abrupt global ocean anoxia during the late ordovician early silurian detected using uranium isotopes of marine carbonates widespread marine anoxia is hypothesized as the trigger for the second pulse of the late ordovician hirnantian mass extinction based on lithologic and geochemical proxies that record local bottom waters or porewaters we test the anoxia hypothesis using 238u values of marine limestones as a global seawater redox proxy the 238u trends at anticosti island canada document an abrupt late hirnantian 0 3 negative shift continuing through the early silurian indicating more reducing seawater conditions the lack of observed anoxic facies and no covariance among 238u values and other local redox proxies suggests that the 238u trends represent a global ocean redox record the hirnantian ocean anoxic event hoae onset is coincident with the extinction pulse indicating its importance in triggering it anoxia initiated during high sea levels before peak hirnantian glaciation and continued into the subsequent lowstand and early silurian deglacial eustatic rise implying that major climatic and eustatic changes had little effect on global ocean redox conditions the hoae occurred during a global 13c positive excursion but lasted longer indicating that controls on the c budget were partially decoupled from global ocean redox trends u cycle modeling suggests that there was a 15 increase in anoxic seafloor area and 80 of seawater u was sequestered into anoxic sediments during the hoae unlike other ocean anoxic events oae the hoae occurred during peak and waning icehouse conditions rather than during greenhouse climates we interpret that anoxia was driven by global cooling which reorganized thermohaline circulation decreased deep ocean ventilation enhanced nutrient fluxes stimulated productivity which lead to expanded oxygen minimum zones

glacial interglacial changes in bottom water oxygen content on the portuguese margin some of the glacial co2 drawdown has been attributed to co2 storage in the deep pacific and southern oceans reconstruction of apparent oxygen utilization suggests that respired co2 storage was also enhanced in the deep northeast atlantic

a far field record of the end ordovician glaciation the ellis bay formation anticosti island eastern canada abstract the end ordovician was a critical period in earth history it was a time of global climatic change with large scale continental glaciations a wealth of recent work has documented the stratigraphical records from the near field areas i e from various gondwanan continental relics where glaciation was more severely felt there is an urgent need to re examine the stratigraphic record from the far field areas at greater levels of resolution than it has been hitherto possible in order to understand the timing duration and character at the glaciation centred around the ordovician silurian o s boundary one of the best exposed and most complete stratigraphic records from paleotropical areas near the boundary is on anticosti island in eastern canada the east west trending 200 km long ellis bay hirnantian outcrop belt with superb coastal exposures at both ends of anticosti island is slightly oblique to the paleoshoreline with near shore siliciclastic dominated facies restricted to the eastern part of the island and more offshore carbonate dominated facies present along the central and western parts these mixed carbonate siliciclastic facies accumulated along a storm dominated beach to offshore profile they are organized into a stacked set of shallowing upward offshore to shoreline facies successions typically coarsening upward separated by deepening upward facies successions typically fining upward formed during intervening transgressions high frequency glacio eustatic fluctuations were the dominant control on the development of tr sequences observed in the ellis bay succession of anticosti island although tectonic subsidence had a substantial secondary effect on thickness five tr sequences are recognized allowing for precise correlation along the ellis bay outcrop belt in the eastern sections relatively thin tr sequences are locally bounded by unconformities a typical sequence begins with thin transgressive limestone overlain by a maximum flooding calcareous shale grading up into regressive storm influenced proximal to transitional zone fine sandstone grainstone and shale which are sharply overlain by shoreface to foreshore sandstone grainstone in the central and western sections the thicker tr sequences have more symmetrical transgressive regressive components reflecting greater subsidence compensated by a higher carbonate sediment supply two discrete glacial periods are recognized during the deposition of the hirnantian ellis bay succession with the younger one recording large sea level changes 50 m in magnitude caused by the development of very large ice sheets over western gondwana abundant ice conditions on the globe during the end ordovician and the resulting high amplitude eustasy appear to have left distinctive stratigraphical imprints on mixed paleotropical siliciclastic carbonate ramps

