditions in ancient environments particularly for geochemical models that utilize isotope mass balance thus accurate estimates of 34sseawater must be measured using a laboratory protocol optimized for the extraction of primary sulfate preserved as carbonate associated sulfate cas by removing any contaminant secondary sulfate e g from diagenetic pyrite oxidation here we use two cas extraction protocols on lower middle ordovician carbonate rocks to test the degree to which similar absolute values and stratigraphic trends in 34scas are produced by different cas extraction methods one method processes carbonate powders once with a single rinse of a 10 nacl solution to remove sulfate minerals or weakly bonded sulfate ions followed by a rinse in 5 6 bleach to remove organically bound sulfur before dissolution in hydrochloric acid hcl the second method treats carbonate powders with three rinses of nacl without a bleach rinse to more aggressively remove secondary sulfate before dissolution in hcl isotopic results show that samples treated in three nacl rinses produce 34scas values that are on average 7 more positive than samples treated with a single nacl rinse but a similar stratigraphic trend is preserved in samples processed with either method we interpret the difference in 34scas between methods to reflect the incomplete removal of secondary sulfate derived from the oxidation of 32s enriched pyrite when employing only a single nacl rinse surprisingly rocks with low cas concentrations 10 mg kg using both methods show no significant difference in 34scas values as well as no significant difference in pyrite concentrations and pyrite 34s 34spy between methods samples with low cas concentrations are likely altered and secondary sulfate sourced from pyrite oxidation has likely become incorporated into recrystallized carbonate minerals or incorporated into pore filling calcite cement all of which is captured as cas results suggest that both cas extraction protocols can produce broadly similar 34scas trends but a single nacl rinse may not completely remove contaminant sulfate sorbed onto carbonate grains and thus will not yield meaningful data for reconstructing 34sseawater this has clear implications for modeling studies aimed at reconstructing paleo redox conditions using 34s data assumed to record global 34sseawater trends if these 34s data are generated using a non optimized cas protocol

did cooling oceans trigger ordovician biodiversification evidence from conodont thermometry the ordovician period long considered a supergreenhouse state saw one of the greatest radiations of life in earth9s history previous temperature estimates of up to 70 c have spawned controversial speculation that the oxygen isotopic composition of seawater must have evolved over geological time we present a very different global climate record determined by ion microprobe oxygen isotope analyses of early ordovician silurian conodonts this record shows a steady cooling trend through the early ordovician reaching modern equatorial temperatures that were sustained throughout the middle and late ordovician this favorable climate regime implies not only that the oxygen isotopic composition of ordovician seawater was similar to that of today but also that climate played an overarching role in promoting the unprecedented increases in biodiversity that characterized this period

isotope fractionation by sulfate reducing natural populations and the isotopic composition of sulfide in marine sediments isotope fractionations during sulfate reduction by natural bacterial populations were measured in seven different marine sediments and compared with the isotopic composition of solid phase sulfides in the same sediments the measured fractionations during sulfate reduction could explain only between 41 and 85 of the 34s depletion in the sedimentary sulfides this result directly demonstrates that the depletion of 34s in solid sulfides is an expression of the combined activity of sulfate reducing organisms with additional fractionations accumulated during the oxidative part of the sulfur cycle the only known process to significantly augment the fractionations created during sulfate reduction is the microbial disproportionation of the intermediate sulfur compounds elemental sulfur thiosulfate and sulfite in a simple model we show how important each of these disproportionation processes could be in generating the isotopic composition of sedimentary sulfides the sulfate reduction rates in the sediments studied varied by a factor of 1000 and did not correlate with the fractionation during sulfate reduction by contrast a correlation was observed between sulfate reduction rate and the extent of 34s depletion into sedimentary sulfides the most 34s depleted sulfides found in sediments supporting the lowest rates of sulfate reduction thus the fractionations imposed during the disproportionating processes are better expressed in sediments with low sulfate reduction rates the direct phototrophic oxidation of sulfide to sulfate with minimal fractionation may have been important in the sediments that had a high sulfate reduction rate

