records of carbon and sulfur cycling during the silurian ireviken event in gotland sweden abstract early silurian 431 ma carbonate rocks record a ca 4 5 positive excursion in the stable isotopic composition of carbonate carbon 13ccarb associated with this isotopic shift is a macroevolutionary turnover pulse known as the ireviken event the onset of this carbon isotope excursion is commonly associated with a shallowing upward facies transition that may have been accompanied by climatic change as indicated by a parallel positive shift 0 6 in the stable isotopic composition of carbonate oxygen 18ocarb however the relationships among carbon cycle perturbations faunal turnover and environmental changes remain enigmatic here we present a suite of new isotopic data across the ireviken event from multiple sections in gotland sweden these samples preserve no systematic change in 18ocarb but show positive excursions of equal magnitude in both carbonate 13ccarb and organic 13corg carbon in addition the data reveal a synchronous perturbation in sulfur isotope ratios manifest as a ca 7 positive excursion in carbonate associated sulfate 34scas and a ca 30 positive excursion in pyrite 34spyr the increase in 34spyr values is accompanied by a substantial concomitant increase in stratigraphic variability of 34spyr the relatively constant offset between the 13ccarb and 13corg excursions throughout the ireviken event could be attributed to increased organic carbon burial or possibly a change in the isotopic composition of co2 sources from weathering however a positive correlation between carbonate abundance and 13ccarb suggests that local to regional changes in dissolved inorganic carbon dic during the shallowing upward sequence may have been at least partly responsible for the observed excursion the positive excursion recorded in 34scas suggests a perturbation of sufficient magnitude and duration to have impacted the marine sulfate reservoir an inverse correlation between cas abundance and 34scas supports the notion of decreased sulfate concentrations at least locally consistent with a concomitant increase in pyrite burial a decrease in the offset between 34scas and 34spyr values during the ireviken event suggests a substantial reduction in the isotopic fractionations epyr expressed during microbial sulfur cycling and pyrite precipitation through this interval decreased epyr and the concomitant increase in stratigraphic variation in 34spyr are typical of isotope systematics observed in modern shallow water environments associated with increased closed system behavior and or oxidative sedimentary reworking during early sediment diagenesis while the isotopic trends associated with the ireviken event have been observed in multiple locations around the globe many sections display different magnitudes of isotopic change and moreover are typically associated with local facies changes due to the stratigraphic coherence of the carbon and sulfur isotopic and abundance records across the ireviken event and their relationship to changes in local depositional environment we surmise that these patterns more closely reflect biogeochemical processes related to deposition and lithification of sediment than global changes in carbon and sulfur burial fluxes

palaeoclimate perturbations before the sheinwoodian glaciation a trigger for extinctiond during the ireviken event telychian to sheinwoodian conodonts pterospathodus eopennatus ssp n 1 zone through the ozarkodina sagitta rhenana superzone were studied in detail for their oxygen isotope composition the upper kockelella ranuliformis and ozarkodina sagitta rhenana zones represent the peak interval of the sheinwoodian positive 18oapatite excursion reflecting the coolest conditions with marine sea water temperatures decreased by more than 6 c in the subtropics the 18oapatite data cover pre through post ireviken event strata the study includes also material from younger levels up to the late wenlock ctenognathodus murchisoni zone however this record is incomplete with one major hiatus reflecting the regression associated with the middle silurian mulde event the stratigraphic levels of the analyzed conodont samples from estonia can be correlated with high precision to the datum points of the ireviken event strata in the lower and upper visby formations on gotland sweden inferred short term climatic changes in the upper telychian part of the studied interval may have been caused by phases of major volcanic activity along the caledonian front across the ireviken event we observe variations in oxygen isotope ratios which allow a direct comparison to the stepwise extinctions and faunal reorganisations in different groups our data suggest that faunal extinctions are connected to time intervals of warming before the establishment of more stable and cooler conditions during the main sheinwoodian glacial the most severe extinctions and faunal turnovers at datum points 2 and 4 occurred during the early warmer interval of the event after the shift into the sheinwoodian icehouse in the lower kockelella ranuliformis zone reflected by rapidly decreasing sea water temperatures faunas restabilized and reefal communities started to flourish sedimentary evidence for the glaciation is discussed for different palaeolatitudes less

carbon and oxygen isotope stratigraphy of the lower mississippian kinderhookian lower osagean western united states implications for seawater chemistry and glaciation a positive carbon isotope 1 3 c excursion has been recognized in upper kinderhookian and early osagean carbonates in three sections in southeast idaho and nevada the oldest 1 3 c peak 7 is dated to the isostichaconodont zone and a younger peak occurs in the typicus zone the shifts are recorded in a range of carbonate lithofacies representing various water depths along the shelf lithofacies sampled for 1 3 c and 1 8 o at the samaria mountain section in southeast idaho record the shallowest water conditions indicated by cross bedded skeletal and peloidal grainstones the deepest water conditions are present in the pahranagat range section in eastern nevada which consists mainly of bioturbated lime mudstone and skeletal wackestone the 1 3 c values from these widely separated sedimentary basins show a consistent trend that correlates with early mississippian curves generated from brachiopod calcite in western europe and the midcontinent of north america as well as dolomites in utah and wyoming 1 8 o values become more positive up section generally paralleling the positive trend in 1 3 c during the late kinderhookian no subaerial exposure surfaces are recognized in the sections examined in southeast idaho and nevada and at least the 1 3 c trends are interpreted as primary seawater fluctuations sea level changes occurred near the beginning of the late kinderhookian 1 3 c shift early to middle parts of the isosticha zone and within the peak of the 1 3 c excursion kinderhookian osagean boundary although tectonic changes associated with the antler orogeny have likely modified the eustatic signature

from tomography to full waveform inversion with a single objective function abstractin full waveform inversion fwi a gradient based update of the velocity model requires an initial velocity that produces synthetic data that are within a half cycle everywhere from the field data such initial velocity models are usually extracted from migration velocity analysis or traveltime tomography among other means and are not guaranteed to adhere to the fwi requirements for an initial velocity model as such we evaluated an objective function based on the misfit in the instantaneous traveltime between the observed and modeled data this phase based attribute of the wavefield along with its phase unwrapping characteristics provided a frequency dependent traveltime function that was easy to use and quantify especially compared to conventional phase representation with a strong laplace damping of the modeled potentially low frequency data along the time axis this attribute admitted a first arrival traveltime that could be compared with picked ones from the observed data such as

full traveltime inversion abstractmany previously published wave equation based methods which attempt to automatically invert traveltime or kinematic information in seismic data or migrated gathers for smooth velocities suffer a common and severe problem the inversions are involuntarily and unconsciously hijacked by amplitude information to overcome this problem we have developed a new wave equation based traveltime inversion methodology referred to as full traveltime i e fully dependent on traveltime inversion fti to automatically estimate a kinematically accurate velocity model from seismic data the key idea of fti is to make the inversion fully dependent on traveltime information and thus prevent amplitude interference during inversion under the assumption that velocity perturbations cause only traveltime changes we have derived the fti method in the data and image domains which are applicable to transmitted arrivals and reflected waves respectively fti does not require an accurate initial velocity model or

high resolution 87sr 86sr chemostratigraphy of the silurian implications for event correlation and strontium flux analyses of 87sr 86sr in silurian conodonts recovered from localities in north america and europe representing 13 of the 14 defined silurian conodont zones provide a high resolution record of seawater chemistry for the silurian period these data which are characterized by little or no scatter depict several high frequency cycles superimposed on a gradual longer term rise in 87sr 86sr for the silurian high frequency cycles have a duration of about one conodont zone and many correlate with sequence boundaries recognized around the world these data provide a much higher resolution image of secular changes in 87sr 86sr during the silurian and may require a rethinking of models of strontium isotope flux in marine basins

early silurian wenlockian δ13c profiles from the cape phillips formation arctic canada and their relation to biotic events geochemical data from the cape phillips formation arctic canada are examined in association with three silurian biotic crises in the graptolite community the early wenlockian ireviken mid wenlockian cyrtograptus lundgreni and end wenlockian colonograptus ludensis extinction events positive 13corg excursions are associated with the ireviken and c lundgreni events but not the co ludensis event the ireviken and c lundgreni excursions are recognized worldwide and are herein interpreted to be the result of carbonate weathering in response to eustatic sea level drop the c lundgreni excursion is of greater magnitude in the more proximal basin margin section at abbott river cornwallis island and is explained by the amplification of a more strongly positive 13c signature in shallower parts of an epeiric basin during increased exposure and weathering of the carbonate shelf excursion c5 within the c praedeubeli c deubeli zone is also of regional significance as it occurs in both the abbott r

integrated silurian conodont and carbonate carbon isotope stratigraphy of the east central appalachian basin abstract the eastern and southeastern margins of the appalachian basin contain extensive exposures of telychian through pridoli upper silurian strata litho and biostratigraphic analyses of these strata over the last century provided a general means for their chronostratigraphic correlation to global series and stage boundaries however a rarity of zonally diagnostic fauna limited precise correlation both within and beyond the appalachian basin recent conodont biostratigraphic studies substantially revised the chronostratigraphic position of these strata by more than a series in some cases much of the interval however remained biostratigraphically unzoned and the precise position of series and stage boundaries remained tentative to improve the chronostratigraphic correlation of these strata to global series and stage boundaries we provide new integrated conodont and high resolution 13ccarb chemostratigraphic analyses from two outcrops located in west virginia and virginia spanning the upper rose hill through tonoloway formations upper telychian through pridoli these new data show that the silurian carbon isotope record for this portion of the appalachian basin is shifted to more negative 13ccarb values than typical global data however the overall isotopic records appear correlative at least within the appalachian basin thereby providing a means for future correlation within the basin for intervals in which biostratigraphy is traditionally lacking these coupled data demonstrate that the ireviken and mulde excursions are contained within our sections the high amplitude lau excursion was however not observed and the present biostratigraphic data suggest that a majority of the ludlow series was either not recorded or was later removed by erosion these coupled analyses therefore indicate that the ludlow pridoli boundary resides within the wills creek formation and we propose that the overlying isotopic record presented here records the late ludlow carbon isotope excursion llcie and silale low previously identified only within the baltic basin