orbital scale climate change and glacioeustasy during the early late ordovician pre hirnantian determined from δ18o values in marine apatite this study focuses on the 10 m y before the latest ordovician hirnantian glaciation we test whether orbital scale climatic fluctuations controlled the growth and melting of continental glaciers resulting in glacioeustatic sea level changes and the development of widespread marine sedimentary cycles 18o values of conodont apatite from 14 late ordovician katian cycles range from 17 to 21 isotopic values decrease and are lowest in the deepest water facies and increase and are highest in shallow water facies supporting the hypothesis that glacioeustasy was the dominant control on water depth changes measured intracycle 18o changes of 0 7 2 5 were controlled by changes in ice volume 60 m sea level changes sea surface temperatures 5 c and potentially local increases in seawater evaporation during drier and or windier glacial stages these interpreted orbital scale climate changes and resultant large glacial ice volume changes support recent interpretations of a dynamic and prolonged ordovician greenhouse to icehouse transition

multispecies stable isotopes of benthic foraminifers reveal past changes of organic matter decomposition and deepwater oxygenation in the arabian sea 1 the stable isotope composition of one epifaunal and three infaunal benthic foraminiferal species of a sediment core from 1800 m water depth of the western arabian sea was determined to evaluate deepwater oxygenation organic matter remineralization and early diagenetic processes during the past 190 000 years the 18o records reveal species specific metabolic effects susceptibility to changes in carbonate ion concentration and supralysoclinal calcite dissolution the foraminiferal 13c records reveal changes in the stable carbon isotope gradients of pore water dissolved inorganic carbon 13cdic and in the microhabitat depth of infaunal species maximum 13cdic offsets between bottom and pore waters ranged between mean values of 0 8 and 1 2 corresponding to estimates of deepwater oxygen concentration between approximately 1 and 2 7 ml l 1 intervals of improved deepwater oxygenation coincided with high benthic foraminiferal diversity and indicate the admixture of well oxygenated deepwater masses during interglacials during interglacial maxima the 13c difference between epifauna and shallow infauna indicates highest organic matter remineralization rates at times of maximum organic matter fluxes

a high resolution continuous δ13c record spanning the ordovician silurian boundary on anticosti island eastern canada1 one of the best exposed and most complete stratigraphic records from paleotropical areas spanning the ordovician silurian o s boundary is located on anticosti island eastern canada our study is

first record of the hirnantian upper ordovician δ13c excursion in the north american midcontinent and its regional implications the most prominent of the two major global delta c 13 excursions in the ordovician the hirnantian delta c 13 excursion hice which is previously recorded from the uppermost ordovician in a few sections in nevada quebec arctic canada baltoscandia scotland and china is documented for the first time from the north american midcontinent samples through the girardeau limestone and leemon formation in missouri and illinois show elevated delta c 13 values of 4 parts per thousand to 5 parts per thousand although not determined precisely the beginning of the hice is likely to be in the upper part of the orchard creek shale and it ends in the upper leemon formation being extraordinarily useful chronostratigraphically the presence of the hice makes it possible to provide a firm dating of the study interval whose age has long been controversial comparison between the study sections and coeval hice sequences in north america and europe show striking similarities especially in sea level history indicating that major local lowstands reflect eustatic sea level changes a comparison with hirnantian diamictite successions in north and south africa and argentina suggests that these lowstands correspond to two major gondwanan glacial episodes less

bathymetric and isotopic evidence for a short lived late ordovician glaciation in a greenhouse period the end ordovician glaciation is distinct among phanerozoic glaciations in that co 2 levels were generally high yet major continental ice sheets accumulated on the gondwana supercontinent new oxygen isotopic data indicate substantial changes in sea water temperatures and ice volume coinciding with glacio eustatic changes in sea level reflecting the growth and decay of the gondwana ice cap major glaciation was apparently confined to the hirnantian and was 0 5 1 m y long rather than the 35 m y of earlier estimates carbon isotope values indicate significant changes in carbon cycling as the oceans changed from a state with warm saline bottom waters to a state with cold deep water circulation and then back again we believe that the changes in the carbon cycle effected a reduction in p co 2 levels in the oceans and atmosphere and thus promoted glaciation but were unable to sustain icehouse conditions in a greenhouse world

