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

late ordovician jaw bearing polychaetes from anticosti island eastern canada and their biogeographic significance the upper ordovician to lower silurian shallow marine succession of anticosti island eastern canada provides one of the most complete records across the hirnantian in the world this study reports a diverse assemblage of scolecodonts polychaete jaws from the upper katian and hirnantian vaureal ellis bay and basal becscie formations of western anticosti the collection of 10 samples includes ca 30 species representing 10 families the fauna is dominated by polychaetaspids mochtyellids paulinitids and polychaeturids the family xanioprionidae and genera pistoprion tetraprion and rakvereprion are documented for the first time from the ordovician of laurentia the anticosti polychaete fauna shows great similarity to the contemporaneous faunas of baltoscandia this is evidenced by a high relative abundance of mochtyellids and polychaeturids and a number of common species thus suggesting that the closing iapetus ocean at that time did not constitute a barrier for the dispersal of jaw bearing polych

gauging the impact of glacioeustasy on a mid latitude early silurian basin margin mid wales uk the early silurian llandovery gondwanan south polar ice sheet experienced episodes of ice retreat and re advance marine base level curves constructed for the interval are widely assumed to provide a record of the associated glacioeustasy in revealing a series of progradational sequences progrades bounded by flooding surfaces recent work on the type llandovery succession in mid wales uk has provided an opportunity to test this hypothesis the grouping of these progrades into three composite sequences underpins the construction of both low order small amplitude high frequency and high order large amplitude low frequency base level movement curves revised biostratigraphical datasets for the type succession permit the accurate dating of base level events the composite sequences record progradational acmes in the acinaces lower convolutus and upper sedgwickii halli graptolite biozones a series of transgressions that postdate the hirnantian glacial maximum culminated in an upper persculptus biozone high stand maximum flooding events also occurred during the revolutus and lower sedgwickii biozones and the base of the early telychian guerichi biozone also marked the onset of a pronounced deepening a review of 62 published datasets including global and other regional base level curves records of glacial activity isotope data patterns of facies and faunal flux and putative climate models permits an evaluation of the origins of these local base level events the concept of a eustasy index is introduced and shows that the impacts of global sea level movements can only be demonstrated within narrow eustatic windows coincident with times of ice sheet collapse at other times the geometry of llandovery area progrades reflects their accumulation across a faulted basin margin where during periods of slow ice sheet advance epeirogenic processes outstripped sea level movements as the dominant forcing factors increased levels of telychian subsidence at first enhanced and then overwhelmed the influence of glacioeustasy as part of the region s response to the north european scandian deformation

rethinking the ancient sulfur cycle the sulfur biogeochemical cycle integrates the metabolic activity of multiple microbial pathways e g sulfate reduction disproportionation and sulfide oxidation along with abiotic reactions and geological processes that cycle sulfur through various reservoirs the sulfur cycle impacts the global carbon cycle and climate primarily through the remineralization of organic carbon over geological timescales cycling of sulfur is closely tied to the redox state of earth s exosphere through the burial of oxidized sulfate and reduced sulfide sulfur species in marine sediments biological sulfur cycling is associated with isotopic fractionations that can be used to trace the fluxes through various metabolic pathways the resulting isotopic data provide insights into sulfur cycling in both modern and ancient environments via isotopic signatures in sedimentary sulfate and sulfide phases here we review the deep time 34s record of marine sulfates and sulfides in light of recent advances in understanding h

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

relations between relative changes in sea level and climate shifts pennsylvanian permian mixed carbonate siliciclastic strata western united states study of pennsylvanian virgilian permian wolfcampian strata of the western united states parts of idaho utah new mexico and kansas reveals complex relations between relative changes in sea level and shifts in climate relative falls in sea level are indicated by subaerial exposure of subtidal sediments and fluvial incision through subtidal sediments changes in the character of pedogenesis and sediment transport deposition and supply and the variable distribution of climatically sensitive lithofacies within cyclothems are interpreted to reflect shifts in climate during deposition of each cyclothem the majority of stratal relationships documented herein from a number of long term climatic sedimentologic and shelf settings are consistent with a hypothesis that relative highs in sea level are accompanied by more seasonal conditions whereas relative lows are characteristically more arid uncommon deviations are present however and may be due to 1 shoaling caused by sedimentation or tectonic uplift independent of a eustatic climatic signal 2 complex sedimentologic or geomorphic responses to climate change 3 leads lags or thresholds in either glacial advance and retreat or the climatic response to such changes this study shows that coupled climatic shifts and changes in sea level exert a pronounced but varied influence on stratigraphic architecture in depositional settings across late paleozoic ramps the proposed climatically enhanced cyclic and reciprocal sedimentation model predicts that during a single eustatic fall updip areas have features indicating a more seasonal or humid climate that are overlain or overprinted by characteristics suggesting more arid climate in downdip areas that were not subaerially exposed until the late stages of a eustatic fall only more arid climatic features are represented in the terrestrial or subaerial exposure features

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

upper cretaceous sequences and sea level history new jersey coastal plain we developed a late cretaceous sea level estimate from upper cretaceous sequences at bass river and ancora new jersey odp ocean drilling program leg 174ax we dated 11 14 sequences by integrating sr isotope and biostratigraphy age resolution 0 5 m y and then estimated paleoenvironmental changes within the sequences from lithofacies and biofacies analyses sequences generally shallow up section from middle neritic to inner neritic paleodepths as shown by the transition from thin basal glauconite shelf sands transgressive systems tracts tst to medial prodelta silty clays highstand systems tracts hst and finally to upper delta front quartz sands hst sea level estimates obtained by backstripping accounting for paleodepth variations sediment loading compaction and basin subsidence indicate that large 25 m and rapid 1 m y sea level variations occurred during the late cretaceous greenhouse world the fact that the timing of upper cretaceous sequence boundaries in new jersey is similar to the sea level lowering records of exxon production research company epr northwest european sections and russian platform outcrops points to a global cause because backstripping seismicity seismic stratigraphic data and sediment distribution patterns all indicate minimal tectonic effects on the new jersey coastal plain we interpret that we have isolated a eustatic signature the only known mechanism that can explain such global changes glacio eustasy is consistent with foraminiferal 18 o data either continental ice sheets paced sea level changes during the late cretaceous or our understanding of causal mechanisms for global sea level change is fundamentally flawed comparison of our eustatic history with published ice sheet models and milankovitch predictions suggests that small 5 10 10 6 km 3 ephemeral and areally restricted antarctic ice sheets paced the late cretaceous global sea level change new jersey and russian eustatic estimates are typically one half of the epr amplitudes though this difference varies through time yielding markedly different eustatic curves we conclude that new jersey provides the best available estimate for late cretaceous sea level variations

exceptionally preserved soft bodied assemblage in ordovician carbonates of anticosti island eastern canada abstract a diverse benthic fauna containing exceptionally preserved soft bodied organisms is described from the new vaureal konservat lagerstatte from the upper ordovician vaureal formation katian of anticosti island canada these fossils occur as pyritic or goethitic if weathered calcitic aggregates or sediment filled voids on micritic bedding planes of a marlstone limestone alternation originated on a shallow tropical carbonate ramp many organisms are preserved in association with their traces e g burrows trails whose changing shapes indicate increasing cohesiveness of the substrate rapid burial obrution under dysoxic bottom conditions must have favoured exceptional preservation although anatomical details of the organisms are often lacking due to recrystallization members of different groups of organisms could be interpreted based on their shape and other characteristic features including those of which represent their oldest fossil record i e acoelomorpha turbellaria nemertea nematoda beside polychaeta sipuncula ostracoda and other arthropoda in parallel with molecular data which recently have changed the phylogenetic status of some clades this soft bodied fauna and their traces not only record their appearance but add to the understanding of their body plan and behaviour the new konservat lagerstatte provides important information about a diverse benthic community in the late ordovician after the great ordovician biodiversification event gobe and represents a calibration point for comparison across the late ordovician mass extinction moreover comparable fossil lagerstatten are probably more common in shallow marine carbonates than currently known but might have been simply overlooked

