paleomagnetism of the silurian volcanism at almaden southern spain the silurian volcanism of the almaden mining district southern spain has been the object of a very detailed paleomagnetic study the 175 cores 25 sites sampled all along the eastern termination of the almaden syncline were subjected to magnetic susceptibility measurements thermomagnetic cycles progressive alternating field and thermal demagnetizations as well as microscopic observations the magnetic mineralogy of these volcanics flows is dominated by magnetite in at least two very distinct forms including small cubic distributed grains and large elongated crystals the observed anisotropy of magnetic susceptibility varies according to the relative abundance of the two magnetite phases showing a complete inversion of the susceptibility tensors this anisotropy has been related to the emplacement of the flows on a gentle slope dipping 10 toward the southwest implying that their volcanic source was located to the northeast the paleomagnetic analysis has revealed two characteristic components of magnetization the first direction d 109 i 11 95 9 pole position at 11 s 69 e dp 5 dm 9 yields a negative fold test and is interpreted as a late paleozoic partial overprint its declination is 46 away of the expected late paleozoic field direction which confirms that rotations are still occurring after the main folding phases of the variscan orogeny the second direction d 62 i 36 95 14 pole position at 8 n 118 e dp 9 dm 13 is prefolding as evident from the significant clustering when applying appropriate tectonic corrections and we interprete it as the original magnetization of silurian age the compilation of all paleozoic paleomagnetic results from spain reveals that the iberian meseta has been moving along with gondwana at high drift rates throughout ordovician and silurian times

palaeomagnetism of devonian ring complexes from the bayuda desert sudan new constraints on the apparent polar wander path for gondwanaland summary the approach of gondwana and or microplates derived from its northern margins towards laurentia and the closure of the intervening palaeo tethys is a controlling factor in the geodynamic evolution of the atlantic bordering continents during the early and mid palaeozoic however the currently available palaeomagnetic data for gondwana especially for the geodynamically critical silurian to devonian interval are still rather sparse and are compatible with a number of palaeogeographic scenarios in order to address this issue 114 samples 18 sites have been collected from a series of high level acidic intrusive rocks rhyolitic lavas and tuffs of mid devonian age 377 5 ma rb sr along the southern escarpment of the gilif hills sudan 17 83 n 32 67 e detailed demagnetization experiments revealed the presence of three directionally distinct components of magnetization labelled a b and c the overall mean direction of component a dec 0 9 inc 29 8 95 4 6 k 91 1 n 12 sites is indistinguishable from either the direction of the present field or the geocentric axial dipole field in northern sudan mixed polarities suggest a recent or pleistocene age of this component component b also displays mixed polarities the site mean directions for this component average to 6 5 declination 40 2 inclination with 95 7 0 and k 48 1 based on 10 sites and yield a palaeomagnetic pole at 48 8 s 23 5 e dp 5 1 1 dm 8 4 maghaemite and haematite have been identified to be the predominant magnetic phases in samples which carry component a and or b both a and b are the probable results of low temperature oxidation of magnetite although the age of component b can not be directly determined it is inferred by correlation with published data to reflect a late carboniferous overprint component c identified in 11 sites 39 samples is characterized by a very steep downward magnetization dec 296 2 inc 79 3 95 10 8 k 18 7 n 11 sites which corresponds to a palaeomagnetic pole position in the southwestern corner of libya 25 9 n 11 6 e dp 19 6 dm 20 6 maximum blocking temperatures of component c generally below 580 c are indicative of magnetite as carrier of this remanence the results of thermal remanent magnetization experiments demonstrate that component c is a thermo remanent magnetization probably primary and suggest this component to be representative for the direction of the earth s magnetic field during emplacement of the gilif hills volcanic rocks in mid devonian times the position of the resulting palaeomagnetic pole lends new support to geodynamic scenarios advocating the existence of a wide approx 5000 km pre hercynian ocean during devonian times the simplest reconciliation of component b with the reference apparent polar wander path is achieved by assuming it to be of carboniferous age and by invoking a westward cusp in the carboniferous part of the apparent polar wander path taken at face value the age of this cusp coincides with large scale post orogenic right lateral strike slip movements in hercynian europe and north africa this interpretation does not require the apparent polar wander apw loop and very high rates of plate motion implied by some alternative models however if it is valid it will require reinterpretation of the remanence age of one west african intrusive suite and of some eastern australian palaeozoic rocks