surface and crater exposed lithologic units of the isidis basin as mapped by coanalysis of themis and tes derived data products 1 the stratigraphy and mineral heterogeneity of surface and crater exposed deposits in the southwestern portion of the isidis basin were investigated using spatial spectral visible and thermophysical data sets from both the mars global surveyor and odyssey missions four themis spectral surface units were identified progressing from the libya montes highlands to the inner lowland plains of isidis planitia lithologic determinations of these spectral units were made by linear deconvolution analysis of tes data olivine rich surface materials correlate with a well documented high thermal inertia unit based on spatial and stratigraphic relationships interpreted from both crater exposed and surface exposures the olivine rich unit was determined to have formed subsequent to the isidis basin forming event morphology occurrence and linear deconvolution of this unit suggests that it may represent noachian age to hesperian age picritic lavas that erupted as early stage lavas originating from sources such as syrtis major beneath isidis and as far as tyrrhena patera these lavas are suggested to have partially filled the basin based on crater exposed occurrences and were subsequently buried by further evolved olivine poor lavas later these lavas were exposed by erosion and subsequent impacts into the basin crater scaling suggests significant infilling of the basin by these lavas that may account for craters with very high d d ratios and the well developed positive mass anomaly associated with the basin this study also demonstrates that the remote sensing of impact craters in conjunction with surface mapping is a potent mapping tool for understanding stratigraphy and the petrogenesis of the martian crust

sequestration of 12c in the deep ocean during the early wenlock silurian positive carbon isotope excursion abstract marine carbonates from three biostratigraphically well dated silurian sections from the mid continent of north america were analyzed for carbon isotopic 13 c carb study the early wenlock positive carbon isotope excursion ireviken excursion is recorded in three sections from tennessee iowa and ohio with values of 4 5 this global shift in the carbon isotopic ratio of marine waters began during a protracted extinction event ireviken event that spanned the llandovery wenlock boundary although several oceanographic models have been developed to account for the coincident changes in lithology biology and carbon isotope stratigraphy during this interval of the silurian their proposed causal connections among glaciation oceanography primary productivity and the global carbon cycle remain a matter of debate this investigation provides carbon isotopic and lithologic evidence that the ireviken excursion occurred during a time of relatively oligotrophic conditions a third order transgressive regressive cycle and increased carbonate production throughout the mid continent of north america the ireviken excursion is recorded in relatively pure carbonate sequences and is immediately preceded by a significant unconformity in each of the measured sections it is proposed that the ireviken excursion was the product of sequestration and burial of 12 c in deep water as a result of altered deep ocean circulation

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

uranium isotopes distinguish two geochemically distinct stages during the later cambrian spice event a b s t r a c t anoxic marine zones were common in early paleozoic oceans 542 400 ma and present a potential link to atmospheric po2 via feedbacks linking global marine phosphorous recycling primary production and organic carbon burial uranium u isotopes in carbonate rocks track the extent of ocean anoxia whereas carbon c and sulfur s isotopes track the burial of organic carbon and pyrite sulfur primary long term sources of atmospheric oxygen in combination these proxies therefore reveal the comparative dynamics of ocean anoxia and oxygen liberation to the atmosphere over million year time scales here we report high precision uranium isotopic data in marine carbonates deposited during the late cambrian spice event at ca 499 ma documenting a well defined 0 18 negative 238 u excursion that occurs at the onset of the spice event s positive 13