the late ordovician mass extinction abstract near the end of the late ordovician in the first of five mass extinctions in the phanerozoic about 85 of marine species died the cause was a brief glacial interval that produced two pulses of extinction the first pulse was at the beginning of the glaciation when sea level decline drained epicontinental seaways produced a harsh climate in low and mid latitudes and initiated active deep oceanic currents that aerated the deep oceans and brought nutrients and possibly toxic material up from oceanic depths following that initial pulse of extinction surviving faunas adapted to the new ecologic setting the glaciation ended suddenly and as sea level rose the climate moderated and oceanic circulation stagnated another pulse of extinction occurred the second extinction marked the end of a long interval of ecologic stasis an ecologic evolutionary unit recovery from the event took several million years but the resulting fauna had ecologic patterns similar to the fauna that had become e

ediacaran growth of the marine sulfate reservoir abstract the sulfur isotope record as preserved in sedimentary sulfate and sulfide minerals reflects the progressive oxidation of earth s surface environment the latest proterozoic to early paleozoic marine sulfur isotope record is distinguished by unusually high 34 s values in marine sulfates and sulfides geological considerations and sulfur isotopic data indicate that the 34 s enrichment of the marine sulfate reservoir was the result of a rapid increase in the net fractionation between sedimentary sulfate and pyrite 34 s without a compensatory decline in the fractional burial rate of pyrite f pyr a simple one box model in which 34 s is linked non linearly to marine so 4 is used to explore the response in sedimentary sulfur isotope records 34 s sulf and 34 s pyr to an increasing marine sulfate reservoir the model results when compared with a compilation of existing 34 s sulf and 34 s pyr data suggest that the marine sulfate reservoir grew rapidly to roughly phanerozoic levels in the middle ediacaran period before the gaskiers glaciation and prior to the onset of the shuram wonoka negative 13 c anomaly it is further hypothesized that p o 2 levels rose prior to the onset of this extreme perturbation to the global carbon cycle permitting the oxidation of a previously euxinic deep ocean and appearance of the first ediacaran fauna by ca 575 ma despite the enormous demand on oxidants implied by this large magnitude and long duration carbon isotope anomaly

a weathering hypothesis for glaciation at high atmospheric pco2 during the late ordovician new paired carbonate and organic carbon isotope analyses from nevada usa together with a consideration of the effects of mountain building and ice sheet coverage of the continents on atmospheric pco2 lead to a new hypothesis for the cause of the late ordovician glaciation we suggest that the taconic orogeny which commenced in the late middle ordovician caused a long term decline in atmospheric pco2 through increased weatherability of silicate rocks ice sheet growth was triggered when pco2 decreased to a threshold of 10 present atmospheric level and proceeded by positive ice albedo feedback in the midst of glaciation atmospheric pco2 began to rise as continental silicate weathering rates declined in response to coverage of weathering terrains by ice sheets at first this enhanced greenhouse effect was overcompensated for by ice albedo effects ultimately however atmospheric pco2 reached a level which overwhelmed the cooling effects of ice albedo and the glaciation ended the isotope results can be interpreted to indicate that atmospheric pco2 rose during the glaciation consistent with other proxy information although alternative interpretations are possible the large positive carbonate isotope excursion observed in late ordovician rocks around the world is explained as the expected response to increased carbonate platform weathering during glacioeustatic sea level lowstand rather than as a response to increased organic carbon burial