glacially megalineated limestone terrain of anticosti island gulf of st lawrence canada onset zone of the laurentian channel ice stream abstract anticosti is a large elongate island 240 km long 60 km wide in eastern canada within the northern part of a deep water trough gulf of st lawrence that terminates at the atlantic continental shelf edge the island s pleistocene glaciological significance is that its long axis lay transverse to ice from the quebec and labrador sectors of the laurentide ice sheet moving south from the relatively high standing canadian shield recent glaciological reconstructions place a fast flowing ice stream along the axis of the gulf of st lawrence but supporting geologic evidence in terms of recognizing its hard bedded onset zone and downstream streamlined soft bed is limited anticosti island consists of gently southward dipping limestone plains composed of ordovician and silurian limestones vaureal becscie and jupiter formations with north facing escarpments transverse to regional ice flow glacial deposits are largely absent and limestone plains in the higher central plateau of the island retain a relict apparently preglacial drainage system consisting of deeply incised dendritic bedrock valleys in contrast the bedrock geomorphology of the lower lying western and eastern limestone plains of the island is strikingly different having been extensively modified by glacial erosion escarpments are glacially megalineated with a distinct zig zag planform reflecting northward projecting bullet shaped noses identified as rock drumlins up to 2 km wide at their base and 4 km in length with rare megagrooved upper surfaces drumlins are separated by southward closing funnel shaped through valleys where former dendritic valleys have been extensively altered by the streaming of basal ice through gaps in the escarpments glacially megalineated bedrock terrain such as on the western and eastern flanks of anticosti island is elsewhere associated with the hard bedded onset zones of fast flowing ice streams and provides important ground truth for the postulated laurentian channel ice stream lcis within the gulf of st lawrence sector of the laurentide ice sheet

magnetic susceptibility evolution and sedimentary environments on carbonate platform sediments and atolls comparison of the frasnian from belgium and alberta canada article i nfo magnetic susceptibility ms measurements on carbonate rocks are considered as a proxy for impurities delivered to the carbonate environments in the absence of strong climatic or tectonic variations bulk ms values have been linked to sea level variations because sea level fall increases clastic supply and therefore increases in magnetic mineral deposition in this paper we explore the relationship between the average magnitude of bulk ms with shallowing up sequences and facies evolution in different devonian carbonate complexes similarities and differences between these parameters have been scrutinized in carbonate attached platform and detached platforms mounds and or atolls from belgium and canada in the carbonate attached platforms from belgium and canada the ms patterns are directly related to depositional environment mean ms values increase from the most distal towards the most proximal facies and towards the top of the majority of fourth order shallowing up sequences these trends are in agreement with theoretical background ms increases with regression

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

calibrating the late ordovician glaciation and mass extinction by the eccentricity cycles of earth s orbit a process based sedimentological analysis of upper ordovician glacial marine rocks in africa suggests that full glaciation of the continental shelf started in the late extraordinarius zone of the hirnantian two cycles of ice sheet growth are represented during full glaciation initial terrestrial ice sheet growth in the early extraordinarius zone influenced the first event of the late ordovician mass extinction retreat of the ice sheet from the shelf ended by the persculptus zone when the second event of the late ordovician mass extinction began eccentricity controlled ice sheet growth is assumed periodicity 0 1 m y hence two cycles of full glaciation lasted 0 2 m y the duration of the extraordinarius zone is estimated as 0 5 m y therefore the minimum duration of the first extinction event was 0 3 m y

how does sea level correlate with sea water chemistry a progress report from the ordovician and silurian following the cambrian explosion and the appearance in the fossil record of most animal phyla associated with a range of new body plans the ordovician and silurian periods witnessed three subsequent major biotic events the great ordovician biodiversification event the end ordovician extinction the first animal extinction and second largest of the five mass extinctions of the phanerozoic and the early silurian postextinction recovery there are currently no simple explanations for these three major events combined extrinsic geological and intrinsic biological factors probably drove the biodiversifications and radiations and the appearance and disappearance of marine habitats have to be analysed in the frame of changing palaeogeography palaeoclimate and sea water chemistry the present paper reviews the relationships of the three biotic events to chemical and physical processes occurring in the ocean and atmosphere during the ordovician and silurian including sea level changes geochemical proxies 13c 18o 87sr 86sr of the ocean waters and the evolution of the atmosphere oxygen and carbon dioxide content

late ordovician sand wave complexes on anticosti island quebec a marine tidal embayment laterally discontinuous mixed carbonate siliciclastic sandstones in the upper vaureal and lower ellis bay formations of anticosti island were deposited on an equatorial carbonate ramp with a slope of less than 1 the 10 18 m thick sandstones are interpreted as subaqueous sand wave complexes analogous to detached parts of modern shoreface connected sand ridges these record storm enhanced tidal modification of a northerly derived shoal retreat massif that may have formed in response to recovery from global sea level lowstands in the late ordovician ashgill late rawtheyan hirnantian the sand wave complexes formed within a tidal embayment that was confined by the precambrian shield to the north and northwest by rising tectonic highlands of the humber zone in newfoundland to the east and by active tectonic highlands in the quebec appalachians gaspesie to the south paleocurrent distributions parallel to the western margins of the strait of belle isle suggest that the north end of the embayment w

subaerial speleothems and deep karst in central sweden linked to hirnantian glaciations the limestones of the upper katian boda mud mounds ordovician of the siljan district in central sweden are deeply fractured the fissures were partly synsedimentary and are often lined with stromatolite like crusts these crusts thus far are the only known subaerial ordovician speleothems they reach depths of up to 30 m below the former mound top macroscopically the crusts form decimetre sized cone shaped domal aggregates stalactites and stalagmites microfabric and morphology identify them as microbially mediated speleothems in a dark environment combined sr and c isotope values indicate a formation of the speleothems from meteoric waters without influence of a significant soil horizon for the first time the age of the speleothems can be precisely constrained by 13 c whole rock and brachiopod shell isotope data to the mid hirnantian repeated and or prolonged subaerial exposure of the boda mud mounds during the hirnantian is evident from karst surfaces and early cements in the mound capping carbonates the speleothems and the karst surfaces record an estimated sea level fall in the range of 80 130 m within the time window of the hirnantian isotopic carbon excursion this massive regression coincides with maximum ice sheet extent inferred from sections in west gondwana supplementary material 87 sr 86 sr isotope ratio of selected brachiopod shells and results of energy dispersive x ray spectroscopy are available from http www geolsoc org uk sup18809

stress triggering and stress memory observed from acoustic emission records in a salt mine summary some of the most interesting questions in geosciences are whether results from laboratory experiments can be applied to processes in the earth crust and whether in situ studies with high spatio temporal resolution can bridge the gap between laboratory work and seismology in this study acoustic emission ae activity caused by stress changes due to the backfilling of a cavity in an abandoned salt mine is studied to answer questions regarding 1 the dependence of ae event rates event distribution and b value on the stress state 2 the stress memory effect of rock kaiser effect 3 the possibility to detect significant changes in the system like the initiation of macrocracks and 4 the possibility to estimate future activity from previous ae records the large number of events studied 3 10 5 allows a spatial resolution of the order of 1 m and a temporal one on the order of 1 hr stress changes are created due to the thermal expansion and contraction of the rock mass in response to the temperature changes caused by the backfilling a roughly 20 50 50 m section of the mining complex just above the backfilled cavity is well covered by a network of 24 piezo electric receivers and poses an optimal volume for the study results of a 2 d finite element thermoelastic stress model are in agreement with the spatio temporal ae event distribution in addition to the initial upward migration of the ae event front which correlates with the calculated stress field the rock salt exhibits a pronounced kaiser effect for the first few thermal loading cycles throughout the whole study region the deviation from the kaiser effect during later loading cycles seems to be caused by the initiation of a planar macroscopic crack which is subsequently reactivated ae activity tends to concentrate along this macrocrack calculated b values decrease before and increase after the supposed initiation of the macrocrack supporting this explanation in intact rock volumes not subjected to macrocracking a linear relation between the maximum event rate and the calculated absolute coulomb stress increase is observed this indicates that future maximum ae event rates can be estimated from expected loading ae activity during stress loading cycles is most prominent in regions with coulomb stress maxima indicating possible shear cracking that has the potential to create macrocracks strong bursts of ae activity observed during thermal unloading phases are concentrated in regions for which the minimum principal stress becomes tensile these regions exhibit significantly higher b values than those active during thermal loading we interpret these weak events with tensile microcracking