global enhancement of ocean anoxia during oceanic anoxic event 2 a quantitative approach using u isotopes during the mesozoic greenhouse world the oceans underwent several oceanic anoxic events oaes characterized by intervals during which organic rich black shales were deposited indicating strong oxygen depletion in the marine realm the cenomanian turonian oae2 ca 93 ma represents one of the most prominent events of the cretaceous with significant perturbations of the global carbon cycle although oae2 likely reached a global scale the spatial extent of seawater anoxia during this oae is poorly constrained here we demonstrate that variations in the 238u 235u isotope ratio 238u a newly developed paleoredox proxy can be used to quantify the extent of marine anoxia for black shales from the mid cretaceous oae2 we find a systematic shift toward lighter 238u and lower u concentrations as compared to modern equivalent organic rich sediments from the black sea this shift translates to a global increase of oceanic anoxia during oae2 by at least a factor of three as compared to the present day or to periods before and after oae2 the constant offset in u concentrations and isotope compositions of black shales throughout oae2 compared to modern black sea sediments indicates an enhancement of oceanic anoxic conditions already prior to the onset of oae2

marine anoxia and delayed earth system recovery after the end permian extinction delayed earth system recovery following the end permian mass extinction is often attributed to severe ocean anoxia however the extent and duration of early triassic anoxia remains poorly constrained here we use paired records of uranium concentrations u and 238u 235u isotopic compositions 238u of upper permian upper triassic marine limestones from china and turkey to quantify variations in global seafloor redox conditions we observe abrupt decreases in u and 238u across the end permian extinction horizon from 3 ppm and 0 15 to 0 3 ppm and 0 77 followed by a gradual return to preextinction values over the subsequent 5 million years these trends imply a factor of 100 increase in the extent of seafloor anoxia and suggest the presence of a shallow oxygen minimum zone omz that inhibited the recovery of benthic animal diversity and marine ecosystem function we hypothesize that in the early triassic oceans characterized by prolonged shallow anoxia that may have impinged onto continental shelves global biogeochemical cycles and marine ecosystem structure became more sensitive to variation in the position of the omz under this hypothesis the middle triassic decline in bottom water anoxia stabilization of biogeochemical cycles and diversification of marine animals together reflect the development of a deeper and less extensive omz which regulated earth system recovery following the end permian catastrophe

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

epipelagic chitinozoan biotopes map a steep latitudinal temperature gradient for earliest late ordovician seas implications for a cooling late ordovician climate abstract the early mid ordovician has long been considered a super greenhouse world based largely on high relative global sea levels and light stable oxygen isotope data from bulk carbonates an alternative and largely untested hypothesis has suggested that at least in equatorial palaeolatitudes there was a steady cooling trend through the early ordovician reaching the range of modern equatorial sea surface temperatures by the mid ordovician this hypothesis though controversial is supported by palaeobiogeographical studies of the early late ordovician sandbian zooplankton graptolite biotopes these are comparable to the modern planktonic foraminifera provinces and suggest a latitudinal temperature gradient that is similar to the modern one chitinozoans are also an important group of palaeozoic marine microfossils but with a poorly known biological affinity here we present an analysis of chitinozoan species palaeobiogeography for the early late ordovician sandbian c 460 ma which confirms that these microfossils and likely their parent organisms were epipelagic unlike the graptolites chitinozoans had their highest diversity and abundance south of 35 s during this time which strongly suggests adaptation to temperate to cold waters the distribution of chitinozoan biotopes during the gracilis time slice allows us to identify the austral subtropical subpolar and polar belts key oceanographic boundaries are identified as the subtropical subpolar transition at 35 s and tentatively the polar front at 55 70 s again these are in a similar position to those in the modern southern ocean chitinozoan biotopes also map a steep latitudinal faunal gradient that is comparable to that of the graptolites and that of modern plankton this likely indicates a steep latitudinal temperature gradient for early late ordovician seas and questions the notion that this interval of earth history had a greenhouse climate

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

rapid expansion of oceanic anoxia immediately before the end permian mass extinction tive of an expansion of oceanic anoxia and higher th u ratios from 0 06 to 0 42 indicative of drawdown of u concentrations in seawater using a mass balance model we estimate that this isotopic shift represents a sixfold increase in the flux of u to anoxic facies implying a corresponding increase in the extent of oceanic anoxia the intensification of oceanic anoxia coincided with or slightly preceded the eh and persisted for an interval of at least 40 000 to 50 000 y following the eh these findings challenge previous hypotheses of an extended period of whole ocean anoxia prior to the end permian extinction