origin and geometry of red river dolomite reservoirs western williston basin remarkably uniform distribution of limestone laminated dolomite and anhydrite as determined from compensated neutron density logs suggests that the entire ordovician red river formation of the central williston basin was deposited in subtidal brining upward sequences study of cores and thin sections verifies this interpretation and reveals that the red river b and c zones consist of from bottom to top 1 highly burrowed locally dolomitized fossiliferous wackestones and packstones 2 laminated to evenly bedded unfossiliferous mudstones dolomitized in many wells and 3 bedded anhydrite no evidence of subaerial exposure was observed in these rocks units dolomitization in the red river c zone is highly localized an empirical study of dolomite distribution using data from well logs reveals the presence of dozens of pods of dolomite immediately beneath the c anhydrite the pods are up to 200 ft 60 m thick and 1 mi 1 6 km in diameter and consist of concentered lenses of 1 tight locally anhydritic cryptocrystalline dolomite up to 40 ft 12 2 m thick and 3 300 ft 1 000 m in diameter 2 fine to medium grained porous dolomite that forms the reservoirs and 3 still farther from the cryptocrystalline dolomite relatively tight partly dolomitized limestones these dolomite pods formed by the subtidal downward migration of magnesium rich brines the brines formed during or immediately after precipitation of the subtidal anhydrite probably originally gypsum brine migration was localized by holes in the anhydrite and or upwelling compaction water from the sediments beneath the anhydrite the holes probably formed by fracturing minor faulting or expulsion of water during compaction where the magnesium rich brines first entered the carbonate sediments they formed cryptocrystalline dolomite but as they migrated away from the holes and lost their potency they produced porous dolomite the nature of which was partly controlled by the host sediment and still farther away partly dolomitized limestones because certain layers of the c zone carbonates were quite permeable to the dolomitizing brines highly irregular reservoirs characterized by interfingering limestones and dolomites formed several oil fields in the western williston basin could have been found through application of this model and others could be more efficiently developed

neogene aquifer properties specified through the interpretation of electrical sounding data salamiyeh region central syria twenty nine schlumberger electrical soundings were carried out in the salamiyeh region in syria using a maximum current electrode separation of 1 km three soundings were made at existing boreholes for comparison aquifer parameters of hydraulic conductivity and transmissivity were obtained by analysing pumping test data from the existing boreholes an empirical relationship between hydraulic conductivity determined from the pumping test and both resistivity and thickness of the neogene aquifer has been established for these boreholes in order to calculate the geophysical hydraulic conductivity a close agreement has been obtained between the computed hydraulic conductivity and that determined from the pumping test the relationship established has therefore been generalized in the study area in order to evaluate hydraulic conductivity and transmissivity at all the points where geoelectrical measurements have been carried out this generalization allows one to derive maps of the hydraulic conductivity and transmissivity in the study area based on geoelectrical measurements these maps are important in future modelling processes oriented towards better exploitation of the aquifers copyright 2006 john wiley sons ltd

calibration of a conodont apatite based ordovician 87sr 86sr curve to biostratigraphy and geochronology implications for stratigraphic resolution the ordovician 87 sr 86 sr isotope seawater curve is well established and shows a decreasing trend until the mid katian however uncertainties in calibration of this curve to biostratigraphy and geochronology have made it diffi cult to determine how the rates of 87 sr 86 sr decrease may have varied which has implications for both the stratigraphic resolution possible using sr isotope stratigraphy and efforts to model the effects of ordovician geologic events we measured 87 sr 86 sr in conodont apatite in north american ordovician sections that are well studied for conodont biostratigraphy primarily in nevada oklahoma the appalachian region and ohio valley our results indicate that conodont apatite may provide an accurate medium for sr isotope stratigraphy and strengthen previous reports that point toward a signifi cant increase in the rate of fall in seawater 87 sr 86 sr during the middle ordovician darriwilian stage our 87 sr 86 sr results suggest that sr isotope stratigraphy will be most useful as a high resolution tool for global correlation in the mid darriwilian to mid sandbian when the maximum rate of fall in sr