variscan tectonics of central europe postaccretionary intraplate deformation of weak continental lithosphere the area occupied by the variscan belt of central europe forms part of gondwana derived microplates essentially avalonia and armorica that according to paleomagnetic data juxtaposed baltica and laurentia from the upper ordovician to the lower devonian the actual structure of this area is a puzzle of rather small fault bounded crustal blocks different in p t t histories which was created by later tectonic processes at around 360 to 320 ma according to a common view and by contrast to paleomagnetic data the structural record of fault bounded blocks is interpreted in terms of mid devonian to lower carboniferous amalgamation of narrow continental plates represented by the fault bounded blocks themselves however this is not supported in this article instead it will be shown that fault systems and fault bounded blocks were created by intraplate deformation of a weak domain of gondwana derived continental lithosphere significantly after its accretion deformation was partitioned into newly formed deep seated strike slip and kinematically connected reverse or normal detachment systems large translation magnitudes caused juxtaposition of crustal areas of variable thickness until they were emplaced along the fault systems the various categories of fault bounded complexes record different tectonic events category 1 metamorphic complexes represent deep seated parts of thickened crust some time before unroofing on the detachment systems between about 340 360 and 320 ma however the record of subduction and extreme crustal thickening during earlier stages of the collision history was erased to a high degree and is only preserved by mineral and structural relics strike slip and detachment systems propagated across and dismembered tectonic boundaries formed during these early events category 2 metamorphic complexes and the barrandian basin recording predominant pre late devonian metamorphic or stratigraphic events represent the brittle upper crustal parts during continuing metamorphism in the category 1 complexes weakly or unmetamorphosed lower paleozoic category 3 basins represent areas of thinned continental crust and record subsidence before and to some degree during activity of detachment systems the complex overall kinematic pattern of block bounding strike slip and detachment systems indicates compression and extension acting simultaneously this probably reflects the effects of continuing oblique plate convergence rotation and wsw extrusion of fault bounded blocks in addition to heating and extensional flow of previously thickened crustal areas

eclogite facies rocks of the monotonous unit clue to variscan suture in the moldanubian zone bohemian massif abstract the formation and exhumation of variscan high to ultrahigh pressure metamorphic rocks from the moldanubian zone in the bohemian massif are the subject of controversy regarding their unclear relationship to possible subduction zones in this paper we present the results of a petrological study of eclogites that occur together with serpentinites within amphibolite facies gneisses in the moldanubian zone east of the tepla barrandian block more than 100 bodies of retrogressed eclogite and serpentinite follow an approximately 250 km long sw ne trending zone in the central part of the bohemian massif together with surrounding gneisses the eclogites share a medium to low pressure amphibolite facies metamorphism p t conditions estimated for the eclogite facies stage indicate a relatively low temperature geothermal gradient similar to those observed in the saxothuringian zone which occurs north and northwest of the tepla barrandian block the presence and distribution of eclogites and comparison of their p t and age data with hp uhpm rocks in other units in the bohemian massif allow us to constrain the variscan suture which straddles the se of the tepla barrandian block the existence of this suture in relation to available geotectonic models and its possible continuation through other allochthonous units along the european variscan belt are discussed