middle eocene paleomagnetic data from the eastern sakarya zone and the central pontides implications for the tectonic evolution of north central anatolia 1 a paleomagnetic investigation has been carried out on middle eocene volcanic rocks at 23 sites in the eastern sakarya zone and the central pontides of north central anatolia in order to better understand the regional tectonic evolution of the main zones of northeastern anatolia in the middle eocene the results constrain the tectonic evolution of the area when evaluated in conjunction with earlier published paleomagnetic directions from 92 localities small counterclockwise rotations in the range of r r 1 3 1 4 to r r 17 2 16 8 with respect to eurasia are observed in the central pontides on the northern side of the north anatolian fault naf clockwise rotations ranging between r r 8 5 15 1 and r r 29 7 12 0 are observed between the naf and the sungurlu fault sf in the eastern sakarya zone counterclockwise rotations of r r 18 9 12 4 to r r 42 2 6 9 are observed to the south of the sf our findings combined with previous data support the indentation model of kaymakci et al 2000 2003a 2003b which postulates the collision and northeast directed indentation of the kirsehir block into the sakarya zone this model which was developed to explain the evolution of the cankiri basin can also explain the variable magnitudes of paleomagnetically determined rotations thrust directions and the curvature of the north anatolian ophiolite belt

the proposed cambrian ordovician global boundary stratotype and point gssp in western newfoundland canada sections exposing the cambrian ordovician boundary interval at broom point in western newfoundland have been proposed earlier for a global systemic boundary stratotype these lie within the cow head group a late middle cambrian to early middle ordovician allochthonous unit of limestone shale and conglomerate deposited at the toe of the ancient continental slope and on the adjacent continental rise several recent studies have further investigated the stratigraphy sedimentology and palaeontology of the cow head group and others are under way on magnetostratigraphy and chemostratigraphy these aspects are reviewed for six key boundary sections representing proximal to distal facies cow head ledge broom point south broom point north st pauls inlet quarry martin point and green point in particular new data are presented from 260 conodont samples that yielded 15500 conodonts this intense sampling has allowed the discrimination of minor hiatuses in the proximal to intermediate facies where conglomerates have eroded and cannibalized underlying strata new conodont data from broom point north have lowered the base of the c lindstromi zone into unit 74 conglomerates thereby making this section unsuitable as a boundary stratotype new collections from green point have yielded abundant conodonts and over 9400 conodonts have been recovered from 77 samples the conodont graptolite and trilobite biostratigraphy through the boundary interval is documented allowing accurate correlation between sections and more precisely revealing small hiatuses in the proximal and intermediate facies the sequence of conodont zones is eoconodontus notchpeakensis cordylodus proavus c caboti c intermedius c lindstromi and c angulatus these can be correlated with trilobite zones established from both in situ and clast faunas from the proximal to intermediate facies and with graptolite assemblages of cooper 1979 especially in the intermediate to distal facies three new species of cordylodus are described c andresi c hastatus and c tortus and the full apparatus of iapetognathus preaengensis is illustrated the criteria for selecting a global boundary stratotype and point gssp are reviewed in terms of the cow head sections the green point section is shown to meet and largely surpass the prerequisites required of a stratotype the green point section is proposed to be the global boundary stratotype with the base of the ordovician system defined at the base of unit 23 which is the base of the broom point member green point formation at a level coincident with the base of the cordylodus lindstromi zone in addition to an abundant and superbly preserved conodont fauna this section preserves the best sequence of earliest planktic graptolites through a 40 m interval the first nematophorous graptolites of assemblage 1 occur in unit 25 6 9 m above the base of the c lindstromi zone this level can be readily correlated into the proximal facies where both deep and shallow water trilobites in situ and in clasts respectively show the base of the c lindstromi zone to lie within the symphysurina brevispicata trilobite subzone