the ordovician chitinozoan biodiversification and its leading factors abstract regional and global dataset now available for almost all of the ordovician fossil groups have led to a search for the causes controlling the major biodiversification events that occurred during the ordovician period a review of the physico chemical state of the ordovician world is presented with an emphasis on the regional or global changes contemporaneous with the origination and extinction events that could have influenced the chitinozoan diversity this study focuses on the chitinozoans because these enigmatic organic walled microfossils are an important component of the ordovician palaeoplankton and provide one of the best documented dataset the intrinsic factors that initiated the ordovician biodiversification of this group are not discussed because the chitinozoans are regarded as reproduction stages of cryptic chitinozoan animals whose biological characteristics are speculative at best this study has two main goals i the evaluation of the impact of regional and global physico chemical events on chitinozoan diversity ii the comparison of the biodiversification patterns of the chitinozoans with other selected benthic and pelagic ordovician fossil groups the chitinozoan diversification was progressive and showed similar patterns in laurentia baltica and northern gondwana from the tremadocian to the late darriwilian when the group reached its acme in baltica and northern gondwana from the middle upper ordovician boundary onward the biodiversification pattern documented in laurentia diverged drastically from the two other regions in the late ordovician the contribution of the laurentian chitinozoans to the global curve was high suggesting major faunal inputs from the two other regions which significantly lowered the endemic character prevalent in the early middle ordovician in the late ordovician large anti clockwise oceanic currents developed as the result of a thermohaline circulation this marine circulation was likely to have been driven by a global cooling concomitant with a major palaeogeographic reorganisation of the southern hemisphere these oceanic climatic changes intervened in the breakdown of the existing chitinozoan endemism globally the chitinozoan biodiversity was not much higher in the mid late ordovician than in the late darriwilian diversity peak the most obvious feature is the progressive decrease in diversity during the late ordovician long before the hirnantian glaciation the influences of cosmic and volcanic parameters are excluded as they appear to have had too low impact on the late ordovician decrease of chitinozoan diversity correlation is noticed between some diversification events and sea level changes at least on a regional scale more globally however a climatic control is favoured a durable greenhouse environment gave an efficient support to the diversification conversely the onset of an icehouse environment in the early late ordovician onward culminating with the hirnantian glaciation is interpreted as a limiting factor for the chitinozoan diversification the reasons behind the onset of this icehouse episode are not fully understood but they appear to be linked to changes in the palaeogeography e g progressive closure of the iapetus northward drift of avalonia and its docking with baltica and in the carbon cycle e g high organic carbon burial with a lowering of the p co 2

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

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

effect of nutrient availability on marine origination rates throughout the phanerozoic eon the diversity of marine life has varied throughout the past 500 million years statistical analyses suggest that fluctuations in the availability of marine nutrients has been one important regulator of rates of origination during this time

the major ion composition of permian seawater abstract the major ion mg2 ca2 na k so42 and cl composition of permian seawater was determined from chemical analyses of fluid inclusions in marine halites new data from the upper permian san andres formation of texas 274 272 ma and salado formation of new mexico 251 ma analyzed by the environmental scanning electron microscopy esem x ray energy dispersive spectrometry eds method along with published chemical compositions of fluid inclusions in permian marine halites from north america two formations of different ages and the central and eastern european basins eight formations of four different ages show that permian seawater shares chemical characteristics with modern seawater including so42 ca2 at the point of gypsum precipitation evolution into mg2 na k so42 cl brines and mg2 k ratios 5 permian seawater however is slightly depleted in so42 and enriched in ca2 although modeling results do not rule out ca2 concentrations close to those in present day seawater na and mg2 in permian seawater are close to slightly below their concentrations in modern seawater permian and modern seawater are both classified as aragonite seas with mg2 ca2 ratios 2 conditions favorable for precipitation of aragonite and magnesian calcite as ooids and cements the chemistry of permian seawater was modeled using the chemical composition of brine inclusions for three periods lower permian asselian sakmarian 296 283 ma lower permian artinskian kungurian 283 274 ma and upper permian tatarian 258 251 ma parallel changes in the chemistry of brine inclusions from equivalent age evaporites in north america central europe and eastern europe show that seawater underwent secular variations in chemistry over the 50 million years of the permian modeled so42 concentrations are 20 mmol per kg h2o mmolal and 19 mmolal in the asselian sakmarian and artinskian kungurian with higher concentrations in the upper permian tatarian 23 mmolal modeled ca2 is at or above its concentration in modern seawater throughout the permian mg2 is close to slightly below its concentration in modern seawater 55 mmolal in the asselian sakmarian 52 mmolal and tatarian 52 mmolal but slightly higher than modern seawater in the artinskian kungurian 60 mmolal mg2 ca2 ratios are 3 5 total range 2 7 to 5 5 in the lower permian and rose slightly to 3 7 total range 3 1 to 5 8 in the upper permian primarily due to decreases in ca2 these results are consistent with models that predict oscillations in the major ion composition of phanerozoic seawater on the basis of changes in the midocean ridge river water flux ratio driven by changes in the rate of midocean ridge crust production the permian was characterized by low sea levels icehouse conditions and southern hemisphere glaciation such conditions analogous to the present ice age and the similarities between permian seawater and modern seawater all suggest that general phanerozoic supercycles driven by mantle convection and global volcanicity also control the major ion chemistry of seawater