lipid biomarkers record fundamental changes in the microbial community structure of tropical seas during the late ordovician hirnantian glaciation the late ordovician mass extinction was linked to climate cooling and glaciation of gondwana during the terminal ordovician hirnantian age 444 7 443 4 ma extinction patterns have been well described for many marine taxa but much less is known about marine microbial communities through this interval to elucidate the structure of microbial communities in tropical marine basins through the late ordovician we analyzed lipid biomarkers in thermally well preserved strata from the taconic foreland anticosti island canada the cincinnati arch midwestern united states and the western continental margin vinini formation nevada united states despite clear oceanographic differences lipid biomarker profiles show similarities between these three localities major shifts in biomarker distributions of anticosti island and the vinini formation mainly hopane sterane ratios record changes in the balance of bacterial versus algal primary production bacterial contributions to sedimentary organic matter were highest during warm intervals both before and after hirnantian cooling in particular 3 methylhopanes likely sourced from aerobic methanotrophic bacteria occur in high relative abundance many times the phanerozoic average across laurentia throughout most of the interval studied 3 methylhopane abundances also reveal an overall positive relationship with paleotemperature proxies implying increased methane cycling during warm intervals these results suggest that enhanced methane cycling could have provided an important positive feedback on climate during extended intervals of early paleozoic time

sequence stratigraphy of the red river formation williston basin usa stratigraphic signature of the ordovician katian greenhouse to icehouse transition abstract the upper katian upper ordovician red river formation is a carbonate evaporite supersequence that formed in the epicratonic williston basin of western north america this study focuses on the upper part of the formation in the southern us part of the basin the studied succession is made up of three transgressive regressive sequences rr1 rr2 and rr3 each 0 5 1 m y in duration and composed of relatively few meter scale high frequency cycles parasequences sequence boundaries are characterized by intraclastic breccia laminated dolomite successions that have been truncated and capped by evaporite deposits subaqueous anhydrites within each sequence represent evaporite lowstand systems tracts lsts that formed during relative sea level lows that lead to the precipitation of gypsum later recrystallized into anhydrite in a quiet salina type environment transgressive tst and early highstand systems tracts hst are composed of subtidal facies of predominantly skeletal mudstone to wackestone packstone with abundant burrow mottling late hsts are predominantly composed of peritidal porous laminated dolomite the study illustrates how the sequence stratigraphic framework of carbonate evaporite deposits from the williston basin relates to the late ordovician glacio eustasy transitional climates and low basin subsidence in an arid setting

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

spatial variation in late ordovician glacioeustatic sea level change abstract mass extinction of late ordovician marine fauna closely coincided with southern hemisphere glaciation the sequence stratigraphic architecture of shallow marine deposits informs estimates of glacioeustatic sea level change at sites both proximal and distal to the reconstructed ordovician ice sheet s and contemporaneous changes in ice volume a recent correlation framework for the stratigraphic architectures of one near and one far field late ordovician margin concluded that the late ordovician glaciation encompassed multiple long term cycles of ice volume growth and retreat with superimposed higher frequency cycles here we posit that similar to cenozoic glacial cycles glacial isostatic adjustment can preclude synchronous and similar magnitude or directional changes in late ordovician sea level between ice proximal and ice distal locations and hence distort a globally correlative sequence stratigraphy we explored whether long duration i e million year late ordovician glacial cycles should produce a globally coherent eustatic record of sea level change between ice proximal and ice distal margins using a gravitationally self consistent theory that accounts for the deformational gravitational and rotational perturbations to sea level on a viscoelastic earth model we adopted a late ordovician paleogeography and a synthetic continental ice sheet distribution and volume informed by the areal extent of glaciogenic deposits and geochemical records respectively we demonstrate that modeled million year late ordovician glacial cycles produce sea level histories on near and far field margins that differ from eustasy and from one another due primarily to elastic flexure and associated gravitational effects while predicted far field sea level histories faithfully preserve the temporal structure of modeled glacioeustasy their amplitude may differ from eustasy by as much as 30 40 the impact of glacial isostatic adjustment is largest at the margins of glaciated continents and these effects can be of the same order of magnitude as the eustatic and even induce a local sea level rise during an episode of ice growth and eustatic sea level fall and vice versa in this regard stratal surfaces of maximum regression and flooding expressed at near field margins need not reflect global eustatic trends in ice sheet growth and decay respectively and thus may not provide chronostratigraphic horizons for correlation with far field sequence stratigraphic architectures

sponge dominated offshore benthic ecosystems across south china in the aftermath of the end ordovician mass extinction abstract offshore benthic communities were sparse after the end ordovician mass extinction the recent discovery of diverse abundant sponges in south eastern china anhui and zhejiang provinces that flourished during the hirnantian late ordovician post extinction interval raises questions over the extent and ecological significance of the sponge community at that time this paper dramatically expands the scale of the hirnantian sponge fauna within south china with seven new occurrences across 2000 km these occurrences cross the ordovician silurian boundary interval in the marginal and submerged platform areas of the yangtze sea in jiangsu hubei hunan chongqing and sichuan provinces in condensed sections platform margin facies the occurrences are normally represented by extensive spiculite layers but several sites yield entire exceptionally preserved body fossils at ganggangshan near nanjing jiangsu province such preservation is abundant the new occurrences are combined with palaeogeographic and tectonic reconstructions to produce a model for the distribution and preservation of the sponge fauna we argue that the exceptional preservation was due largely to nepheloid layers that were generated during rapid post glacial transgression that flooded the weathered land surface of the uplifted cathaysia block nepheloid layer collapse led to rapid deposition of suspended sediment within the deeper parts of the intra plate restricted basin and on adjacent platform margins and slopes the suspended sediment influx also introduced abundant nutrients to the water column as indicated by a significant 13c excursion during the sponge bearing mudstone interval at ganggangshan the combined sponge assemblage was dominant over large areas of the hirnantian offshore sea floor across south china we predict that similar occurrences with exceptional preservation will also be found in other continental blocks abundant articulated sponges may be encountered particularly in restricted basin successions containing rapidly deposited graptolitic mudstones near the ordovician silurian boundary

revisiting the appalachian structural front and offshore anticosti basin northern gulf of st lawrence canada by integrating old and new geophysical datasets abstract the anticosti basin is a large predominantly marine sedimentary basin centered within the northern gulf of st lawrence poor quality vintage seismic reflection data collected more than 25 years ago in the offshore part of the basin failed to depict its geometry however recently acquired multibeam bathymetry and magnetic data can be used in combination with high resolution seismic lines to map the paleozoic succession within the honguedo strait the upper part of the offshore sedimentary succession consists of approximately 1140 m of sediments younger than those exposed on anticosti island the offshore part of the basin includes a 40 km wide fold and fault belt sub parallel to the coastline of the gaspe peninsula the structural style is characterized by broad open synclines narrow anticlines and nw striking faults with dextral strike separations which bears similarities to that of the gaspe belt that formed mainly during the middle devonian acadian orogeny this study indicates that the appalachian deformation front is located to the north of its classically inferred location moreover it suggests that the curvature of the early paleozoic assemblages in the gulf of st lawrence is partly a late devonian feature an improved understanding of the offshore architecture of the basin may help to develop new paleogeographic reconstructions and hydrocarbon exploration models in this under explored frontier basin