early palaeozoic cooling events peri gondwana and beyond the short lived end ordovician hirnantian glaciation allied to marine mass extinction is variously considered as a short lived event or as the peak of long drawn out climatic cooling through at least late ordovician early silurian times evidence from early palaeozoic facies faunas and stable isotope excursions used to interpret climatic cooling events ranges farther from late mid cambrian to late silurian times glacigenic sediments structures and geomorphology provide direct evidence of glacial episodes cool water carbonate deposition which is particularly widespread during the late ordovician boda event in high latitude peri gondwana gondwana and beyond into mid low palaeolatitudes is interpreted as indicating global cooling not warming as has been proposed such carbonates also characterize mid latitude continents widely at horizons earlier in the ordovician and more locally in the mid silurian in high latitude gondwana cool water carbonate mounds have distinctive facies controlled mound faunas across palaeocontinents other facies evidence for palaeoclimates includes black shale deposition including deglacial organic rich hot shales which indicate transgression in epeiric seas and sea level curves interpreted from facies and faunal successions correlation is shown between facies evidence and positive c isotope excursions from which cyclicities are apparent the possible interface of orbitally controlled rhythms is considered against evolving palaeobiogeography and changes in global sea level and in pco2 facies and faunal evidence from peri gondwanan terranes armorica central europe alborz is assessed with that from gondwana mostly north africa south america and correlatives in avalonia baltica and laurentia to establish a wider picture of early palaeozoic cooling events

the graptolite conodont and sedimentary record through the late ludlow kozlowskii event silurian in the shale dominated succession of bohemia the shale dominated hemipelagic succession exposed in the southwestern part of the prague synform preserves the most complete ludfordian graptolite record so far encountered from peri gondwanan europe four graptolite biozones the neocucullograptus inexpectatus nc kozlowskii pseudomonoclimacis latilobus slovinograptus balticus and pristiograptus fragmentalis biozones are recognized in the middle and late ludfordian between the bohemograptus tenuis biozone and the base of the pridoli series conodont occurrences are restricted to scattered limestone beds but enable tentative integration of the graptolite and conodont biozonal schemes particular attention was paid to faunal and sedimentary changes and the carbon isotope record across the middle ludfordian kozlowskii extinction event the kozlowskii event caused the almost simultaneous extinction of graptolites with ventrally curved rhabdosomes the genera bohemograptus polonograptus and neocucullograptus along with pseudomonoclimacis dalejensis disappeared from the fossil record the offshore conodont fauna recorded in the section was not strongly affected and similarly the pelagic orthocerids and nektonic ceratiocaris passed unaffected through the extinction interval the abundant and widespread pelagic myodocopid ostracod entomis however became extinct the late ludfordian graptolite recovery gave origin to a novel fauna of pridoli type from taxa that emerged or just reappeared above the kozlowskii crisis in vseradice and elsewhere in the prague synform the recovery manifested by the appearance of pseudomonoclimacis latilobus and slovinograptus balticus closely postdates the end of the isotope excursion but pre dates the first appearance of the conodont index ozarkodina snajdri here the graptolite recovery was delayed relative to the recovery of the benthic fauna a canalized intraformational limestone conglomerate corresponds with a gap in the sedimentary record above the kozlowskii extinction and just below the graptolite recovery the benthic faunas from the conglomerate matrix and pebbles permit correlation with the shallower part of the basin indicating a distinct fall in sea level the present data demonstrate the coincidence of the graptolite crisis with benthic faunal change and eustatic fall in sea level manifested by facies change and the carbon isotope excursion polonograptus chlupaci sp nov from the nc kozlowskii biozone is described and several other graptolite taxa are redescribed

integrated sedimentary biotic and paleoredox dynamics from multiple localities in southern laurentia during the late silurian ludfordian extinction event abstract the silurian was a time of major climatic transition punctuated by multiple biotic crises and global carbon cycle perturbations the most severe of these biotic events was the late silurian ludfordian lau kozlowskii extinction event lke and the associated lau carbon isotope excursion cie although the extinction event and lau cie are globally documented the only records thus far of local and global marine paleoredox conditions through this interval are from a single region in scandinavia here we examine four sections along a bathymetric transect of mixed carbonate siliciclastic sediments from western tennessee usa a novel approach using a multi proxy dataset combining high resolution geochemical data and microfacies analyses from multiple localities explores the possibilities of local regional scale redox heterogeneities during a time of widespread environmental upheaval on a global scale paired positive excursions recorded in carbonate carbon isotopes and carbonate associated sulfate sulfur isotopes support recent work from carbonate and siliciclastic successions from scandinavia suggesting a global enhancement of organic carbon and pyrite burial driven by an expansion of euxinic anoxic and sulfidic water column conditions in the oceans during the mid ludfordian furthermore positive excursions in organic carbon isotopes and pyrite sulfur isotopes reflect the global changes in redox stratigraphic trends in i ca ratios imply a local expansion of low oxygen conditions with low but non zero values during the rising limb and peak of the cie the fossil assemblages vary across the shelf and through the cie interval stratigraphic changes in fossil assemblages and i ca are closely associated with local and global changes in oxygenation and sea level during the mid ludfordian the collective data indicate significant biotic reorganization in response to changes in marine redox conditions and in conjunction with sea level variation during the lke interval but detailed macroscopic biodiversity is currently unconstrained for this region

carbon isotope excursion near the wenlock ludlow silurian boundary in the anglo welsh area oxygen and carbon isotope analyses have been performed at close sampling intervals on rocks from several exposures that show the boundary between the wenlock and ludlow series in the classic silurian areas of wales the welsh borderland and the west midlands of england our data indicate a monotonic decline in 13 c from positive to negative values across the boundary between the much wenlock limestone formation wenlock series and the lower elton formation ludlow series and correlatives in all sites investigated a slightly earlier negative excursion is also present in the upper wenlock strata of two of these localities both negative 13 c excursions appear to be contemporaneous with episodes of increased ocean ventilation and eustatic sea level lowstand postulated by kemp at least one and possibly both of the late wenlock carbon isotope depletions can also be related to the decline in pelagic graptolite diversity the distribution of the carbon isotope depletion in both carbonate platform and off shelf facies implies that this trend is unlikely to have been caused by local diagenetic alteration isotope chemostratigraphy may also provide an extra correlation tool that can be integrated with traditional biostratigraphic methods for silurian strata

velocity model building from seismic reflection data by full waveform inversion full waveform inversion is re emerging as a powerful data fitting procedure for quantitative seismic imaging of the subsurface from wide azimuth seismic data this method is suitable to build high resolution velocity models provided that the targeted area is sampled by both diving waves and reflected waves however the conventional formulation of full waveform inversion prevents the reconstruction of the small wavenumber components of the velocity model when the subsurface is sampled by reflected waves only this typically occurs as the depth becomes significant with respect to the length of the receiver array this study first aims to highlight the limits of the conventional form of full waveform inversion when applied to seismic reflection data through a simple canonical example of seismic imaging and to propose a new inversion workflow that overcomes these limitations the governing idea is to decompose the subsurface model as a background part which we seek to update and a singular part that corresponds to some prior knowledge of the reflectivity forcing this scale uncoupling in the full waveform inversion formalism brings out the transmitted wavepaths that connect the sources and receivers to the reflectors in the sensitivity kernel of the full waveform inversion which is otherwise dominated by the migration impulse responses formed by the correlation of the downgoing direct wavefields coming from the shot and receiver positions this transmission regime makes full waveform inversion amenable to the update of the long to intermediate wavelengths of the background model from the wide scattering angle information however we show that this prior knowledge of the reflectivity does not prevent the use of a suitable misfit measurement based on cross correlation to avoid cycleskipping issues as well as a suitable inversion domain as the pseudo depth domain that allows us to preserve the invariant property of the zero offset time this latter feature is useful to avoid updating the reflectivity information at each non linear iteration of the full waveform inversion hence considerably reducing the computational cost of the entire workflow prior information of the reflectivity in the full waveform inversion formalism a robust misfit function that prevents cycle skipping issues and a suitable inversion domain that preserves the seismic invariant are the three key ingredients that should ensure well posedness and computational efficiency of fullwaveform inversion algorithms for seismic reflection data

eutrophication by decoupling of the marine biogeochemical cycles of c n and p a mechanism for the late devonian mass extinction the late devonian mass extinction was unusually protracted and ecologically selective with preferential diversity losses among reefbuilding organisms and tropical shallow water faunas in general we have investigated the link between the extinction s unique characteristics and changes in biogeochemical cycling through analyses of the 13 c and c n p atomic ratios of organic matter buried across the kellwasser horizons in western new york state each horizon is characterized by 1 a long term 4 5 excursion in 13 c 3 of which occurs within the horizon and 2 a dramatic increase in the burial ratios of c n p from values of 100 15 1 to an average of 5000 170 1 on the basis of these results we propose that 1 increased efficiency of biolimiting nutrient recycling resulting from cyclic water column stratification and mixing promoted eutrophication during kellwasser deposition in new york and 2 the isotope excursions represent the composite effect of long term global organic c burial and local changes in photosynthetic c isotope fractionation related to nutrient availability this eutrophication model forges a mechanistic link between proposed late devonian climatic cooling and the selective demise of taxa likely to have been narrowly adapted to oligotrophic conditions