climate change and the selective signature of the late ordovician mass extinction selectivity patterns provide insights into the causes of ancient extinction events the late ordovician mass extinction was related to gondwanan glaciation however it is still unclear whether elevated extinction rates were attributable to record failure habitat loss or climatic cooling we examined middle ordovician early silurian north american fossil occurrences within a spatiotemporally explicit stratigraphic framework that allowed us to quantify rock record effects on a per taxon basis and assay the interplay of macrostratigraphic and macroecological variables in determining extinction risk genera that had large proportions of their observed geographic ranges affected by stratigraphic truncation or environmental shifts at the end of the katian stage were particularly hard hit the duration of the subsequent sampling gaps had little effect on extinction risk suggesting that this extinction pulse cannot be entirely attributed to rock record failure rather it was caused in part by habitat loss extinction risk at this time was also strongly influenced by the maximum paleolatitude at which a genus had previously been sampled a macroecological trait linked to thermal tolerance a model trained on the relationship between 16 explanatory variables and extinction patterns during the early katian interval substantially underestimates the extinction of exclusively tropical taxa during the late katian interval these results indicate that glacioeustatic sea level fall and tropical ocean cooling played important roles in the first pulse of the late ordovician mass extinction in laurentia

large climate driven changes of oceanic oxygen concentrations during the last deglaciation during the last glacial termination the solubility of gases in the ocean decreased as ocean temperatures rose however marine sediments have not unanimously recorded ocean deoxygenation throughout this time some records show increasing oxygenation since the last glacial maximum particularly in the deep sea while many document abrupt oxygenation changes often associated with apparent changes in the formation rate of north atlantic deep water here we present a global compilation of marine sediment proxy records that reveals remarkable coherency between regional oxygenation changes throughout deglaciation the upper ocean generally became less oxygenated but this general trend included pauses and even reversals reflecting changes in nutrient supply respiration rates and ventilation the most pronounced deoxygenation episode in the upper ocean occurred midway through the deglaciation associated with a reinvigoration of north atlantic deep water formation at this time the upper indo pacific ocean was less oxygenated than today meanwhile the bulk of the deep ocean became more oxygenated over the deglaciation reflecting a transfer of respired carbon to the atmosphere the observed divergence from a simple solubility control emphasizes the degree to which oxygen consumption patterns can be altered by changes in ocean circulation and marine ecosystems

terminal ordovician carbon isotope stratigraphy and glacioeustatic sea level change across anticosti island quebec canada globally documented carbon isotope excursions provide time varying signals that can be used for high resolution stratigraphic correlation we report detailed inorganic and organic carbon isotope curves from carbonate rocks of the ellis bay and becscie formations spanning the ordovician silurian boundary on anticosti island quebec canada strata of the anticosti basin record the development of a storm dominated tropical carbonate ramp these strata host the well known hirnantian positive carbon isotope excursion which attains maximum values of 4 5 in carbonate carbon of the laframboise member or the fox point member of the becscie formation the excursion also occurs in organic carbon and 13 c carb and 13 c org values covary such that no reproducible 13 c 13 c carb 13 c org excursion is observed the most complete stratigraphic section at laframboise point in the west shows the characteristic shape of the hirnantian stage excursion at the global stratotype section and point gssp for the hirnantian stage in china and the silurian system in scotland we therefore suggest that the entire hirnantian stage on anticosti island is confined to the laframboise and lower fox point members by documenting discontinuities in the architecture of the carbon isotope curve at multiple stratigraphic sections spanning a proximal to distal transect across the sedimentary basin we are able to reconstruct glacioeustatic sea level fluctuations corresponding to maximum glacial conditions associated with the end ordovician ice age the combined litho and chemostratigraphic approach provides evidence for the diachroneity of the oncolite bed and becscie limestones the former transgresses from west to east and the latter progrades from east to west the sea level curve consistent with our sequence stratigraphic model indicates that glacioeustatic sea level changes and the positive carbon isotope excursion were not perfectly coupled although the start of the isotope excursion and the initial sea level drawdown were coincident the peak of the isotope excursion did not occur until after sea level had begun to rise carbon isotope values did not return to baseline until well after the anticosti ramp was reflooded the sea level 13ccarb relationship proposed here is consistent with the weathering hypothesis for the origin of the hirnantian 13 c carb excursion

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

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

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