a late ordovician age for the whirlpool and power glen formations new york a restudy of the palynology of the whirlpool formation and power glen formation in new york usa yielded a diverse fossil assemblage with cryptospores glomalean fungi acritarchs chitinozoans scolecodonts and small carbonaceous fossils these new data and particularly the presence of the chitinozoan index fossil hercochitina crickmayi combined with emerging stable carbon isotope data suggest a late ordovician katian or hirnantian age for these formations which is older than their previously suggested silurian rhuddanian age

maximum extent of ice sheets in morocco during the late ordovician glaciation new field data demonstrate that during the late ordovician hirnantian glaciation an ice sheet expanding northwestwards over the anti atlas range reached into the southern meseta of northern morocco its growth to a glacial maximum position resulted in extensive subglacial erosion and deformation including the development of soft sediment striated surfaces and streamlined subglacial bedforms preserved between the high atlas of marrakech and rehamna these features imply that this ice mass extended 200 km further than previously thought and increase its size by at least ca 190 000 km2 comparable in area to the uk correlation between a measured section in the high atlas of marrakech and that of the southern meseta identifies sedimentary evolution within an ice contact system common to both these findings imply that the west african craton and northern morocco were in full glaciological communication during the latest ordovician palaeogeographic reconstruction shows that beyond the ice sheet south and southeastward palaeoslopes persisted on the shelf a palaeohigh beyond the main ice sheet was a major source for sand feeding delta systems that grew along the shelf as far as the shelf break this palaeohigh probably formed as a result of rift shoulder uplift and supported a satellite ice mass in the eastern meseta a thick 350 m underflow dominated deep marine fan was fed both from this shelf delta system and from glaciogenic debris derived from the main ice sheet the occurrence of this unexpected deep marine area in northern morocco implies that continued northward advance of the ice sheet was hampered by a dramatic break in bathymetry two depositional units are recognised across the meseta containing four distinct sedimentary cycles each recognised as a glacioeustatic response to the waxing and waning of ice masses elsewhere in west gondwana

conodonts and the position of the ordovician silurian boundary at the eastern end of anticosti island quebec canada conodonts have been recovered from a section spanning the ordovician silurian boundary at the eastern end of anticosti island they indicate that the systemic boundary lies slightly above or at the top of a pisolitic limestone bed this placement of the boundary compares closely with that based on a previous interpretation of brachiopod faunas from the same section and also with the position of the systemic boundary based on conodonts elsewhere on anticosti island the conodonts are the first to be reported from the eastern end of anticosti island

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

episodic sedimentation on an early silurian storm dominated carbonate ramp becscie and merrimack formations anticosti island canada the 150 160 m thick lowermost silurian rhuddanian becscie and merrimack formations of anticosti island canada represent continuous deposition on a shallow open marine carbonate ramp several rock types are identified a laminated and homogenous mudstone b laminated and homogenous packstone c argillaceous mudstone and packstone d calcareous shale e laminated calcisiltite f medium to fine grained grainstone and g bio intraclastic rudstone these rock types are arranged into five distinct lithofacies lf1 calcareous mudstone shale lf2 laminated homogenous mudstone lf3 calcareous grainstone shale lf4 laminated mudstone grainstone and lf5 laminated calcisiltite grainstone the sequence reflects deposition on a low energy muddy carbonate to argillaceous ramp subject to short lived episodic high energy storms these events produced fining upwards storm units 5 80 cm thick or tempestites consisting of a sharp scoured base overlain by intra bioclastic rudstone grading upwards into medium grained grainstone finely laminated calcisiltite and mudstone or shale these are interbedded with low energy fairweather mudstones and calcareous shales deposition progressed from a carbonate mud dominated ramp in the becscie formation to an argillaceous mud dominated ramp in the merrimack formation lateral tempestite proximality trends and lithofacies distribution indicate that the anticosti basin deepened to the south east into the iapetus ocean and shallowed towards a sw ne orientated shoreline to the north west vertical tempestite proximality trends and lithofacies changes identify third order eustatic sea level changes after an initial deepening at the base of the formation a shallowing deepening event dominated the sequence several higher order fluctuations defined by lithofacies and tempestite proximality trends are superimposed on these changes the fluctuations identified with the aid of tempestite proximality trends are of an order of magnitude higher than those identified by either lithofacies or palaeontological methods

climatic and oceanic controlled deposition of late ordovician early silurian black shales on the north yangtze platform south china abstract black shales deposited across the late ordovician and early silurian transition are considered to be the most important source rock and shale gas reservoirs in the yangtze region of south china however the origin of these sediments remains contentious in this study we investigated the systematic changes in total organic carbon toc organic carbon isotopes 13corg major elements and trace elements on the ordovician silurian boundary strata from qiaoting north yangtze sea to reconstruct the climatic conditions redox changes primary productivity and other factors there promoting the understanding for the formation mechanism of the organic rich black shales element compositions and their ratios suggest that the black shales studied here were dominantly sourced by the felsic igneous rock which is similar to granodiorite in composition paleoclimatic proxies cia ciw 13corg suggest that the global climate system experienced significant changes from warm humid to cold dry and then to warm humid climates during the ordovician silurian transition paleoredox indices s c u th ni co and v v ni ratios also demonstrate fluctuating redox variations from anoxic to oxygenated and then to anoxic states during this interval in the yangtze sea which describe an oxygenated guanyinqiao strata sandwiched between the stagnant and anoxic wufeng and longmaxi black shales paleoproductivity parameters toc babio contents and p al ratio suggest that the organic rich wufeng and longmaxi black shales were deposited with a high biological productivity while the organic lean guanyinqiao sediments were deposited with a low biological productivity these data demonstrate large climatic and oceanic fluctuations during the ordovician silurian transition providing essential controlling factors on the oceanic anoxia primary productivity and subsequent organic rich black shale depositions in the yangtze region during the late ordovician and early silurian intervals

revision of the position of the ordovician silurian boundary in southern ontario regional chronostratigraphic implications of δ13c chemostratigraphy of the manitoulin formation and associated strata delta c 13 values of 142 samples from the manitoulin formation and subjacent strata collected from 14 exposures and two drill cores on manitoulin island bruce peninsula and the region south of georgian bay suggest that the manitoulin formation is latest ordovician hirnantian rather than earliest silurian in age a delta c 13 excursion identified as the hirnantian isotope carbon excursion hice which has a magnitude of nearly 2 5 parts per thousand above baseline values is present in an interval from the upper queenston formation to the lower to middle part of the manitoulin formation in most of bruce peninsula and in the area south of georgian bay whereas on manitoulin island the rice appears to be absent this indicates that a significant part of the manitoulin formation is older on the bruce peninsula and in its adjacent region than on manitoulin island the chemostratigraphically based conclusions are consistent with biostratigraphic data from conodonts and brachiopods the hirnantian delta c 13 curve from anticosti island quebec is closely similar to those of southern ontario traditionally the ordovician silurian boundary has been placed at the base of the manitoulin formation but the new results suggest that it is more likely to be at or near the base of the overlying cabot head formation these new results have major implications for the interpretation of the geologic history and marine depositional patterns of the latest ordovician of a large part of the north american midcontinent

late ordovician hirnantian diploporitan fauna of anticosti island quebec canada implications for evolutionary and biogeographic patterns hirnantian latest ordovician localities containing echinoderm fossils are rare the few that have been discovered primarily contain disarticulated crinoid ossicles therefore relatively little is known about echinoderm evolutionary dynamics across the late ordovician early silurian boundary especially noncrinoid echinoderms new diploporitan echinoderms holocystites salmoensis and an unidentified holocystitid from reefal facies of the upper ordovician ellis bay formation of anticosti island provide a critical data point concerning diploporitan biogeography and evolutionary pathways undertaken during the ordovician and silurian these fossils also provide a crucial link in understanding the ancestry of the silurian holocystites fauna an unusual diploporitan fauna from the middle silurian of north america whose origination dates back at least 15 million years earlier than previously thought with the discovery of taxa described here new fossil data such as these stress the importance of uncovering