hirnantian ordovician through wenlock silurian conodont biostratigraphy bioevents and integration with graptolite biozones cape phillips formation slope facies cornwallis island canadian arctic islands cornwallis island in the canadian arctic archipelago provides one of the world s best areas for establishing an integrated graptolite conodont biozonation for the late ordovician silurian given the well exposed interfingering relationship of the basinal shale and carbonate platform facies building on earlier graptolite work 180 samples were collected from nine sections of the cape phillips formation of which 118 yielded approximately 7600 conodont elements representing 54 species in 25 genera including one new genus and species mayrodus melchini the conodonts are well preserved with minimal thermal alteration colour alteration index cai 1 nine conodont zones are recognized and defined for this region namely the amorphognathus ordovicicus distomodus kentuckyensis aspelundia expansa as fluegeli and pterospathodus celloni interval zones pt a amorphognathoides taxon range zone pt pennatus procerus and kockelella ranuliformis highest occurrence zones and k ortus absidata taxon range zone an integration of graptolite and conodont zones is documented earlier paleontological and isotope geochemical studies have demonstrated the dynamic nature of the silurian ocean climate system and identified major faunal turnovers or events five of these are recognized in the cornwallis fauna and related to oceanographic and climate changes in part referring to recent oxygen isotope data from conodonts from the cornwallis collections hirnantian extinction event sandvika event late aeronian llandovery snipklint primo episode early telychian llandovery ireviken event late telychian llandovery sheinwoodian wenlock and mulde event early homerian wenlock

near surface velocity estimation using source domain full traveltime inversion and early arrival waveform inversion accurate estimation of near surface velocity is a key step for imaging deeper targets we present a new workflow to invert complex early arrivals in land seismic data for near surface velocities this workflow is composed of two methods source domain full traveltime inversion fti and early arrival waveform inversion ewi source domain fti automatically generates the background velocity that kinematically matches the reconstructed plane wave sources from early arrivals with true plane wave sources this method does not require picking first arrivals for inversion which is one of the most challenging and labor intensive steps in ray based first arrival traveltime tomography especially when the subsurface medium contains low velocity zones that cause shingled multi value arrivals moreover unlike conventional born based method source domain fti can determine if the initial velocity is slower or faster than the true one according to the gradient sign in addition the computational cost is reduced co

wave equation migration velocity analysis i theory we present a migration velocity analysis mva method based on wavefield extrapolation similarly to conventional mva our method aims at iteratively improving the quality of the migrated image as measured by the flatness of angle domain common image gathers adcigs over the aperture angle axis however instead of inverting the depth errors measured in adcigs using ray based tomography we invert image perturbations using a linearized wave equation operator this operator relates perturbations of the migrated image to perturbations of the migration velocity we use prestack stolt residual migration to define the image perturbations that maximize the focusing and flatness of adcigs our linearized operator relates slowness perturbations to image perturbations based on a truncation of the born scattering series to the first order term to avoid divergence of the inversion procedure when the velocity perturbations are too large for born linearization of the wave equation we do not invert directly the image perturbations obtained by residual migration but a linearized version of the image perturbations the linearized image perturbations are computed by a linearized prestack residual migration operator applied to the background image we use numerical examples to illustrate how the backprojection of the linearized image perturbations i e the gradient of our objective function is well behaved even in cases when backprojection of the original image perturbations would mislead the inversion and take it in the wrong direction we demonstrate with simple synthetic examples that our method converges even when the initial velocity model is far from correct in a companion paper we illustrate the full potential of our method for estimating velocity anomalies under complex salt bodies

early silurian paired δ13ccarb and δ13corg analyses from the midcontinent of north america implications for paleoceanography and paleoclimate abstract paired 13ccarb and 13corg samples from well preserved marine carbonates of the wayne formation in central tennessee were analyzed to test a previously proposed model of relative changes in atmospheric pco2 during the well known ireviken early wenlock positive carbon isotope 13ccarb excursion our investigation provides the first detailed 13corg stratigraphy through this interval and documents a negative excursion in 13corg associated with the positive 13ccarb excursion this is consistent with the notion that the ireviken positive 13ccarb excursion was associated with elevated global temperatures that began during an icehouse greenhouse transition although our data are consistent with predictions made by the silurian oceanographic model jeppsson l 1990 an oceanic model for lithological and faunal changes tested on the silurian record j geol soc lond 147 663 674 a link to global changes in atmospheric pco2 remains tenuous until it can be shown that the 13corg trends shown herein can be reproduced globally

upper ordovician mohawkian carbon isotope δ13c stratigraphy in eastern and central north america regional expression of a perturbation of the global carbon cycle abstract the guttenberg carbon isotope excursion gice documented from eastern north america demonstrates the effects that regional geochemically distinct water masses upwelling and ocean circulation have on the carbon isotope record from carbonate platforms late turinian chatfieldian carbonates from oklahoma kentucky virginia and west virginia record a positive carbon isotope excursion 3 0 the gice excursion the gice excursion has relations to established biostratigraphy beginning in the north american midcontinent phragmodus undatus conodont zone and continuing through the plectodina tenuis zone sequence and event stratigraphy previously established models for positive carbon isotope shifts on carbonate platforms have been tested during the gice excursion where geochemically distinct water masses are defined for the upper ordovician a major eustatic sea level rise before the gice promoted a greater exchange of open ocean waters onto the carbonate platform of laurentia however restricted or sluggish circulation and exchange between water masses within the epeiric seas and the adjacent iapetus ocean were still apparent local variations documented in the gice excursion are directly related to upwelling of nutrient rich isotopically light waters increased primary productivity and the subsequent organic carbon production and burial

palynology organic geochemistry and carbon isotope analysis of a latest ordovician through silurian clastic succession from borehole tt1 ghadamis basin southern tunisia north africa palaeoenvironmental interpretation abstract palynological and palynofacies analyses combined with organic carbon isotope measurements have been performed on terminal ordovician through silurian clastic sediments from the north african margin of gondwana southern tunisia a first carbon isotopic signal 13corg from gondwanan silurian sedimentary sequences is presented showing interesting correlation with existing coeval isotopic curves from other areas changes in lithology palynofacies characteristics palynomorph diversity carbon isotope developments and organic geochemistry parameters appear to be all causally linked and to reflect changes in palaeoceanographic conditions in particular the detailed chronostratigraphic correlation and the observed similarities in patterns of carbon isotopic developments and palyno and litho facies changes through the study section has permitted to identify the supposedly global earliest wenlock ireviken event and late ludlow lau event isotopic excursions for the first time in high latitude gondwana this confirms that these excursions were linked to global changes in the oceanic system the present results suggest that from rhuddanian to early wenlock times an extended period of black shale deposition occurred over the north african gondwanan margin progressively transgressing from basin palaeodepressions to basin palaeohighs palynofacies and organic geochemistry support a coastal upwelling promoted productivity increase during this interval associated to a decrease in diversity of the marine microplanktonic communities the earliest wenlock strong positive isotopic shift appears associated with this protracted period of massive black shale deposition and thus of organic carbon burial on continental platforms located in high latitude settings this could well explain the apparent paradox between excessive carbonate deposition simultaneous to a carbon isotopic shift towards lighter values observed in low palaeolatitude localities laurentia and baltica the strong distinct isotope excursion occurring in late ludlow times is possibly linked to the well know lau event evidence for extensive organic carbon burial is lacking to explain this strong excursion some significant changes in the marine palynomorph communities are recorded in connection to the increase in stable isotope values 13corg additionally a strong correlation between abundance of terrestrially derived palynomorphs miospores and 13corg development is recorded which seems to support increased clastic input from a terrestrial source

carbon isotope stratigraphy across the silurian devonian transition in podolia ukraine evidence for a global biogeochemical perturbation the carbon and oxygen isotope composition of marine carbonates 13 c and 18 o respectively are studied in the fossiliferous stratigraphically well constrained and remarkably expanded successions of podolia sw ukraine spanning the silurian devonian transition significant isotopic shifts are directly comparable to previously published global secular trends in well preserved brachiopod calcite isotopic ratios from this region and therefore may be taken as a reliable primary record of seawater 13 c changes the sections reveal a major positive 13 c excursion with an amplitude above 6 beginning in the upper pridoli and reaching peak values as heavy as 4 2 in the lowermost lochkovian this turnover in carbon cycling is followed by a general trend toward more negative 13 c values in the upper lochkovian the podolian isotopic signals provide strong support for the previously inferred global biogeochemical perturbation across the silurian devonian transition reflecting a complex combination of palaeogeographical biogeochemical and evolutionary processes in the late caledonian geodynamic setting with a likely undervalued role of the expanding vegetation in vast near coastal shallows and deltas

a global biogeochemical perturbation across the silurian devonian boundary ocean continent biosphere feedbacks abstract the large scale global biogeochemical perturbation across the silurian devonian s d boundary recorded in the major positive excursion of the 13c time curve klonk event of an amplitude of 2 5 to 3 0 max 4 0 in europe and up to 5 0 in north america reflects a unique combination of palaeogeographic biogeochemical and evolutionary processes in the late caledonian geodynamic setting the steady sulfur isotopic ratios show an overall stability of the s d oceanic geochemical system as a whole and do not indicate any synchronous changes in anoxic deep oceanic and sediment processes therefore this led us to a hypothesis that the crucial changes that contributed to the recorded carbon cycling turnover are related to rapidly evolving ocean continent biosphere feedback coastal zones of the latest silurian epicontinental seas accumulated considerable quantities of organic carbon from early vascular vegetation which explosively expanded to inhabit vast near coastal shallows and deltas large primary production of these early terrestrial plants and rapidly enhanced sedimentary burial of organic carbon were also responsible for co2 drawdown which have resulted in reversed greenhouse effect and a global climatic cooling tendency this feedback was blocked when the sea level gradually dropped and led to shrinking of the silurian epicontinental seas and the growing climatic deterioration during the s d transition limited primary production furthermore continued processes of regressive abrasion and erosion limited the storage of organic carbon as well as the efficiency of the carbonate factory during the following early devonian greenhouse interval marine regression and active latest caledonian tectonism promoted progressive weathering of the sedimentary organic matter the oxidation of c rich deposits caused the subsequent growth of co2 levels in the early devonian culminating in the warming of global climate

vertebrate extinctions and reorganizations during the late silurian lau event during the silurian period as shown here by data from gotland sweden and other regions of the baltoscandian paleocontinent fish faunas were struck hard by extinctions caused by the late ludlow lau event also known for being associated with the largest positive stable carbon isotope excursion of the phanerozoic this event had a profound impact on the early evolutionary history of vertebrates wiping out two thirds of the fish taxa and causing significant ecological reorganizations over an estimated time span of similar to 200 ka our data show that immediately prior to the event jawed acanthodians dominated the fish faunas whereas the event led to a diverse fauna and a brief but marked dominance of the jawless thelodonts the stepwise changes observed mimic those of conodonts suggesting a similar mode of life and response to atmospheric oceanic perturbations for these clades less