the middle darriwilian ordovician δ13c excursion mdice discovered in the yangtze platform succession in china implications of its first recorded occurrences outside baltoscandia abstract the middle darriwilian 13 c excursion mdice one of the least known of the ordovician 13 c excursions has previously been recorded only from the middle ordovician of baltoscandia analysis of many recently collected limestone samples from the guniutan formation at two yangtze platform localities show elevated 13 c values in the same biostratigraphic interval microzarkodina ozarkodella conodont subzone as the mdice in baltoscandia which justifies identification of the chinese 13 c excursion as the mdice these occurrences which are in strata that show striking lithological and conodont faunal similarity to the swedish holen limestone and some coeval units in estonia indicate that the mdice the stratigraphically oldest of the named ordovician 13 c excursions is likely to have a world wide distribution and to have great potential for local and long range chemostratigraphic correlations

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

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

stratification and mixing of a post glacial neoproterozoic ocean evidence from carbon and sulfur isotopes in a cap dolostone from northwest china abstract to improve our knowledge about the geochemical and environmental aftermath of neoproterozoic global glaciations we analyzed stable isotopes 13 c 18 o 34 s and elemental concentrations ca mg s sr fe and mn of the 10 m thick zhamoketi cap dolostone atop the tereeken diamictite in the quruqtagh area eastern chinese tianshan available chemostratigraphic data suggest that the tereeken diamictite is probably equivalent to the marinoan glaciation our new data indicate that organic and carbonate carbon isotopes of the zhamoketi cap dolostone show little stratigraphic variations averaging 28 2 and 4 6 respectively in contrast sulfur isotopes show significant stratigraphic variations carbonate associated sulfate cas abundance decreases rapidly in the basal cap dolostone and 34 s cas composition varies between 9 and 15 in the lower 2 5 m in the overlying interval cas abundance remains low while 34 s cas rises 5 and varies more widely between 10 and 21 the range of 34 s py of the cap dolostone overlaps with that of 34 s cas but direct comparison shows that 34 s py is typically greater than 34 s cas measured from the same samples hypotheses to explain the observations must account for both the remarkable sulfur isotope enrichment of pyrites and the inverse fractionation we propose that cas and pyrite were derived from two isotopically distinct reservoirs in a chemically stratified basin or a basin with a sulfate minimum zone in this model cas was derived from shallow oxic surface waters with moderate sulfate concentration and depleted in 34 s due to the post glacial influx of sulfur from continental weathering in contrast pyrite was derived from anoxic bottom waters or a sulfate minimum zone with low sulfate concentration and 34 s enrichment due to long term syn glacial sulfate reduction the rapid shift in cas abundance and sulfur isotope composition within the cap dolostone is interpreted to reflect the mixing of the two reservoirs after initial deglaciation comparison with other post marinoan cap carbonates shows significant spatial heterogeneity in 34 s cas which together with strong temporal variation in 34 s cas points to generally low sulfate concentrations in post marinoan oceans

a major drop in seawater 87sr 86sr during the middle ordovician darriwilian links to volcanism and climate a large drop in seawater 87 sr 86 sr during the middle ordovician was among the most rapid in the entire phanerozoic new 87 sr 86 sr measurements from nevada indicate that the rapid shift began in the pygodus serra conodont zone of the upper darriwilian stage we use a numerical model to explore the hypothesis that volcanic weathering provided the flux of nonradiogenic sr to the oceans a close balance between volcanic outgassing and co 2 consumption from weathering produced steady p co 2 levels and climate through the middle katian consistent with recent ordovician paleotemperature estimates in the late katian outgassing was reduced while volcanic weathering continued and resulted in a cooling episode leading into the well known end ordovician glaciation