reefs during the multiple crises towards the ordovician silurian boundary anticosti island eastern canada and worldwide multiple latest ordovician rawtheyan hirnantian glaciations in central africa with concomitant global sea level lowstands and cooler restricted equatorial carbonate shelves and ramps interrupted by warmer interstadial highstands had a dramatic global impact on the tropical shallow water reef ecosystem and carbonate production with the ordovician silurian boundary strata on anticosti island as a global standard for a carbonate shelf ramp setting the latest ordovician and earliest silurian reveal three reef phases ended by three extinctions the first extinction towards the end of the rawtheyan affected the last richmondian type reefs vaureal formation mill bay member the second extinction was less pronounced ending with reefs at the base of the prinsta member ellis bay formation interpreted as the top of the normalograptus extraordinarius graptolite subzone the third and most severe extinction phase capped the laframboise patch reef complex ellis bay formation at the top of the no

late ordovician early silurian cryptospore occurrences on anticosti island ile d anticosti quebec canada residues from hirnantian upper ordovician and aeronian lower silurian strata of anticosti island quebec canada have yielded the oldest occurrence of terrestrial based palynomorphs cryptospores from eastern north america a low abundance low diversity cryptospore assemblage containing specimens of pseudodyadospora velatitetras and alete monads has been recovered from the velleda member of the ellis bay formation upper ordovician hirnantian the gun river formation lower silurian aeronian contains a low abundance low diversity assemblage composed of specimens of velatitetras laevolcancis and alete monads the occurrence of these assemblages provides evidence for early land plants in this part of north america during the late ordovician and early silurian and documents the existence of a terrestrial based flora during and after the saharan late ordovician glaciation

nd isotope records of late ordovician sea level change implications for glaciation frequency and global stratigraphic correlation abstract detailed records of ice sheet advances and retreats are reconstructed for the hirnantian and katian ages of the late ordovician using nd isotopes e nd as a sea level proxy in three study sections from the western margin of laurentia two shallow water platform sections located south and north of the paleo equator and one deep water section located in a continental slope rise setting when sea level was high and paleo shorelines had migrated eastward the e nd value of seawater in the vicinity of each of the study sections shifted toward the e nd value of the eastern panthalassa ocean 4 0 by contrast when sea level was low and paleo shorelines had migrated westward the e nd value of seawater shifted toward the e nd value of the continental weathering flux from laurentia 8 5 0 2 2 mean these stratigraphic patterns of changing e nd values are interpreted to reflect the eustatic sea level fluctuations that previous studies have documented in response to gondwanan ice sheet advances and retreats thus linking the e nd sea level proxy to late ordovician global scale climate changes the e nd profiles for the two platform sections yielded similar proxy sea level curves with five cycles of oscillation recorded during the latest katian and hirnantian three additional cycles of late katian sea level change are recognized in the e nd profile of the deep water continental slope rise section the combination of e nd 13 c and graptolite biostratigraphic data facilitates a precise interregional correlation of the hirnantian age and the paleoclimate changes that took place during this interval the new correlations support previous findings that the hirnantian ice age comprised two major glacial periods separated by a minor interglacial during the early part of the metabolograptus persculptus biozone the younger glacial confined to mid m persculptus biozone time led to more extensive sea surface cooling than did the earlier one and resulted in extensive eustatic sea level drawdown and c cycle changes it records the strata most often recognized as hice the hirnantian carbon isotope excursion in sedimentary successions worldwide such as anticosti island scotland estonia siberia and south china the results of this study support and strengthen the view that glaciation predated the hirnantian age in the late ordovician and that the records of small positive 13 c excursions in katian successions from baltica and eastern north america are themselves proxy indicators of glaciation frequency and eustatic sea level changes

regional depositional changes and their controls on carbon and sulfur cycling across the ordovician silurian boundary northwestern guizhou south china abstract the controls of regional depositional environment on stable isotope records spanning the ordovician silurian boundary on the yangtze platform have seldom been investigated the objectives of this study include reconstruction of regional depositional settings and assessment of how sedimentary processes may have influenced carbon and sulfur isotopic fractionation seven shale facies have been recognized in the wufeng longmaxi interval from two locations completely bioturbated claystone of the basal wufeng formation accumulated on a shallow oxygenated muddy shelf overlying faintly banded black siliceous shale suggests deposition under deep anoxic conditions interrupted by episodes of dysoxia muddy fossiliferous facies of the guanyinqiao formation reflects a glacial sea level lowstand setting and the observed proximal to distal heterogeneity of facies matrix supports shallowing in the proximal area overlying faintly banded black shale of the longmaxi formation tells of a post glacial transgression the carbonaceous deposits are overlain by banded gray and dark gray muddy siltstones and suggest a shoaling upward trend the shallow water facies display relatively heavier 13 c org values whereas deep water facies are characterized by lower 13 c org values discrepancies between hirnantian positive 13 c org excursions in proximal and distal areas probably reflect spatial gradient in seawater 13 c dic induced by glacioeustasy deposits accumulated under oxic and physically dynamic conditions display strong positive 34 s sulfide excursions perhaps a consequence of diagenetic sulfate reduction occurred at some depth below the sediment water interface and removed from the sulfate pool of overlying water column negative relationship between 34 s sulfide and toc indicates that redox conditions and physical reworking of sediment at the sediment water interface controlled the rate of microbial sulfate reduction and sulfur isotope fractionation overall co variation of lithofacies and isotopic records suggests that local heterogeneity of seawater chemistry and sedimentary dynamics may exert partial controls on biogeochemical processes and amplify the magnitude of the hirnantian isotopic excursions in the proximal area

paleogeography of the matapedia basin in the gaspe appalachians initiation of the gaspe belt successor basin the matapedia basin consists of the uppermost ordovician lowermost silurian deep water fine grained carbonate siliciclastic rocks of the honorat garin formation and matapedia groups pabos and white head formations the lower rock assemblage of the gaspe belt in the gaspe appalachians paleogeographic maps of eight time slices from the caradocian to the llandoverian are presented to better understand the tectonosedimentary evolution of the matapedia basin deposition evolved from siliciclastic garin fm to argillaceous limestones pabos fm to limestones white head fm the overall change from terrigenous garin fm to limestone facies white head fm reflects a change in the source area paleocurrent directions and composition of sandstones indicate an orogenic source area to the south for the garin formation which is believed to be the inliers of the humber and dunnage zones in the southern gaspe and new brunswick appalachians lime muds deposited by turbidity currents coming from the nor

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

stratigraphy of the upper ordovician upper vaureal and ellis bay formations eastern anticosti island quebec marked facies changes occur in late ordovician strata assigned to the uppermost vaureal and ellis bay formations ashgill rawtheyan hirnantian on anticosti island quebec western anticosti features shales and carbonates whereas outcrops along the eastern coast contain prominent discontinuous mixed siliciclastic carbonate units detailed section measurement along the northeast coast allows for the first time accurate definition of seven new members within this uninterrupted sequence sands present in the upper vaureal and lower ellis bay formations in the east appear to have deterred the growth of muddy bottom brachiopod communities comparable to those in the western and central regions of anticosti sand units within the upper vaureal formation contain 1 m diameter colonies of paleofavosites coeval small coral patch reefs are found in the central part of the island where sands are absent the uppermost ellis bay formation of northeast anticosti is marked by a shallow subtidal coral algal onco