a decade of tomography over the past 10 years ray based postmigration grid tomography has become the standard model building tool for seismic depth imaging while the basics of the method have remained unchanged since the late 1990s the problems it solves have changed dramatically this evolution has been driven by exploration demands and enabled by computer power there are three main areas of change first standard model resolution has increased from a few thousand meters to a few hundred meters this order of magnitude improvement may be attributed to both high quality complex residual moveout data picked as densely as 25 m to 50 m vertically and horizontally and to a strategy of working down from long wavelength to short wavelength solutions second more and more seismic data sets are being acquired along multiple azimuths for improved illumination and multiple suppression high resolution velocity tomography must solve for all azimuths simultaneously to prevent short wavelength velocity heterogeneity from being mistaken for azimuthal anisotropy third there has been a shift from predominantly isotropic to predominantly anisotropic models both vti and tti with four component data anisotropic grid tomography can be used to build models that tie pz and ps images in depth

carbon oxygen and strontium isotopes in paleozoic carbonate components an evaluation of original seawater chemistry proxies abstract brachiopods trilobites cements and whole rock matrix material were evaluated for their reliability as proxies of original seawater carbon oxygen and strontium isotope chemistry in this process coeval material was evaluated from individual horizons of formations spanning the permian to cambrian unaltered shells of low mg calcite articulated brachiopods retain original seawater oxygen carbon and strontium isotope compositions as old as late ordovician assessments of older specimens are hampered by a lack of suitable material coeval brachiopod whole rock sets a definitive assessment of cement marine chemistry is difficult due to the paucity of material but despite this caveat cements hold some promise for retaining original carbon and possibly strontium isotope seawater values in contrast the potential of whole rock material as a proxy of original seawater chemistry is quite complex in some but not all instances the carbon isotopic composition of whole rock after detailed scrutiny and evaluation appears to represent an original seawater chemistry signal in a few instances their oxygen isotope compositions reflect original seawater values unlike the stable isotope compositions the strontium isotopes of all studied whole rock material permian to upper cambrian appear not to reflect original seawater chemistry values thus the potential for retaining original seawater isotope chemistry and serving as specific proxies in order of decreasing reliability are 1 unaltered low mg calcite brachiopods c o and sr isotopes 2 pristine marine cements c and sr isotopes and 3 whole rock material c isotopes carbon oxygen and strontium isotope compositions of ordovician trilobites mirror those of coeval unaltered brachiopods cambrian trilobites hold significant promise as an important proxy of original seawater isotope chemistry but further studies are needed to ascertain their full potential the survey of isotopes in some ordovician and cambrian intermediate low mg calcite trilobites demonstrates their potential as an important proxy of original seawater chemistry

testing the limits of paleozoic chronostratigraphic correlation via high resolution 500 k y integrated conodont graptolite and carbon isotope δ13ccarb biochemostratigraphy across the llandovery wenlock silurian boundary is a unified phanerozoic time scale achievable the resolution and fidelity of global chronostratigraphic correlation are direct functions of the time period under consideration by virtue of deep ocean cores and astrochronology the cenozoic and mesozoic time scales carry error bars of a few thousand years k y to a few hundred k y in contrast most of the paleozoic time scale carries error bars of plus or minus a few million years m y and chronostratigraphic control better than 1 m y is considered high resolution the general lack of paleozoic abyssal sediments and paucity of orbitally tuned paleozoic data series combined with the relative incompleteness of the paleozoic stratigraphic record have proven historically to be such an obstacle to intercontinental chronostratigraphic correlation that resolving the paleozoic time scale to the level achieved during the mesozoic and cenozoic was viewed as impractical impossible or both here we utilize integrated graptolite conodont and carbonate carbon isotope delta c 13 carb data from three paleocontinents baltica avalonia and laurentia to demonstrate chronostratigraphic control for upper llandovery through middle wenlock telychiansheinwoodian similar to 436 426 ma strata with a resolution of a few hundred k y the interval surrounding the base of the wenlock series can now be correlated globally with precision approaching 100 k y but some intervals e g uppermost telychian and upper shein woodian are either yet to be studied in sufficient detail or do not show sufficient biologic speciation and or extinction or carbon isotopic features to delineate such small time slices although producing such resolution during the paleozoic presents an array of challenges unique to the era we have begun to demonstrate that erecting a paleozoic time scale comparable to that of younger eras is achievable less

strontium and carbon isotope stratigraphy of the llandovery early silurian implications for tectonics and weathering abstract a high resolution 87sr 86sr curve and paired 13c carbonate organic data set is generated for the llandovery series from the ikla drill core in estonia a 13c carbonate curve is also presented from the pancake range in nevada observed 87sr 86sr values in the ikla drill core are at a minimum in the early llandovery rhuddanian stage 0 7079 to 0 7080 and then trend to more radiogenic ratios in the basal part of the telychian stage an 87sr 86sr high near 0 7084 is observed in the telychian at the top of the studied section the range of values is in general agreement with the data from previous sample sets of brachiopods and conodonts recovered from localities in north america and europe that record a rising trend in the 87sr 86sr ratio throughout the llandovery from approximately 0 7080 to 0 7084 the major increase in the 87sr 86sr ratio during the late llandovery may be due to weathering of radiogenic source rocks that were uplifted during early silurian continent continent collisions the sr rise potentially coincides with the occurrence of an unusually thick sequence of k bentonite beds representing large magnitude ash falls in the early telychian a previously documented negative 13c excursion in marine carbonates in the lower telychian interval of the ikla core is quasi synchronous with the increase in 87sr 86sr our new organic matter 13c data from the ikla core confirm that this negative 13c carbonate excursion is not a result of diagenesis furthermore a negative 13c excursion in carbonates from the early telychian portion of the pancake range section in nevada seems to confirm the global scope of this carbon cycle perturbation

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

a late silurian extinction event and anachronistic period resurgence of microbialites and anachronistic facies in the aftermath of phanerozoic extinction events has hitherto only been documented among the major five based on the classical silurian carbonate platform strata of gotland sweden a minor biotic event is herein for the first time related to a substantial increase in anachronistic facies strata deposited during and shortly after the lau event in the baltic paleobasin eke and burgsvik formations include evidence for reduced grazing and infaunal activity in contemporaneous shallow low latitude seas e g flat pebble conglomerates normal marine stromatolites mass occurrence of oncoids and subtidal wrinkle structures e g kinneyia and microripples the latter structures have recently been suggested to be restricted to proterozoic cambrian and early triassic time and therefore are regarded as anomalous for the phanerozoic the co occurrence of these facies and global extinctions among pentamerid brachiopods and conodonts suggests that minor extinction events also resulted in anachronistic periods less devastating but fully comparable to those in the aftermath of the end ordovician and end permian mass extinctions a global positive stable carbon isotope excursion and the last appearance datum of the conodont polygnathoides siluricus may facilitate identification of the late silurian anachronistic period elsewhere less

shot and angle domain wave equation traveltime inversion of reflection data synthetic and field data examples abstractfull waveform inversion requires the accurate simulation of the dynamics and kinematics of wave propagation this is difficult in practice because the amplitudes cannot be precisely reproduced for seismic waves in the earth wave equation reflection traveltime tomography wt is proposed to avoid this problem by directly inverting the reflection traveltime residuals without the use of the high frequency approximation we inverted synthetic traces and recorded seismic data for the velocity model by wt our results demonstrated that the wave equation solution overcame the high frequency approximation of ray based tomography was largely insensitive to the accurate modeling of amplitudes and mitigated problems with ambiguous event identification the synthetic examples illustrated the effectiveness of the wt method in providing a highly resolved estimate of the velocity model a real data example from the gulf of mexico demonstrated these benefits of wt but also found the limitations in traveltime

carbon chemostratigraphy of the cambrian ordovician transition in a midlatitude mixed platform montagne noire france the cambrian drumian carbon isotope excursion dice and the steptoean positive carbon isotope excursion spice have been so far reported in subtropical platforms the dice event is a large negative excursion in 13ccarb record that nearly coincides with the beginning of the drumian cambrian series 3 whereas the spice event is a large positive shift in 13ccarb identified close to the base of the paibian furongian series the chemostratigraphic excursions have been recognized in carbonate dominated platforms and their application to mid and high latitude siliciclastic dominated platforms has been problematic this paper offers for the first time a high resolution stratigraphic analysis of this time span in a temperate water platform montagne noire france that recorded episodes of carbonate productivity in nearshore environments two sections were analyzed in detail a complete cambrian series 3 tremadocian succession devoid of carbonate interbeds and paleogeographically located in the distal part of the platform pardailhan nappe and a furongian tremadocian succession which has limestone interbeds and represents the proximal part of the platform minervois nappe the lower part of the la gardie formation in the pardailhan nappe displays background 13corg values of 22 punctuated by a large negative middle languedocian regional substage shift peaking at 24 5 this negative excursion is similar to the dice event reported close to the base of the drumian although the latter is exclusively based on 13ccarb values stable isotopes of 13ccarb and 13corg from both sections also indicate a common chemostratigraphic shift at the onset of the furongian in nearshore deposits of the minervois nappe the 13ccarb background values for the interbedded limestone strata of the val d homs formation gradually increase from 3 to 1 and are punctuated by a sharp increase in 13ccarb values to 3 0 by contrast in basinal shales of the la gardie formation in the pardailhan nappe the 13corg background values decrease from 22 to 27 1 this trend is directly controlled by total organic carbon toc contents and is interpreted to have been initiated by changes in the degree of biodegradation of organic matter sulfate reduction and methanogenesis before or soon after burial despite this overprinting and the very low degree of metamorphism reached by the shales the 13corg background trend is also punctuated by a sharp positive shift in 13corg values to 4 0 similar to the spice event in addition the asymmetric shape of the spice excursion in the montagne noire shows evidence for the herb event a late furongian negative carbon isotope excursion not yet accepted as a worldwide chemostratigraphic anomaly the recognition of both chemostratigraphic shifts in mixed and clayey strata opens new possibilities of chemostratigraphic correlation in mid and high latitude platforms where carbonate factories did not widely develop and strata only contain endemic fauna this implication has a major consequence because the global stratotype section and point gssp of the drumian and paibian stages occurs near the base of both excursions