sulfur isotopic profile through the oceanic crust sulfur mobility and seawater crustal sulfur exchange during hydrothermal alteration measurements of the abundance of anhydrite and pyrite veins and the distribution of alteration types in oceanic basement enable estimates of global mass and isotopic fluxes of sulfur during hydrothermal alteration of oceanic crust fluxes of 2 5 10 12 g yr s 34 s 2 7 from the volcanic section to seawater and 2 1 10 12 g yr of seawater s 21 into the crust result in an insignificant change in the s content of altered ocean crust these are clearly second order fluxes compared to the riverine input of s to the oceans and the output of sedimentary pyrite most of the anhydrite that forms in the high temperature axial convection cell must later be dissolved at lower temperatures during off axis circulation the mean 34 s of altered oceanic basement is 0 9 only slightly changed from the primary value of 0 1 unless s is preferentially mobilized from 34 s rich sulfide deposits or mineralized zones subduction of altered basaltic crust is unlikely to be the source of high 34 s values of arc volcanic rocks 4 but could contribute to isotopic heterogeneities of s in the mantle

depositional and diagenetic constraints on the abundance and spatial variability of carbonate associated sulfate abstract marine carbonate rocks are composed in varying abundance of cement micrite abiotic grains and fossils which can provide information about the physical and chemical environments in which they formed geochemical analyses of these carbonates are not always interpreted alongside the wealth of geologic including petrographic information available resulting in potentially faulty reconstructions of biogeochemical and environmental conditions these concerns have prompted closer scrutiny of the effect of depositional lithofacies and diagenesis on carbonate proxies here we have combined x ray absorption near edge structure xanes spectroscopy and x ray fluorescence xrf imaging to map the speciation and abundance of sulfur in carbonate petrographic thin sections in upper ordovician carbonates from anticosti island canada and early silurian carbonates from gotland sweden across multiple depositional facies lithofacies and fossil communities between anticosti island and gotland are similar which allows for comparison of changes in the dominant s species and their abundance in separate basins associated with variations in glacio eustatic sea level sulfide abundance is greatest in mudstone wackestone and packstone facies where interstitial micrite hosts abundant pyrite sulfate abundance as carbonate associated sulfate cas varies within individual fossil fragments as well as within the same fossil phylum and is particularly high in unaltered brachiopods in contrast sulfate abundance is generally very low in micrite near the detection limit and generally arises in situ from sulfide that has been oxidized as opposed to true cas in different cement fabrics sulfate abundance is greatest in drusy pore filling cements organic sulfur compounds are also detected and although low in abundance are mostly found within micrite the detection and characterization of both inorganic sulfur and organic sulfur compounds provides a platform to understand early processes of biomineralization this approach will broaden our understanding of the source of inorganically bound sulfate in ancient carbonates as well as the effect of depositional setting and diagenesis on cas incorporation re mobilization and ultimate abundance in sedimentary carbonates additionally this work has implications for the cas isotopic value of individual carbonate components that may affect interpretations of stratigraphic variability of numerous cas sections throughout earth history