scarcity of the c30 sterane biomarker 24 n propylcholestane in lower paleozoic marine paleoenvironments abstract 24 n propylcholestane 24 npc a c30 sterane compound derived from sterol precursors which are the major sterol constituents of modern pelagophyte microalgae occurs in certain neoproterozoic rocks and oils and throughout the phanerozoic rock record this broad distribution leads 24 npc to be widely considered a reliable indicator of open to partially restricted marine depositional conditions for source rocks and oils here we report two significant hiatuses in the occurrences of 24 npc in the lower paleozoic marine rock record the first in the middle late cambrian and the second in the late ordovician early silurian transition for a range of lithofacies carbonates and siliciclastic rocks organic carbon contents both organic lean and organic rich and paleoceanographic environments shelf and deeper water marine settings and observed offshore of two paleocontinents laurentia and baltica the ordovician silurian gap is at least 9 million years and possibly up to 20 million years in duration robust older occurrences of 24 npc steranes in some neoproterozoic rocks and oils suggest that oceanographic conditions in our intervals of lower paleozoic time were unfavorable for the proliferation of pelagophyte algae as phytoplankton caution should therefore be applied when interpreting a lacustrine versus marine depositional environmental setting for source rocks and oils in these intervals of early paleozoic time using lipid biomarker assemblages

tempestite frequency curves a key to late ordovician and early silurian subsidence sea level change and orbital forcing in the anticosti foreland basin quebec canada following partial closure of the northern iapetus ocean along the newfoundland segment of the st lawrence promontory subsidence along the anticosti platform was influenced by residual thermal subsidence renewed tectonic loading by thrust sheets to the south and sediment loading basement subsidence calculated by removing the effects of sediment loading was between 2 and 5 cm ka in the caradocian increasing to 8 6 cm ka in the pusgillian and cautleyan and reaching a maximum of 17 7 cm ka in the rawtheyan during deposition of the princeton lake and vaureal formations a marked decline in subsidence beginning in the hirnantian and continuing into the early silurian may reflect decoupling of thrust loads to the south although a further stage of thrust loading may have occurred in the aeronian during deposition of the lower part of the jupiter formation storm frequency curves produced using tempestite abundance thickness and grain size through more than a kilometre of carbonate strata on antico

late ordovician obliquity forced glacio eustasy recorded in the yangtze block south china abstract a high precision time framework is critical for improving our understanding of three major events the late ordovician mass extinction the hirnantian glaciation and the hirnantian isotopic carbon excursion hice in the present study we conducted detailed cyclostratigraphic analysis of a high resolution 1 2 cm intervals anhysteretic remanent magnetization series obtained from the wanhe section in south china covering the lower katian through hirnantian power spectra and derived average spectral misfit results show that the section records the ensemble of milankovitch cycles with 405 kyr long eccentricity 93 125 kyr short eccentricity 33 8 kyr main obliquity and 17 22 kyr precession periods the obliquity period of 33 8 0 46 kyr indicates a precession constant k of 57 19 0 53 arcsec yr a length of day of 22 37 0 12 h and an earth moon distance of 375 330 722 km vs 384 000 km today during the late ordovician 445 ma the ratios of obliquity band power to total power reveal periodicities of 1 32 myr for the s4 s3 term and 173 4 kyr for the s3 s6 term aided by the highly fluctuating magnetic susceptibility ms we recognized a series of 1 32 myr obliquity forced glacio eustatic cycles with three intervals of sea level rise and climate warming trend characterized by high ms values and two episodes of sea level fall and climate cooling trend before the hirnantian the 405 kyr and subsequent 33 8 kyr calibrations produce an 7 36 myr floating astronomical time scale fats for the studied succession indicating that the estimated durations for the hirnantian stage the worldwide hirnantian glaciation and the first and second phases of the late ordovician mass extinction were 1225 kyr 830 kyr 440 kyr and 540 kyr respectively this work provides a reliable high resolution time framework for further investigation of the major geological events occurred in the late ordovician and improves our knowledge of solar system behaviour and earth moon history during that period

fluctuations in epeiric sea carbonate production during silurian positive carbon isotope excursions a review of proposed paleoceanographic models positive excursions in the carbon isotopic ratio of marine carbonates 13ccarb are commonly attributed to an increase in the burial ratio of organic versus carbonate carbon in the global oceans regardless of whether this increased deposition of organic carbon was oceanographically or biologically induced the fact that high carbon isotope values are most often attributed to the sequestration and burial of organic carbon remains the notion that high 13ccarb values should be the result of organic rich deposition in epeiric sea settings is not surprising considering that the overwhelming majority of organic carbon burial in the modern oceans occurs in continental shelf and margin settings what is surprising is that major positive carbon isotope excursions during the silurian are recorded in clean carbonate successions with little to no appreciable black shale deposition in epeiric seas in fact silurian 13ccarb excursions occurred during intervals of prolific reef development and an overall expansion of carbonate platform environments globally this discrepancy has sparked considerable debate among silurian carbon isotope specialists but has offered a valuable opportunity to re examine the intricate connections among nutrients primary productivity carbonate sedimentology and the global carbon cycle in the lower paleozoic here we consider several sedimentological and geochemical features of one of these silurian positive carbon isotope 13ccarb excursions and discuss the broader implications of current silurian 13ccarb models to the interpretation of lower paleozoic carbonate sequences

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

hirnantian sea level changes in the baltoscandian basin a review abstract correlation of hirnantian sections of the baltoscandian basin reveals four major transgressive regressive sea level cycles related to waxing and waning of ice sheets at high southern hemisphere paleolatitudes the correlation between stratigraphic members in estonia and latvia shows transgressions regressions in the first half of the hirnantian when sea level fluctuated between shallow and deep shelves during the second half of the hirnantian the sea regressed into the former deep shelf leaving north estonia as a mainland after that the sea retreated to the central baltoscandian basin and the whole baltic shelf stayed subaerial this final regression brought the coastline to the vicinity of scania in the west and of lublin basin in the east in the latest hirnantian during ice cap melting sea level began to ascend the baltic shelf submerged again in the early silurian this transgression of latest hirnantian earliest silurian indicates fast melting of ice in gondwanan ice sheets we are proposing that the destabilisation of great amount of gas hydrates due to diminishing of water column pressure during the last deepest regression acted as a trigger for rapid warming of climate the ascertainment of sea level changes presumes correlations between different parts of the baltoscandian basin comparison of published data on stratigraphy lithology faunal finds chemical elements and stable carbon isotopes was the ground for tracking hirnantian changes

polygenetic polyphase karsted hardground omission surfaces in lower silurian neritic limestones anticosti island eastern canada abstract exquisitely preserved and well exposed rocky paleoshoreline omission surfaces in lower silurian chicotte formation limestones on anticosti island quebec are interpreted to be the product of combined marine and meteoric diagenesis the different omission features include 1 planar erosional bedding tops 2 scalloped erosional surfaces 3 knobs ridges and swales at bedding contacts and 4 paleoscarps an interpretation is proposed that relates specific styles of omission surface to different diagenetic depositional processes that acted in separate terrestrial peritidal shallow neritic zones such processes were linked to fluctuations in relative sea level with specific zones of diagenesis such as 1 karst corrosion 2 peritidal shallow subtidal erosion 3 subtidal seawater flushing and cementation and 4 shallow subtidal deposition most surfaces are interpreted to have been the result of initial extensive shallow water synsedimentary lithification forming hardgrounds that were as sea level fell planed by inner neritic abrasion successively altered by exposure and subaerial corrosion only to be buried by sediments as sea level rose again this succession was repeated several times resulting in a suite of recurring polyphase omission surfaces through many meters of stratigraphic section synsedimentary cloudy marine cements are well preserved and are thus interpreted to have been calcitic originally aragonite components are rare and thought to have been dissolved just below the silurian seafloor large mollusks that survived such seafloor removal were nonetheless leached and the resultant megamolds were filled with synsedimentary calcite cement these silurian inner neritic strandline omission surfaces are temporally unique they are part of a suite of marine omission surfaces that are mostly found in early to middle paleozoic neritic carbonate sedimentary rocks these karsted hardgrounds formed during a calcite sea time of elevated marine carbonate saturation and extensive precipitation of marine cement the contemporaneous greenhouse atmosphere was supercharged with co2 leading to profound surface karst under strongly acid rain younger peritidal omission surfaces although potentially formed during aragonite sea or calcite sea times would have been subject to very different terrestrial diagenetic process with lower atmospheric pco2 values but increasingly complex biogenic soils producing dissimilar alteration features