carbonate platform evolution and conodont stratigraphy during the middle silurian mulde event gotland sweden evidence from sedimentology and conodont biostratigraphy is used to reinterpret the mid homerian late wenlock succession on gotland sweden a new conodont zonation includes from below the ozarkodina bohemica longa zone including five subzones the kockelella ortus absidata zone and the ctenognathodus murchisoni zone two taxa are named ozarkodina bohemica longa and pseudooneotodus linguicornis these new zones are integrated with facies in order to correlate strata and infer the major depositional environments and the controls on deposition during the mid homerian mulde event reef associated and skeletal carbonate deposition predominated before and after the event i e during the uppermost o s sagitta zone and again in the c murchisoni zone these periods are characterized by the expansion of reefs and shoal facies across marls in the topmost slite group on eastern gotland and in the lower parts of the klinteberg formation on western gotland respectively the intervening o b longa and k o absidata zones are initially characterized by rapid facies changes including siliciclastic deposition and later stabilisation of a carbonate depositional system the composition of sediments and depositional rates are closely related to the creation and destruction of accommodation space and reflects a classical case of depositional bias of the carbonate and siliciclastic depositional systems based on coastline migration stratal boundaries and the stratigraphic position of major reef belts several facies associations can be fitted into a sequence stratigraphic model for platform evolution a highstand systems tract hst situation prevailed prior to and during the early part of the event the upper slite group including the lower frjel formation this hst was characterized by prolific skeletal production and regional reef development except for during the latest stage when carbonate production declined at the onset of the mulde event platform growth was inhibited during a following regressive systems tract rst when regional siliciclastic deposition predominated the gannarve member the subsequent lowstand resulted in regional emersion and karstification i e a complete termination of the platform the post extinction transgressive systems tract tst is exclusively composed of non skeletal carbonates the bara member of the halla formation re occurrence of reefs and a prolific skeletal production marks platform recovery during a second hst the remaining halla and the lower klinteberg formations integration of high resolution biostratigraphy and sequence stratigraphy reveals that the major physical control on platform evolution was a 5th order eustatic sea level change during an early part of the mulde event and that the bulk of the strata accumulated when the platform aggraded and prograded during the highstand systems tracts thus silurian oceanic events and associated sea level changes had profound impact on the neritic carbonate system the gotland based middle and late homerian sea level curve shows two rapid regressions both leading to truncation of highstand systems tracts the first lowstand occurred at the very end of the c lundgreni chron and the second at the end of the co ludensis chron the intervening interval was characterized by stillstand or possibly slow transgression

evidence for rapid environmental changes in low latitudes during the late silurian lau event the burgen 1 drillcore gotland sweden erosional outliers are an important component of the ludlow late silurian stratigraphy of gotland sweden however due to the poor degree of exposure outcrop studies have not revealed a detailed section from any of these outliers the first complete stratigraphy of such an erosional remnant the burgen outlier is presented here based on the recently recovered burgen 1 drillcore stratigraphical units encountered are from oldest to youngest the uppermost hemse group 35 m eke formation 0 4 m burgsvik formation 3 4 m and the basal hamra formation 12 in preserved based on carbon stable isotopes and lithological correlation to nearby outcrops that are zoned by conodonts it can be concluded that the most profound late ludlow sedimentary changes took place shortly after the onset of the globally recognized lau event delta 13 c values reach 8 7 parts per thousand in the lower hamra formation sedimentary changes include development of discontinuity surfaces widespread occurrence of an intraclastic conglomerate increased microbial activity rothpletzella and wetheredella and influx of clastics to the basin these changes are linked to substantial base level changes during the event a general comparison with the prague basin suggests contemporaneous sedimentary changes on baltica and on cratonic elements from peri gondwana during the event

carbon isotope event markers through the wenlock pridoli sequence at ohesaare estonia and priekule latvia abstract the ohesaare borehole in the northern part of the silurian baltic gulf penetrates shallow and deep shelf marlstones limestones and dolostones the priekule borehole in the deep shelf belt of the basin was drilled through marlstones and graptolitic mudstones most of the successions except the upper part are characterized by a high sr ca ratio close to marine calcites suggesting good preservation of carbonate material conodont alteration indices are low 1 2 carbon isotopes were measured using whole rock samples in the ohesaare core section two distinctive 13 c positive excursions were established one in the early wenlock riccartonensis and low rigidus ellesae standard graptolite zones 4 2 and the second in the late wenlock nassa zone 4 6 before and after the latter there are also intervals with low 13 c values the first just above the lundgreni zone 1 8 and another in the ludensis perhaps reaching into nilssoni zone 3 8 in the priekule section the early wenlock 13 c peak is well represented 3 1 but in the late wenlock early ludlow 13 c values show little variation from 2 0 to 1 in this section a high peak of 13 c values 5 9 was determined in the nova beds correlated with the topmost gorstian and lowermost ludfordian ludlow the carbon isotope positive excursions show good correlation with lows of the global sea level curve and with most of the biotic and supposed climatic events glaciation ocean state

middle palaeozoic extinction events faunal and isotopic data abstract at least 9 and possibly as many as 12 extinction events of global or near global impact can be discriminated in the mid palaeozoic earliest silurian to early carboniferous on the basis of brachiopod coral conodont and ammonoid data and the history of carbonate build ups isotopic data from whole rock samples are presented for three of these events based on australian carbonate sequences constrained by conodont data these data represent the initial phase of a more extensive investigation using c o s sr and nd isotopic signatures derived from conodonts articulate and inarticulate brachiopod shell and fish remains from numerous australian and european sequences the aim of the project is to identify isotopic responses to extinction events and address causes for these changes in limestone sections analysed so far variations in carbon isotope compositions on a whole rock scale are most marked at horizons that can be correlated with times of significant reduction in biomass and diversity this is despite the fact that the whole rocks are in fact multicomponent systems with respect to carbon in part arising from diagenetic rearrangements of carbon distribution within the scale of the whole rock thus the carbon isotope data do provide evidence on a whole rock scale for fundamental changes in the global carbon cycle that are correlative with extinction events it is however unlikely that the magnitude of the isotopic shift will be precisely documented from such whole rock analyses the oxygen isotope results are less obviously related to the times of carbon isotope excursion this contrast in apparent resilience to post depositional modification of pristine isotope compositions exemplifies the extreme caution needed in evaluating isotopic data from carbonates

late cretaceous reactivation of the norumbega fault zone maine evidence from apatite fission track ages apatite fission track ages from opposite sides of the norumbega fault zone in southern maine reveal a significant early cretaceous time temperature discontinuity across the structure ages west of the fault zone range from 113 to 89 ma while those to the east are significantly older ranging from 159 to 140 ma modeling of track length distributions reveals that at 120 ma an 55 c difference in temperature existed between opposite sides of the fault zone currently exposed at the surface this discontinuity is best explained by an episode of significant to 2 km of net vertical separation east side down vertical movement along the norumbega fault zone in late cretaceous time the modeling suggests contemporaneous cooling and that final movement along the fault zone occurred ca 80 ma these results when combined with earlier structural and thermochronological studies suggest that faults mapped within the norumbega fault zone were intermittently active from middle devonian to late cretaceous time and that the structure is a fundamental zone of crustal weakness the prolonged period of activity likely reflects stresses associated with late paleozoic dextral transgression early mesozoic rifting associated with the initial opening of the north atlantic ocean basin and processes associated with the late mesozoic uplift and exhumation of the northern appalachian orogen

contrasting thermal histories across the flying point fault southwestern maine evidence for mesozoic displacement 40 ar 39 ar analysis of hornblende muscovite biotite and k feldspar across the flying point fault in southwestern maine reveals important information on the timing of thermal events and subsequent cooling history of the region the flying point fault is the most significant structure associated with the norumbega fault zone in southwestern maine it forms the boundary between two different lithotectonic sequences offsets metamorphic isograds and marks a remarkable time temperature discontinuity southeast of the flying point fault in the casco bay group early carboniferous hornblende ages and late carboniferous muscovite and biotite ages reflect slow cooling following middle to late devonian acadian metamorphism and deformation no significant 300 c late paleozoic thermal event affected these rocks in contrast rocks northwest of the flying point fault were affected by a significant 500 c late paleozoic alleghanian thermal event as indicated by latest carboniferous permian hornblende cooling ages this represents the northernmost occurrence of alleghanian high grade metamorphism reported in the appalachian orogen cooling below muscovite and biotite closure temperatures northwest of the flying point fault did not occur until the early triassic the thermochronologic data indicate that the rocks currently juxtaposed across the flying point fault underwent drastically different thermal histories during permian and triassic time approximately 4 km of post paleozoic west side up displacement along the flying point fault can account for the presently observed time temperature discontinuity

evaluation of the salinic i tectonic cancaniri glacial and ireviken biotic events biochemostratigraphy of the lower siluriansuccession in the niagara gorge area canada and u s a abstract well preserved brachiopods from the niagara gorge area anticosti island britain gotland and estonia were utilised to delineate a complex isotopic evolution for llandovery wenlock seawater the sr isotope record reflects the salinic i tectophase of the late llandovery in the continuous increase in 87sr 86sr values from 0 708070 to 0 708346 the salinic ii tectophase is marked by relative constancy of sr isotope values until the late wenlock when it rises from 0 708345 to 0 708430 the second tectonic phase was therefore likely only of a regional nature the carbon isotopes during the llandovery fall within a band of about 1 to 3 a range comparable to modern low latitude brachiopods a large positive 13c excursion of about 3 identifies the ireviken event excursion characterizes the early wenlock the biotic crisis and the isotope excursion itself may be ultimately related to the onset and duration of the cancaniri glaciation although a direct causative scenario is as yet unknown the oxygen isotopic trends of well preserved brachiopods clearly reflect a warm climate interval during the latest llandovery associated with the silurian sea level highstand subsequently in the early wenlock the sea level fell with the onset of the cancaniri glaciation in the southern hemisphere this is reflected in a significant positive 18o excursion particularly in brachiopods from the niagara gorge area brachiopods from lower latitudes were awash in warm tropical currents and therefore exhibit somewhat more negative 18o values indicating a lesser cooling gradient