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

isotopic evidence for the early meteoric diagenesis of the reef facies permian reef complex of west texas and new mexico abstract multiple isotopic analyses of former aragonitic marine cements from a single sample of permian reef complex reef facies define a linear covariant trend with a positive slope each individual analysis represents a different mixture of two isotopically distinct phases which are too finely intergrown for separate analysis in order to assess the systematic variation of the isotopic composition of these calcite phases over a paleobathymetric range samples were collected from four sites a through d along a single upper capitan paleoslope reflecting the transition from reef massive through reef foreslope facies on a plot of 13c versus 18o the convergence f four marine cement trends from these four sites indicates that the isotopically enriched phase is invariant whereas the depleted phase a luminescent calcite intergrowth along original marine cement intercrystalline boundaries varies systematically with position on the paleoslope the emplacement of the calcite intergrowth in the marine cements is intimately related to the earliest episodes of equant calcite spar precipitation at the shallowest paleodepths sites a through c the compositions of the first generation of calcite spar spar i clusters around a mean 18o of 8 2 and is progressively less depleted in 13c from site a to site c as this variation is also present in the marine cement trends it is concluded that both the intergrowth and spar i represent successive episodes of calcite precipitation during a single diagenetic event at the deepest paleodepth site d sp r i is absent and the calcite intergrowth is interpreted to have the same composition as the second generation of spar spar ii 12 3 18o and 0 0 13c which can be found throughout the reef facies on the basis of its relatively constant oxygen composition the systematic enrichment of its carbon composition with increasing paleo depth and its absence from the deepest site on the paleoslope we conclude that the spar i diagenetic event represents the establishment of a meteoric phreatic system in the reef facies characterized by well defined downdip gradients in rock water interaction furthermore isotopic evidence of fresh water alteration of allochthonous massive facies reef debris deposited in foreslope strata near site d and below the limit of the spar i diagenetic event suggests that this meteoric system was active contemporaneously with the growth of the reef complex interpretation of the spar ii diagenetic event is less clear cut the associated diagenetic fluid probably did not have marine or hypersaline oxygen signatures as this would require the persistence of aragonite mineralogies to temperatures over 70 c and burial depths of at least i kin spar il was probably precipitated from meteoric water with a 18o similar to that of the spar i meteoric water at temperatures between 30 65 c unlike the spar i diagenetic event the spar ii event shows no association with depositional systematics hence we tentatively suggest that spar ii may have been related to a presumed ochoan or younger regional meteoric or shallow burial phreatic system

middle late ordovician darriwilian sandbian decoupling of global sulfur and carbon cycles isotopic evidence from eastern and southern laurentia abstract middle late ordovician sequences from the appalachian basin and arbuckle mountain regions of north america were analyzed for carbonate associated sulfate 34 s cas and pyrite 34 s pyr paired with carbonate 13 c carb and organic matter 13 c org chemostratigraphy two major negative drops in 34 s cas 12 excursions are recognized the older decline in 34 s cas occurs within the histiodella holodentata phragmodus polonicus conodont zones and the younger drop is within the cahabagnathus sweeti amorphognathus tvaerensis baltoniodus gerdae subzone zones these overall these negative shifts in 34 s cas have an antithetical relationship with positive shifts in 34 s pyr 10 and 13 c carb 2 recorded in the same successions the older negative 34 s cas shift is coincident with the widely documented mid darriwilian 13 c excursion mdice and the younger negative 34 s cas shift is coincident with another positive 13 c carb shift in the early sandbian geochemical modeling of these sulfur isotope shifts suggests that a decrease in the global rate of pyrite burial or isotope fractionation between seawater sulfate and sedimentary pyrite could account for these negative 34 s cas trends additionally a substantial increase in the weathering flux of pyrite to the global oceans could also explain these secular sulfur isotope trends while increased crustal weathering is broadly consistent with a sea level lowstand and the seawater 87 sr 86 sr isotope record of change in continental weathering in the late darriwilian stage of the ordovician geologic and geochemical proxy evidence do not support distinct pulses of continental weathering required to generate two separate negative shifts in 34 s cas these antithetical isotope trends may be best explained by changes in the marine redox state that significantly reduced microbially mediated pyrite burial and organic matter remineralization rates pulses of oceanic ventilation would have expanded habitable environments for marine organisms and thus is broadly consistent with major increases in biodiversification during this period of the ordovician