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

chemostratigraphy of an ordovician silurian carbonate platform δ13c records below glacioeustatic exposure surfaces the use of carbon isotope stratigraphy to construct time lines for stratigraphic correlation requires synchronous changes in carbon isotope ratios 13 c to be preserved in carbonate dominated strata such changes are commonly interpreted to reflect primary secular variation in ocean chemistry however negative 13 c anomalies developed in pliocene pleistocene carbonate platforms following glacioeustatic sea level fall due to remineralization of terrestrial biomass during meteoric diagenesis these anomalies are similar in structure and magnitude to some neoproterozoic 13 c records opening the possibility that the neoproterozoic 13 c anomalies have a meteoric origin derived from a large terrestrial biosphere here we test the hypothesis that a large terrestrial biosphere existed prior to the silurian devonian land plant radiation by examining 13 c records of subaerial exposure surfaces formed in a shallow water carbonate platform during the ordovician silurian icehouse the exposure surfaces include an unconformity at the ordovician silurian boundary with terra rossa and dissolution collapse breccia and a lower silurian quartz sand layer feeding a 50 m deep system of karst pipes there is no evidence for 13 c depletion beneath either exposure surface strontium concentrations in the rocks are low 10 120 ppm and covary with 18 o oxygen isotope ratios however do not positively correlate with 13 c our results suggest that there was no significant terrestrial biosphere during ordovician silurian time and by extension that neoproterozoic negative carbon isotope anomalies cannot be explained by meteoric diagenesis

did changes in atmospheric co2 coincide with latest ordovician glacial interglacial cycles article i nfo the late ordovician hirnantian stage 444 million years ago was one of three time periods during the past half billion years in which large continental glaciers formed over earth s polar regions the effects of this glaciation were far reaching and coincided with one of the largest marine mass extinction events in earth history the cause of this ice age is uncertain and a paradoxical association with evidence for high atmospheric co2 levels has been debated precise linkages between sea level ice volume and carbon isotope 13 ccarb and 13 corg proxy records of pco2 have been poorly understood due in part to uncertainties in stratigraphic correlation and the interpretation of globally important sections although correlation difficulties remain recent hirnantian biostratigraphic studies now allow for improved correlations here we show that consistent trends in both 13 ccarb and 13 corg from two well dated stratigraphic sequences in estonia and anticosti island canada coincide with changes in late ordovician hirnantian climate as inferred from sea level and the extent of ice sheets the integrated datasets are consistent with increasing pco2 levels in response to ice sheet expansion that reduced silicate weathering ultimately the time period of elevated pco2 levels is followed by geologic evidence of deglaciation

the stratigraphic and geochemical imprints of late ordovician glaciation on far field neritic carbonates anticosti island eastern canada abstract the storm dominated open marine neritic carbonate facies continuously exposed in coastal cliffs and wide rocky tidal flat exposures at the western end of anticosti island form a 315 m thick expanded section spanning the ordovician silurian boundary these carbonate facies display deepening shallowing trends of different orders resulting in a hierarchical stacking of sedimentary cycles the carbonate facies within these multi order cycles represent sediments that were deposited from shoreline to deeper outer carbonate ramp 80 m in water depth and exhibit sea level changes of several tens of metres in magnitude field petrographic and geochemical evidence support a well preserved primary 18o signal in close equilibrium with the original paleoenvironmental conditions the 18o intracycle variations up to 1 0 in magnitude reveal repeated decreasing and increasing values during the transgressive and regressive portions of decametre tr cycles respectively although temperature salinity and ice volume influenced the recorded primary 18o signal orbital scale glacioeustasy ultimately controlled facies and cycle development and 18o signal during the katian immediately before the latest ordovician hirnantian glaciation the 3 5 myr long 18o signal recorded by our entire succession supports a protracted period of cooling leading into the main phase of latest ordovician hirnantian glaciation spectral analysis of the 18o record also reveals decametre variations which are associated with decametre scale transgressive regressive tr cycles possibly reflecting eccentricity orbital signals our study shows the importance of a thick expanded stratigraphic succession sampled at the highest resolution possible to decipher potential linkages between facies changes multi order stratigraphic cycles 18o signal and the earth s orbital parameters

greenschist facies regional and contact metamorphism of the thalanga volcanic hosted massive sulfide deposit northern queensland australia the conditions of two separate greenschist facies metamorphic events have been reconstructed at the syn genetic thalanga massive sulfide deposit northern queensland australia using scarce garnet biotite and garnet chlorite bearing samples from the felsic volcano sedimentary host rock succession regional metamorphism peak temperature 440 to 480 c generated a foliation defined by aligned chlorite muscovite and biotite and rare euhedral spessartine almandine grossular rich garnets the emplacement of a post tectonic diorite pluton was associated with contact metamorphism and scarce anhedral spessartine almandine rich garnet poikiloblasts crystallized within a 650 m wide aureole at temperatures of 500 c both metamorphic stages occurred under pressure conditions of 3 5 kbar our study documents a useful method for geothermobarometric reconstruction of metamorphosed syn genetic ore deposits where calculations based on sulfide assemblages failed to produce unequivocal constraints furthermore we show that hydrothermally altered footwall rhyolite have higher rm x rm mg rm chlorite rm and rm x rm mg rm biotite than unaltered rocks which may help to identify fossil metamorphosed hydrothermal systems associated with massive sulfide deposits similar to thalanga

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

the queenston formation shale dominated mixed terrigenous carbonate deposits of upper ordovician semiarid muddy shores in ontario canada the upper ordovician queenston formation constitutes the westernmost part of the taconic clastic wedge of eastern north america in ontario the queenston formation consists of a mixed terrigenous carbonate succession composed primarily of red silty shales which are locally gypsiferous alternating in the middle and lower parts of the formation with thin grey shales calcareous siltstone and sandstone and bioclastic limestone brachiopods some ostracodes and in the lower beds bryozoans constitute the body fossil assemblage diplocraterion skolithos chondrites and occasional large vertical burrows of unknown origin form the ichnofauna plane beds ripple marks and occasional swaley and hummocky cross stratifications are present in the calcareous units mudcracks gypsum nodules occasional halite hopper casts and gilgai like structures typically occur in horizons where shales alternate with siltstone and limestone these features are indicative of prograding muddy and storm and tide affected

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

ordovician and silurian sea water chemistry sea level and climate a synopsis following the cambrian explosion and the appearance in the fossil record of most animal phyla associated with a range of new body plans the ordovician and silurian periods witnessed three subsequent major biotic events the great ordovician biodiversification event the end ordovician extinction the first animal extinction and second largest of the five mass extinctions of the phanerozoic and the early silurian postextinction recovery there are currently no simple explanations for these three major events combined extrinsic geological and intrinsic biological factors probably drove the biodiversifications and radiations and the appearance and disappearance of marine habitats have to be analysed in the frame of changing palaeogeography palaeoclimate and sea water chemistry the present paper reviews the relationships of the three biotic events to chemical and physical processes occurring in the ocean and atmosphere during the ordovician and silurian including sea level changes geochemical proxies 13c 18o 87sr 86sr of the ocean waters and the evolution of the atmosphere oxygen and carbon dioxide content less

refining 2 km of ordovician chronostratigraphy beneath anticosti island utilizing integrated chemostratigraphy new high resolution chemostratigraphy in combination with updated biostratigraphy refines the chronostratigraphic resolution of the nearly 2 km thick ordovician section below anticosti island a total of 1414 horizons spaced at 1 0 1 5 m intervals were sampled from the new associated consolidated paper nacp drill core and analyzed for major and trace elemental composition by portable x ray fluorescence analyzer pxrf select micrite and calcareous shale powders were then analyzed for 13ccarb number of samples n 364 and 87sr 86sr n 25 our results indicate a floian to early darriwilian f3 dw1 age for the romaine formation a middle darriwilian to sandbian dw2 sa1 age for the mingan formation a lower to mid katian ka1 ka2 age for the macasty formation an upper katian ka3 age for the lower vaureal formation an uppermost katian ka4 age for the upper vaureal and a hirnantian h1 age for the ellis bay formation this integrated chemostratigraphic and biostratigraphic synthesis es