wave equation traveltime inversion this paper presents a new traveltime inversion method based on the wave equation in this new method designated as wave equation traveltime inversion wt seismograms are computed by any full wave forward modeling method we use a finite difference method the velocity model is perturbed until the traveltimes from the synthetic seismograms are best fitted to the observed traveltimes in a least squares sense a gradient optimization method is used and the formula for the frechet derivative perturbation of traveltimes with respect to velocity is derived directly from the wave equation no traveltime picking or ray tracing is necessary and there are no high frequency assumptions about the data body wave diffraction reflection and head wave traveltimes can be incorporated into the inversion in the high frequency limit wt inversion reduces to ray based traveltime tomography it can also be shown that wt inversion is approximately equivalent to full wave inversion when the starting velocity model is

the late cambrian spice δ13c event and the sauk ii sauk iii regression new evidence from laurentian basins in utah iowa and newfoundland carbon isotope data from upper cambrian sections in three laurentian basins in northern utah central iowa and western newfoundland record a large positive d 13 c excursion spice event of up to 1 5 peak d 13 c ratios are well dated by trilobite collections to the middle of the steptoean stage dunderbergia zone and occur during maximum regression associated with formation of the sauk ii sauk iii subsequence boundary on the north american craton max imum regression was marked by an influx of quartz sand into carbon ate platform settings in all three widely separated basins in northern utah this quartz sand formed a thick sequence known as the worm creek quartzite which marks a conspicuous interruption of carbonate deposition during the middle to late cambrian in the region in west ern newfoundland the thickness of the quartz sand unit is much re duced but still marks a brief shutdown of the carbonate factory that is unique to the cambrian shelf succession of the area in the central iowa area of the cratonic interior an upward shallowing carbonate succession culminates in cross stratified trilobite grainstones at the peak of the spice in dunderbergia zone time and the lowest point on the relative sea level curve is associated with the occurrence of coarse quartz sand derived from the encroaching shoreface although it is difficult to determine precisely the departure from baseline d 13 c that marks the beginning of the spice excursion in the stratigraphic successions analyzed our results are consistent with a rise and subsequent fall in d 13 c tracking a major regressive transgressive event recorded across northern laurentia the correlation of a major d 13 c excursion with regression is similar to that described for the late ordovician for which the pattern has been attributed to either in creased carbonate relative to terrigenous weathering rates as ice sheets covered up organic matter containing silicates at high latitudes or high productivity and organic carbon burial driven by oceanic overturn the lack of known steptoean age ice sheets that could have affected the ratio of carbonate to silicate weathering rates suggests that organic carbon burial was the likely cause of the spice event we suggest that increased weathering and erosion rates during relative sea level fall sauk ii iii increased the burial fraction of organic carbon in an expanded region of fine grained siliciclastic deposits in shelf and upper slope environments during the steptoean

assessing mechanisms of environmental change palynological signals across the late ludlow silurian positive isotope excursion δ13c δ18o on gotland sweden abstract the silurian is characterised by strong environmental changes as indicated by several pronounced positive 13c and 18o excursions the mechanisms responsible for these isotopic shifts are a matter of much debate the purpose of the present study is a quantitative high resolution evaluation of the palynomorph distribution across the late ludlow ludfordian isotope excursion on gotland marine and terrestrial palynomorphs have been chosen as palaeoenvironmental indicators because they are widely distributed and exceptionally well preserved moreover the combined analysis of marine and terrestrial palynomorphs allows the reconstruction of contemporaneous palaeoenvironmental change in the marine and terrestrial realms to correlate our results with the carbon isotope stratigraphy we have sampled only localities with published isotope data from diagenetically unaltered brachiopod shells our results show that fluctuations in the composition of the acritarch assemblages are closely correlated with the stable isotope development low abundances of acritarchs occur in times of high stable isotope values and vice versa indicating that an increase in marine productivity cannot have been the reason for the positive 13c excursion the results are in good agreement with climatic models for the silurian assuming alternating humid and arid climatic conditions in low latitudes times of high stable isotope values correspond to arid climatic conditions in low latitudes with low input of terrestrial nutrients resulting in impoverished acritarch and conodont communities whereas times of low stable isotope values correlate with humid climate high nutrient input and abundant and diverse acritarch communities the major change in acritarch communities took place during the increase of the stable isotope values after hemipelagic planktonic or nektonic organisms graptolites conodonts have been affected this indicates that environmental changes connected with the stable isotope excursion first affected deeper water settings and later the photic zone the absolute abundance of terrestrial spores closely mirrors the marine 13c and 18o development showing that both the marine and the terrestrial realm were synchronously affected by the climatic changes the high abundance of spores during the isotope excursion might be explained by increased aeolian input

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

carbon and oxygen isotope composition of oxfordian early kimmeridgian belemnite rostra palaeoenvironmental implications for late jurassic seas abstract stable isotope analyses have been undertaken on oxfordian early kimmeridgian belemnite rostra derived from three palaeogeographic provinces of europe 1 the boreal subboreal province isle of skye in scotland 2 the submediterranean province central poland and southern germany and 3 the mediterranean province pieniny klippen belt in the western carpathians the samples have been screened for possible diagenetic alteration by using cathodoluminescence and trace element analyses and have been found to be largely well preserved calculated palaeotemperatures 10 15 c and analyses of belemnite habitats suggest that the boreal subboreal belemnites were living in a relatively shallow sea not deeper than 100 150 m while the other belemnites possibly inhabited deeper neritic environments despite the influence of a vital effect the 13c values of belemnite rostra are interpreted to reflect primary variations in the oceanic carbon isotope signal the fairly uniform submediterranean 13c values indicate a similar isotope composition of dissolved inorganic carbon dic in the area of sedimentation of sponge megafacies in the deep tethyan shelves the slightly lowered 13c values of some mediterranean belemnites might occur due to upwelling providing 12c enriched bottom waters onto the area of the czorsztyn ridge in the oxfordian the discernibly more positive 1 2 5 13c values of boreal subboreal belemnites are related to a high biologic productivity and a high organic matter burial in the partly isolated boreal sea the stated differentiation in carbonate 13c values distinctly exceeds present day differences in isotope composition of seawater dic

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

palaeozoic low oxygen high latitude carbonates silurian and lower devonian nautiloid and scyphocrinoid limestones of the anti atlas morocco abstract high latitude silurian and lowermost devonian basinal limestones of northern gondwana are characterised by mass accumulations of orthocone nautiloids bivalves and planktonic crinoids scyphocrinoids extensive exposures in the anti atlas of morocco reveal this basinal facies passes up gradient into an iron rich bioclastic facies associated with distal shelf siliciclastics and into crinoid brachiopod dominated bioclastic blankets and shoals the limestones formed in a cool water setting in which upwelling of nutrient rich waters during sea level lowstands is interpreted to have triggered the abundance of nautiloids and scyphocrinoids preservation of organic matter in under and overlying graptolite shales as well as iron precipitation in the neighbouring shallower water sub oxic zone the nautiloid limestones proved to be good correlation markers on a regional to global scale contemporaneous siliciclastic supply into the neighbouring facies belt as well as new biostratigraphic data are taken as evidence that these limestones can be correlated to 2nd order eustatic sea level lowstands and the following transgression the low oxygen basinal facies disappeared more or less abruptly during the early devonian pragian and was replaced by a well oxygenated basinal lowstand facies suggesting the end of upwelling at the same time the first reefal build ups coral biostromes and mud mounds appear in north africa indicating a significant rise in water temperatures the change from low oxygen lowstand mass accumulations to well oxygenated basinal facies with a concomitant rise in water temperature is in accordance with recent models of a cool silurian to earliest devonian lochkovian ocean with oscillating oxic to anoxic ventilated to poorly circulated oceanic states and a warm ocean with anoxic to euxinic poorly ventilated to stagnant ocean waters from the beginning of the pragian onwards

discovery of the ordovician millbrig k bentonite bed in the trenton group of new york state implications for regional correlation and sequence stratigraphy in eastern north america the ordovician chatfieldian millbrig k bentonite bed is a key stratigraphic marker horizon that is regionally synchronous over much of eastern and central north america this prominent marker is an independent source of correlation among the major chronostratigraphic and sequence stratigraphic units in this region the general stratigraphic position of the millbrig k bentonite has suggested to some authors that it is identical with the hounsfield k bentonite at dexter new york the traditional type area of the middle ordovician or mohawkian series in north america but previously available geochemical and biostratigraphical evidence has been insufficient to confirm this correlation analyses of apatites and melt inclusions in quartz phenocrysts from the millbrig k bentonite at eight localities in kentucky missouri virginia and pennsylvania and the hounsfield k bentonite at its type locality at dexter new york using high precision electron microprobe analysis techniques shows that the millbrig and the hounsfield have identical apatite and melt inclusion chemistry indicating a geochemical correlation this correlation is supported by conodont biostratigraphy and 13c isotope chemostratigraphy the new data demonstrate that the millbrig k bentonite and therefore the base of the chatfieldian stage of the north american mohawkian series by definition lies very close to the base of the traditional rocklandian stage of new york furthermore the millbrig k bentonite bed lies in close proximity to the base of the taconic supersequence over much of the midcontinent region and in particular lies just below the m5 sequence boundary recognized in kentucky and tennessee our results permit extension of the chatfieldian sequences into new york state and southern ontario and contributes to the resolution of the long standing uncertainty about the position of the base of the trenton group in ontario furthermore viewed in a broad context of regional stratigraphic relations we conclude that our results suggest that the persistent correlation difficulties reflect diachronous effects of widespread changes in oceanographic circulation patterns that emerged during taconic orogeny finally regional differences in the timing and character of sequence bounding surfaces and facies similarities summarized here suggest that the causes of relative sea level change during the chatfieldian may have been primarily tectonoeustatic mechanisms