milankovitch band cyclicity in bedded halite deposits contemporaneous with late ordovician early silurian glaciation canning basin western australia abstract the mallowa salt of the early palaeozoic carribuddy group canning basin western australia is a halite mudstone anhydrite dolomite evaporite sequence of late ordovician to early silurian age deposited in barred marginal marine to ephemeral saltpan and saline mudflat environments bhp utah minerals potash exploration well gingerah hill no 1 obtained drill core of a 477 m stratigraphic interval of the mallowa salt which permitted the study of geochemical variations in a thick halite sequence contemporaneous with late ordovician early silurian glaciation on other continents a strip of uniform width was continuously ground from the core and the powdered rock bagged at 1 m intervals for geochemical assay the resulting geochemical stratigraphic series display conspicuous cyclicity several orders of cycles ranging from 3 m to100 m stratigraphic thickness are discernible in the plots of minor constituents br mgo and k2o in extracted soluble salts and also in the plots of na2o and total salts content fast fourier transform fft of the br mgo and k2o series gives principal spectral peaks in the 0 00 0 10 cycles m frequency band at 250 113 35 4 22 1 and 19 7 m as well as peaks at 65 m and between 2 8 and 13 5 m fft of the na2o and total salts series shows dominant spectral peaks at 220 259 and 109 114 m if the strongest spectral peak at 113 m is taken to represent the relatively stable eccentricity period of 100 ka and a constant net rate of accretion assumed for long sections the other main periods in the mallowa salt as exemplified by the br spectrum would be31 3 3 0 19 6 1 1 and17 4 1 1 ka these figures are consistent with predicted late ordovician early silurian 440 ma periods for obliquity 30 5 ka and precession 19 3 and 16 4 ka based on evolutionary change in the earth moon system additional spectral peaks identified with implied periods of approximately 206 233 57 5 8 12 and 2 5 3 5 ka also may be of palaeoclimatic relevance the relative amplitudes and structure of spectral peaks in the 0 00 0 10 cycles m frequency band support the identification of climatic oscillations forced by orbital cycles the data indicate a precession eccentricity dominated pattern which accords with the know low palaeolatitude

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

stable carbon isotope record of shallow marine evaporative epicratonic basin carbonates ordovician williston basin north america secular variations in stable carbon isotope values of marine carbonates are used widely to correlate successions that lack high resolution index fossils various environmental processes however commonly may affect and alter the primary marine carbon isotope signal in shallow epicratonic basins this study focuses on the marine carbon isotope record from the carbonate evaporite succession of the upper katian upper ordovician red river formation of the shallow epicratonic williston basin usa it documents the carbon isotope signal between the two major ordovician positive shifts in 13c the early katian guttenberg and the hirnantian excursions eight 13c stages are identified based on positive excursions shifts from positive to negative values and relatively uniform 13ccarb values a correlation between carbon isotope trends and the relative sea level changes based on gross facies stacking patterns shows no clear relation based on the available biostratigraphy and 13c trends the studied williston basin curves are tied to the isotope curves from the north american midcontinent quebec anticosti island and estonia which confirm the late katian age aphelognathus divergens conodont zone of the upper red river formation the differences in the 13c overall trend and absolute values coupled with the petrographic and cathodoluminescence evidence suggest that the carbon isotope record has been affected by the syndepositional environmental processes in the shallow and periodically isolated williston basin and stabilized by later burial diagenesis under reducing conditions and the presence of isotopically more negative fluids

a global carbon isotope excursion spice during the late cambrian relation to trilobite extinctions organic matter burial and sea level abstract the steptoean positive carbon isotope excursion spice marks a global oceanographic event that confirms intercontinental correlations between different biogeographic realms based on agnostids and other blue water trilobites the spice excursion is documented from sections in laurentia kazakhstan china and australia where it begins with the mass extinction at the base of the pterocephaliid biomere steptoean stage in laurentia and at coeval extinction horizons in gondwana and perigondwana terranes the peak of spice 5 coincided with a time of maximum regression in laurentia spice is similar in this regard to excursions that coincide with glacio eustatic falls such as in the late ordovician a plausible scenario involves the transformation of ocean circulation between two states which led to enhanced coastal upwelling and benthic extinctions the lack of evidence for glaciation indicates that the coeval sea level fall sauk ii sauk iii event resulted from tectonic or hydrologic changes that remain poorly understood at this time