fungal event and palynological record of ecological crisis and recovery across the permian triassic boundary the end of the permian period was marked by the most severe mass extinction in the geologic record detailed quantitative study of pollen and spores from shallow marine deposits spanning the permian triassic p tr boundary in israel reveals a sequence of palynological ecological stages reflecting a major crisis among land plants the disappearance of the gymnosperm dominated palynoflora of the late permian lueckisporites virkkiae zone is recorded at a claystone horizon containing almost exclusively abundant fungal remains and carbonized terrestrial plant debris this fungal spike is followed by a zone dominated by marine acritarchs and a succession showing ecological recovery with abundantlycopodsporesandeventualreappearanceofbisaccategymnospermpolleninthe earlytriassic thelatestpermianproliferationoffungiisrecognizableworldwideandcan be correlated with other paleontological and geochemical markers of a global ecological disaster

the magnitude of late paleozoic glacioeustatic fluctuations a synthesis a comprehensive literature review shows that the magnitude of eustatic fluctuations varied throughout the carboniferous and permian and that at least eight distinct phases can be recognized facies juxtapositions in carbonate successions and erosional relief in clastic successions indicate that glacioeustatic fluctuations of 20 25 m and occasionally as much as 60 m took place throughout the early mississippian tournaisian a widely recognized glacial period middle mississippian mid chadian through holkerian shallow marine carbonate and clastic successions indicate that eustatic fluctuations were 10 25 m a decrease that matches well with the paucity of coeval glacial deposits late visean asbian through mid brigantian glacioeustatic fluctuations of 10 50 m record the initial phases of ice accumulation in advance of the widespread mid carboniferous glacial event the latest mississippian earliest pennsylvanian mid brigantian through langsettian was a time of widespread glaciation and strata of this age commonly exhibit evidence of glacioeustatic fluctuations of as much as 40 100 m although middle pennsylvanian duckmantian through asturian glacial deposits are present in eastern australia paleovalley depths suggest that coeval glacioeustatic fluctuations were less than 40 m glacioeustatic fluctuations of as much as 100 120 m have been widely reported from late pennsylvanian earliest permian stephanian through mid sakmarian successions in north america an increase that corresponds to the growth of large ice sheets across much of gondwana and the accumulation of ice in the northern hemisphere incision and facies juxtaposition in early middle permian mid sakmarian through kungurian successions in eastern australia indicate that glacioeustatic fluctuations of 30 70 m occurred during the waning stages of major glaciation erosional relief in paleoequatorial carbonates and the presence of coeval glacial deposits in australia suggests that eustatic fluctuations of 10 60 m occurred during the final stages of glaciation in the middle to late permian roadian through capitanian but the modest size of most of these fluctuations makes it difficult to isolate the glacioeustatic signature this review demonstrates that far field cyclic successions record changing glacial conditions in gondwana that the magnitude of glacioeustatic fluctuations was directly related to the volume of glacial ice that carboniferous permian glacioeustasy was more variable than previously recognized and that generalizations from short temporal intervals are probably not representative of the late paleozoic ice age as a whole although any attempt to quantify the magnitude of ancient eustatic changes is based on caveats and assumptions this review incorporates the results of over 100 published papers on the topic in an attempt to minimize the errors inherent in any one study

chitinozoan biostratigraphy of a new upper ordovician stratigraphic framework for anticosti island canada thickness and facies variations in the upper ordovician succession of anticosti island have made correlations between the western and eastern ends of the island difficult this chitinozoan study reveals that most strata of the ellis bay formation at the eastern end of the island correlate with the vaureal formation at the western end in the eastern end chitinozoans typical of the ellis bay formation occur only in the upper part of the lousy cove member whereas chitinozoans diagnostic of the mid upper vaureal occur in the rest of the formation moreover the eastern schmitt creek and mill bay members of the vaureal formation are older than the uppermost vaureal eponym members in the western end new east west correlations supported by geochemical sedimentological and sequence stratigraphic data are proposed comparison with coeval upper ordovician successions suggests a hirnantian age for the gamachiana zone that occurs in the lower members of the ellis bay formation it also indicates that the hirnantian isotopic carbon excursion hice on anticosti begins in the d anceps zone just below the base of the hirnantian stage reaches a first peak in the lower n extraordinarius zone a second modest peak high in the n extraordinarius zone and a third more important one in the lower part of the n persculptus zone these results are in agreement with the biostratigraphic and chemostratigraphic data at the global boundary stratotype section and points gssps for the base of the hirnantian and silurian in south central china and at dob9s linn in scotland

late cretaceous chronology of large rapid sea level changes glacioeustasy during the greenhouse world we provide a record of global sea level eustatic variations of the late cretaceous 99 65 ma greenhouse world ocean drilling program leg 174ax provided a record of 11 14 upper cretaceous sequences in the new jersey coastal plain that were dated by in tegrating sr isotopic stratigraphy and biostratigraphy backstripping yielded a late cre taceous eustatic estimate for these sequences taking into account sediment loading com paction paleowater depth and basin subsidence we show that late cretaceous sea level changes were large 25 m and rapid k1 m y suggesting a glacioeustatic control three large d 18 o increases are linked to sequence boundaries others lack sufficient d 18 o data consistent with a glacioeustatic cause and with the development of small 10 6 km 3 ephemeral ice sheets in antarctica our sequence boundaries correlate with sea level falls recorded by exxon production research and sections from northwest europe and russia indicating a global cause although the exxon record differs from backstripped estimates in amplitude and shape

large carbonate associated sulfate isotopic variability between brachiopods micrite and other sedimentary components in late ordovician strata carbonate associated sulfate cas is trace sulfate incorporated into carbonate minerals during their precipitation its sulfur isotopic composition is often assumed to track that of seawater sulfate and inform global carbon and oxygen budgets through earth s history however many cas sulfur isotope records based on bulk rock samples are noisy to determine the source of bulk rock cas variability we extracted cas from different internal sedimentary components micro drilled from well preserved late ordovician and early silurian age limestones from anticosti island quebec canada mixtures of these components whose sulfur isotopic compositions vary by nearly 25 can explain the bulk rock cas range large isotopic variability of sedimentary micrite cas 34 s depleted from seawater by up to 15 is consistent with pore fluid sulfide oxidation during early diagenesis specimens recrystallized during burial diagenesis have cas 34 s enriched by up to 9 from hirnantian seawater consistent with microbial sulfate reduction in a confined aquifer in contrast to the other variable components brachiopods with well preserved secondary layer fibrous calcite a phase independently known to be the best preserved sedimentary component in these strata have a more homogeneous isotopic composition these specimens indicate that seawater sulfate remained close to about 25 v cdt through hirnantian end ordovician events including glaciation mass extinction carbon isotope excursion and pyrite sulfur isotope excursion the textural relationships between our samples and their cas isotope ratios highlight the role of diagenetic biogeochemical processes in setting the isotopic composition of cas

evolution of the late ordovician plaesiomyid brachiopod lineage in laurentia multivariate analysis of 197 well preserved specimens among 11 species of late late ordovician katian and hirnantian plaesiomyid brachiopods from north america revealed two trends of morphological changes 1 increasing globosity and dorsal convexity from the early to late katian and 2 coarser but fewer ribs in species from the paleoequatorial intracratonic seas compared with species from the mid to high tropical pericratonic shelves and platforms another notable change albeit not quantified was the enlargement of cardinal process in plaesiomyids from early to late katian characterized by a predominantly trilobed cardinal process in the intracratonic species versus a bilobed cardinal process in pericratonic species increases in dorsal convexity and shell globosity are interpreted to have allowed for development of a larger lophophore improving the capacity and efficiency of filter feeding and gas exchange smaller but more frequently bifurcating ribs would have led to more numerous setae nea