sequence development on a sediment starved low accommodation epeiric carbonate ramp silurian wabash platform usa mid continent during icehouse to greenhouse transition abstract twelve cored wells through the silurian section of the wabash platform indiana u s a and 2 non cored basinal wells with cuttings were studied to define the sequence stratigraphy of a sediment starved ramp within a low latitude epeiric sea during the latest ordovician icehouse to silurian greenhouse transition the wabash platform approximately 200 000 km2 area was bounded to the north by the michigan basin to the east by the appalachian foreland basin and passed to the southwest into the vincennes basin which was open to the ocean facies developed include platform fringing buildups stromatactis wackestone core stromatoporoid skeletal wackestone caps and crinoidal rudstone packstone flank facies and non buildup crinoid grainstone packstone sheets shoals skeletal wackestone variably cherty between fair weather and storm wave base deep lagoon and deep ramp and carbonate mudstone clean to argillaceous sub storm wave base deep lagoon and deep ramp basin the eleven sequences 2 m to 30 m thick and 1 to 4 m y durations average 2 5 m y generally can be correlated to global sea level cycles the lower three sequences are disconformity bounded reflecting moderate ice sheets but the others are relatively conformable the most easily mapped regional surfaces are the transgressive surfaces because the correlative conformities are cryptic the wide well spacing prevented mapping of lowstand system tracts the transgressive systems tracts are upward deepening upward fining carbonate units that become more argillaceous and silty upward the highstand systems tracts are upward coarsening carbonate mudstone to wackestone packstone and rare grainstone barrier bank complexes and isolated buildups aggraded during deposition of the upper 4 sequences to form a discontinuous raised rim 40 m relief to the ramp while the interior of the platform remained sediment starved subsidence rates in the basin were very low 0 6 to 1 2 cm k y but the low platform accumulation rates 0 2 cm k y to 0 8 cm k y prevented the seafloor from building to sea level in all but the lower three sequences and later over barrier banks accumulation rates 2 cm k y parasequence development in high accommodation settings elsewhere in the silurian in north america records transition from moderate ice sheets to an ice free world this is weakly expressed on the wabash platform due to the dominantly deeper subtidal setting the remarkably low sedimentation rates resulted from the epeiric sea being isolated from the open ocean and they likely are typical of the large epeiric seas of the paleozoic

δ13c records across the late silurian lau event new data from middle palaeo latitudes of northern peri gondwana prague basin czech republic abstract during the late silurian the prague basin was located in middle southern latitudes in contrast to palaeocontinents positioned in tropical and subtropical latitudes like baltica no reefs are developed which is in accordance with the predicted cooler water the prague basin represents a relatively restricted and shallow rift basin with a complex tectonic history sections in different palaeoenvironments have been studied to document the most prominent silurian stable carbon isotope excursion recorded during the late silurian ludfordian lau event from this part of peri gondwana deeper water deposits of the kopanina formation investigated in the present study were deposited on the slope to basin transition near the kosov volcanic centre in the western part of prague basin the sediments are developed as an alternation of dark partly laminated limestones and marls with an increase of the limestone marl ratio in the upper part of the succession a pronounced positive carbon isotope excursion starts in the neocullograptus kozlowskii graptolite and in the upper polygnathoides siluricus conodont zone the maximum of the shift is observed in the lower part of an interval characterised by the ananaspis fecunda cyrthia postera community the maximum values scatter around 8 which represent the highest values reported hitherto from the prague basin in low latitudes often a decrease of 13c values towards deeper water settings is reported in contrast in the present study the 13c values of about 8 are much higher than those recorded from the contemporaneous shallow water sections studied in the classical muslovka and po ary quarries the most reasonable explanation is the presence of stratigraphical gaps in the shallow parts of the basin as indicated by karstification these gaps were caused by a sea level drop another effect of this sea level fall was a strongly reduced sedimentation of the cephalopod limestone facies around volcanic and tectonic elevations

paleoenvironmental changes in the silurian indicated by stable isotopes in brachiopod shells from gotland sweden ratios of stable carbon and oxygen isotopes in brachiopod shells more than 370 specimens esp atrypa reticularis from the silurian of gotland sweden have been analysed preservation of biological skeletal ultrastructures observed in sem micrographs and cathodoluminescence analyses indicate that usually no diagenetical alteration occurs the silurian of gotland consists of 440 m carbonate deposits spanning the late llandovery to late ludlow epochs 431 411 m y repeatedly uniform sequences of micritic limestones and marks are interrupted by complex structured reefs and adjacent platform sediments previously the alteration of facies is interpreted as the result of sea level fluctuations caused by a gradual regression with superimposed minor transgressive pulses the silurian sequence of gotland exhibits principally parallel carbon and oxygen isotope records corresponding closely to the topostratigraphic units lower values occur in periods dominated by deposition of marly sequences higher values are observed in periods dominated by reefs and extended carbonate platforms the isotope ratios are influenced by local as well as global factors the oxygen isotope ratios are interpreted to reflect paleosalinity changes due to varying freshwater input rather than changes in paleotemperature consequently the facies distribution of gotland is interpreted as resulting from changes in terrigenous input caused by different rates of continental weathering and freshwater runoff rather than by sea level fluctuations periods of arid climate and therefore anti estuarine downwelling of oxygenated surface water appear as short episodes of reef growing 1 5 m y in an epoche characterized by a tropic humid climate which causes an estuarine circulation and the upwelling of co2 rich deep water carbon isotope ratios are obviously connected to these changes in circulation by the advection of 13c rich surface water arid episodes or upwelling of 13c depleted deep water humid episodes of a silurian ocean which itself reveals generally euxinic deep water conditions due to the burial of organic carbon in black shales

traveltime information based wave equation inversion we propose a new wave equation inversion method that mainly depends on the traveltime information of the recorded seismic data unlike the conventional method we first apply a p transform to the seismic data to form the delayed shot seismic record back propagate the transformed data and then invert the velocity model by maximizing the wavefield energy around the shooting time at the source locations data fitting is not enforced during the inversion so the optimized velocity model is obtained by best focusing the source energy after a back propagation therefore inversion accuracy depends only on the traveltime information embedded in the seismic data this method may overcome some practical issues of waveform inversion in particular it relaxes the dependency of the seismic data amplitudes and the source wavelet

extinction and migration in silurian trilobites and conodonts of northwestern canada appearances and disappearances lineage turnover of trilobite and conodont evolutionary species clades in silurian shelf marginal sections of the mackenzie mountains are concentrated at particular stratigraphical levels a depth energy curve generated for this region shows three major and several minor transgressive regressive cycles in the llandovery and wenlock series a comparison of this curve with conodont and trilobite distributional data supports the thesis that there are cause effect relationships between changes in sea level and migration evolution and extinction events of these marine organisms the rate amount and direction of change in depth were important factors in the migration and extinction of clades the depth energy curve for the region fits the pattern expected for a passive downdropping shelf responding to global eustatic sea level changes emigrations and extinctions are associated with rapid transgressive episodes and concentrated at three levels lower llandovery lower wenlock and middle or lower part of the upper wenlock the early wenlock event was the most impressive a further concentration of extinctions and or migrations not considered here occurred later in the ludlow associated with a regional regression phyletic evolution is most convincingly demonstrated in fossil collections from intervals of gradual regression a model is proposed that predicts that a similar pattern of extinctions occurred around the silurian world in regions of similar palaeotectonic regime and that global extinction patterns of clades between episodes of mass extinction were not random episodes of high extinction turnover pulses were caused by perturbations of the environment foremost of which were rapid changes in the depth of the seas on the continental shelves because taxonomic loss during these extinction emigration episodes was generally at low below generic levels resolution of silurian turnover patterns requires data collected at the evolutionary species level

automatic velocity analysis via shot profile migration we described an implementation of differential semblance velocity analysis based on shot profile migration and illustrated its ability to estimate complex strongly refracting velocity fields the differential semblance approach to velocity analysis uses waveform data directly it does not require any sort of traveltime picking the objective function minimized by the differential semblance algorithm can measure either focusing of the image in offset or flatness of the image in scattering angle we showed that the offset variant of differential semblance yields somewhat more reliable migration velocity estimates than does the scattering angle variant and we explain why this is so we observed that inconsistency with the underlying model born scattering about a transparent background can lead to degraded velocity estimates from differential semblance and we showed how to augment the objective function with stack power to enhance ultimate accuracy a 2d marine survey over a target obscured by the lensing effects of a gas chimney provides an opportunity for direct comparison of differential semblance with reflection tomography the differential semblance estimate yields a more data consistent model flatter angle gathers than does reflection tomography in this application resulting in a more interpretable image below the gas cloud

velocity model building by waveform inversion of early arrivals and reflections a 2d case study with gas cloud effects joint full waveform inversion jfwi combines reflection waveform inversion rwi and early arrival waveform inversion ewi to build a large scale velocity model of the subsurface from long offset data the misfit function of jfwi requires an explicit separation between the short spread reflections and early arrivals the feasibility of which is illustrated with a real data case study jfwi is alternated with a waveform inversion migration of short spread reflections to provide a short scale impedance model this model is needed for building the sensitivity kernel of rwi along the two way reflection paths the large scale velocity macromodel built by jfwi can be used as the initial model for classical fwi to enrich the high wavenumber content of the subsurface model we present an application of this workflow to a real 2d obc profile across a gas cloud in the north sea to review its main promises and pitfalls viscoacoustic vti seismic modeling allows us to account for attenuation and anisotropy effects

carbon and oxygen isotopic composition of silurian brachiopods gotland sweden palaeoceanographic implications abstract one hundred and seventy two non luminescent brachiopod shells from the silurian sequence of gotland were investigated for their carbon and oxygen isotopic composition diagenetic alteration of the shell calcite was excluded by means of petrographic trace element and cathodoluminescence analysis oxygen and carbon isotopic signals vary considerably within the uppermost llandovery to the ludlow average 18o and 13c values of well defined stratigraphic units range from 2 4 to 5 9 and 1 to 8 respectively we observed parallel trends in the carbon and oxygen isotope curves with the most prominent shifts occurring in the lower wenlock upper wenlock and middle ludlow average 18o values 2 4 to 2 9 recorded in the middle ludlow are among the highest 18o values reported for the early palaeozoic the recorded 18o variations cannot solely be explained by changes in sea surface temperature and salinity the variations in the isotopic signals correlate negatively with changes in sea level heavy carbon and oxygen signals coincide with sea level lowstands whereas light signals are recorded during sea level highstands sea level highstands were probably characterized by the formation of warm saline waters on subtropical epicontinental shelves inducing oxygen deficiency in the basins alumshale facies the low 18o values are attributed to 18o depletion in surface waters owing to the storage of 18o enriched saline waters in the basins low 13c values are explained by a low surface water productivity since nutrients were not supplied to the photic zone but rather stored in the deeper ocean during sea level lowstands colder waters that formed in higher latitudes may have represented the main intermediate water source a better ventilation of the basins is documented by the deposition of well aerated sediments whereas the high 18o values are explained by the change in oceanic circulation the high 13c values are attributed to an enhanced primary productivity and possibly to an enhanced burial of corg the model discussed here may provide an attractive scenario for accounting for the recorded short term variations in the isotopic signals without the necessity of introducing an isotopic correction factor for silurian sea water

oxygen and carbon isotopic composition of silurian brachiopods implications for coeval seawater and glaciations we collected 236 calcitic brachiopod shells covering the entire silurian period 30 m y at high temporal resolution from stratotype sections from anticosti island canada wales united kingdom the oslo region norway gotland sweden and podolia ukraine estonia latvia and lithuania data from petrography scanning electron microscopy cathodoluminescence isotopes and trace elements all confirmed that there was excellent preservation in most shells thus arguing for retention of primary isotope signals exceptions were samples from the oslo region the d18o and d13c values for the well preserved samples range from 2 to 6 5 and from 1 to 7 5 peedee belemnite respectively in terms of temporal trends oxygen and carbon isotopes vary in parallel with a slight decrease with declining age of 1 through silurian time with superimposed short term oscillations that are negatively correlated with sea level changes three successive positive d18o shifts in early aeronian latest aeronian and early wenlock time correlate with sea level lowstands and with glacial diamictite deposits in the amazon basin and in africa the high d18o and d13c values are attributed to cold episodes with low sea levels and low values to warm episodes with high sea levels during warm silurian episodes the d18o value of seawater is suggested to have been about 3 5 standard mean ocean water smow and the global tropical temperatures of about 20 30 c similar to the present day values in summer during glacial episodes seawater is proposed to have had a d18o value of 2 5 smow and temperature of about 14 23 c comparable to tropical temperatures proposed for the last glacial episode 14 to 28 c three positive d13c peaks in early and late wenlock and late ludlow time likely of regional to global significance appear to coincide with sea level lowstands but we are as yet unable to propose a convincing causative geologic scenario that would explain their origin

assessing the role of orogen parallel faulting in post orogenic exhumation low temperature thermochronology across the norumbega fault system maine as a part of a regional effort to determine the extent of low temperature thermochronological discontinuities across major orogen parallel faults in northern new england 41 apatite fission track

velocity inversion by differential semblance optimization differential semblance optimization dso is an approach to inversion of reflection seismograms which avoids the severe convergence difficulties associated with nonlinear least squares inversion the method exploits both moveout and amplitude characteristics of reflections we have implemented a version appropriate to plane wave p tau seismograms and layered constant density acoustic earth models theoretical and numerical analyses of this version of dso indicate that stable and reasonably accurate estimates of both velocity trend and reflectivity can be derived to test dso further we applied it to a marine data set from the gulf of mexico where the method produced results which compare favorably to well log information the method can be extended to incorporate laterally heterogeneous velocity models

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

stable isotope geochemistry of the lower cambrian sekwi formation northwest territories canada implications for ocean chemistry and secular curve generation abstract three curves delineating 13 c variations across the early cambrian sekwi formation carbonate ramp mackenzie mountains northwest territories canada are incorporated into a biostratigraphic and sequence stratigraphic framework that provides a higher resolution temporal and spatial framework for the sekwi formation the three 13 c curves were correlated across a range of depositional environments from tidal flat to deep subtidal on the sekwi carbonate ramp eight 13 c cycles in the curves a h were correlated across the basin complimenting the sequence stratigraphic framework the most complete 13 c curve from the sekwi formation correlates well with a composite early cambrian 13 c curve from the siberian platform however the differing magnitude and absolute timing of some of the correlative isotopic excursions records the influence of regional tectonism sea level changes and diagenesis on the sekwi ramp additionally the sekwi formation 13 c curve has more high frequency variations than the siberian curve due in part to the higher sample density used to construct the sekwi curve the early middle cambrian boundary was not sampled in this study but a large negative isotopic excursion in the very latest early cambrian coincides with a regional drowning that may be correlative throughout western laurentia but recognizing this event globally is problematic

u pb monazite geochronology of granitic rocks from maine implications for late paleozoic tectonics in the northern appalachians crystallization ages of monazites from peraluminous granites and granitic pegmatites in southwestern maine constrain the timing of final ductile motion of a major shear zone to approximately 280 ma u pb monazite ages of granite at brunswick 278 pm 2 ma and evolved granitic pegmatites in topsham 275 269 ma provide evidence that synkinematic intrusion was followed by final emplacement at the end of ductile deformation these granitic rocks were evidently generated during the final high temperature event experienced by this portion of the appalachian orogen during this time the host rocks record initial rapid cooling c 16 c m y followed by slower cooling c 4 c m y into the mesozoic the similarity of ages of granite in brunswick and pegmatites in topsham permit the rocks to be related to the same magmatic system the spatially associated sebago batholith the largest exposed pluton in new england has a concordant u pb monazite age of 293 pm 2 ma thus the sebago batholith cannot be

wave equation migration velocity analysis ii subsalt imaging examples subsalt imaging is strongly dependent on the quality of the velocity model however rugose salt bodies complicate wavefield propagation and lead to subsalt multipathing illumination gaps and shadow zones which cannot be handled correctly by conventional traveltime based migration velocity analysis mva we overcome these limitations by the wave equation mva technique introduced in a companion paper and demonstrate the methodology on a realistic synthetic data set simulating a salt dome environment and a gulf of mexico data set we model subsalt propagation using wave paths created by one way wavefield extrapolation those wave paths are much more accurate and robust than broadband rays since they inherit the frequency dependence and multipathing of the underlying wavefield we formulate an objective function for optimization in the image space by relating an image perturbation to a perturbation of the velocity model the image perturbations are defined using linearized prestack residual migration thus ensuring stability relative to the first order born approximation assumptions synthetic and real data examples demonstrate that wave equation mva is an effective tool for subsalt velocity analysis even when shadows and illumination gaps are present

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

silurian δ13c stratigraphy a view from north america three positive shifts in 13 c are recognized in well dated carbonate successions in north america and can be confidently correlated with shifts in the baltic region by using conodont and graptolite zonations the 13 c excursions reach distinct peaks of 3 to 4 at the ends of the amorphognathoides and siluricus conodont zones and within the ludensis graptolite zone the three events are associated with the most significant extinction horizons in the silurian and occur near clastic carbonate transitions that mark eustatic or oceanographic changes the magnitudes of the two oldest 13 c peaks compare well with the baltic region however the end siluricus excursion reaches far greater values of as much as 11 in sweden likely reflecting local modification of the global seawater signature

the ireviken event in the lower silurian of gotland sweden relation to similar palaeozoic and proterozoic events abstract for a long time the silurian was thought to represent a time of stable environmental conditions in the greenhouse period that followed the late ordovician glaciation during the past decade knowledge about the silurian has increased markedly and today it is known that the conditions in the silurian were much more variable than previously assumed detailed isotopic investigations have revealed several distinct positive excursions in both carbon and oxygen isotope values in low latitudes these periods of high c and o isotope values are in many cases characterised by the growth of reefs and the formation of extended carbonate platforms the sediments deposited during these excursions contain impoverished fossil assemblages especially with respect to conodonts graptolites and trilobites a conspicuous isotope excursion coincident with facies changes and a marked mass extinction is observed near the llandovery wenlock boundary this event is called the ireviken event after its type locality on the island of gotland sweden jeppsson l 1987 in palaeobiology of conodonts ellis horwood ltd chichester pp 129 145 here isotope data from nine sections at the nw coast of gotland are presented that cover the time interval of the ireviken event the 13c mean values rise from 1 4 to 4 5 and the 18o values increase from 5 6 to 5 0 the relative timing of stable isotope development extinctions and facies development is discussed it is shown that first extinctions precede the isotope excursion this indicates that extinction events and stable isotope development are only indirectly connected but might reflect the same causes other events characterised by similar relationships between positive isotope excursions mass extinctions and facies development are found in younger parts of the silurian late wenlock and ludlow in the late ordovician the late cambrian and with some reservations in the proterozoic the similarities between these events indicate analogous controlling mechanisms for the silurian climatic changes between humid and arid conditions in low latitudes were postulated in an earlier study of the authors a palaeoceanographic climatic model was postulated which is consistent with most of the sedimentological palaeontological and geochemical data here we demonstrate that this model may be applicable also for the older events

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

elastic reflection traveltime inversion with decoupled wave equation elastic full waveform inversion fwi updates high resolution model parameters by minimizing the residuals of multicomponent seismic records between the field and model data fwi suffers from the potential to converge to local minima and more serious nonlinearity than acoustic fwi mainly due to the absence of low frequencies in seismograms and the extended model domain p and s velocities reflection waveform inversion can relax the nonlinearity by relying on the tomographic components which can be used to update the low wavenumber components of the model hence we propose an elastic reflection traveltime inversion erti approach to update the low wavenumber component of the velocity models for both p and s waves in our erti algorithm we take the p and s wave impedance perturbations as elastic reflectivity to generate reflections and a weighted cross correlation as the misfit function moreover considering the higher wavenumbers lower velocity value of the s wave velocity compared to the p wav