drowning unconformities palaeoenvironmental significance and involvement of global processes abstract drowning unconformities are stratigraphic key surfaces in the history of carbonate platforms they mostly consist in the deposition of deep marine facies on top of shallow marine limestones although large scale depositional geometries mimic lowstand systems track architecture these sedimentary turnovers are developed in relation with major sea level rise inducing an increase in the rate of creation of accommodation space that outpaces the capacity of carbonate to keep up this so called paradox of carbonate platform drowning implies that parameters other than purely eustatic fluctuations are involved in the demise of shallow marine ecosystems worldwide and at different times during earth history in depth studies of drowning unconformities revealed that changes in nutrient input clastic delivery temperature or a combination of them may be responsible for a decrease in light penetration in the water column and the progressive suffocation and poisoning of photosynthetic carbonate producers the examination of such case examples from various stratigraphic intervals and palaeogeographical settings thus helps in identifying and hierarchizing potential triggering mechanisms for drowning unconformities this is complemented by new data from early cretaceous successions from the helvetic alps during this time period the helvetic carbonate platform developed along the northern tethyan margin using both photozoan and heterozoan communities phases of healthy production were interrupted by several drowning episodes the latter are marked in the sedimentary record by condensation and associated phosphogenesis and glauconitisation from the earliest valanginian to the early to late barremian three drowning unconformities reflect the intermittent installation of a more humid climate and subsequent enhanced trophic conditions which first induced a switch from photozoan to heterozoan communities and then to long lasting drowning phases the latter encompass several sea level rise and fall cycles and may be linked to strengthened upwelling currents with the return to more oligotrophic conditions during the late barremian photozoan urgonian type communities took up again their development has been abruptly stopped at the end of the early aptian by a major emersion phase the subsequent drowning is documented in various peritethyan areas this initial crisis is followed by three other drowning phases that ultimately led to the replacement of shallow ecosystems by a deeper marine sedimentation in the cenomanian this long term trend in the evolution of the helvetic carbonate platform and of other peritethyan ecosystems may have been driven by more global phenomena in particular the progressive opening of the northern and equatorial atlantic may have impacted sea level by creating new oceanic basins the emplacement of submarine volcanic plateaus may have triggered sea level rise and fertilized deep oceanic waters through hydrothermal processes drowning unconformities thus record the interplay of local with long term processes and constitute regional sedimentary archives of global phenomena

the relationship between u s east coast sea level and the atlantic meridional overturning circulation a review scientific and societal interest in the relationship between the atlantic meridional overturning circulation amoc and u s east coast sea level has intensified over the past decade largely due to 1 projected and potentially ongoing enhancement of sea level rise associated with amoc weakening and 2 the potential for observations of u s east coast sea level to inform reconstructions of north atlantic circulation and climate these implications have inspired a wealth of model and observation based analyses here we review this research finding consistent support in numerical models for an antiphase relationship between amoc strength and dynamic sea level however simulations exhibit substantial along coast and intermodel differences in the amplitude of amoc associated dynamic sea level variability observational analyses focusing on shorter generally less than decadal timescales show robust relationships between some components of the north atlantic large scale circulation and coastal sea level variability but the causal relationships between different observational metrics amoc and sea level are often unclear we highlight the importance of existing and future research seeking to understand relationships between amoc and its component currents the role of ageostrophic processes near the coast and the interplay of local and remote forcing such research will help reconcile the results of different numerical simulations with each other and with observations inform the physical origins of covariability and reveal the sensitivity of scaling relationships to forcing timescale and model representation this information will in turn provide a more complete characterization of uncertainty in relevant relationships leading to more robust reconstructions and projections

dynamics of a stepped carbon isotope excursion ultra high resolution study of early toarcian environmental change abstract in the toarcian early jurassic 183 myr ago the ocean atmosphere system was subject to one of the largest perturbations of the carbon cycle in the last 250 myr known as the toarcian oceanic anoxic event t oae this event was accompanied by a 6 negative carbon isotope excursion cie caused by massive injection of isotopically light carbon into the ocean atmosphere system possibly from destabilisation of gas hydrates this study reveals the pacing and sequence of events leading up to the cie and to widespread deposition of organic rich sediments the very high resolution isotopic record from exceptionally well preserved carbonate and organic matter from the paris basin enables recognition of increased co 2 levels 130 kyr in advance of the major negative cie an accelerated increase in the p co 2 is registered 25 kyr before the onset of this negative excursion and was so rapid and so intense that it led to a water column undersaturated with respect to calcium carbonate in the paris basin undersaturation is expressed as a dramatic drop in the accumulation of the biogenic calcite produced by the surface dwelling calcifiers these environmental perturbations representing precursor phases of co 2 injection predate the first step towards relatively light carbon isotopes in carbonate and organic matter and are tentatively attributed to karoo ferrar magmatism this negative shift was registered slightly earlier in terrestrial carbon than marine carbonate subsequent global warming is credited with liberating isotopically light carbon and ultimately fostered anoxia in the paris basin the response of these cumulative inputs of carbon to the earth system isotopic and sedimentological evidence indicates continuously elevated phytoplanktonic productivity throughout the first step of the negative cie suggesting that the biological pump accelerated the drawdown of excess carbon leading to temporary recovery of carbonate sedimentation 45 kyr after the first step of the cie this re establishment of the saturation state of the water column was only fleeting before the later stepwise release of isotopically light carbon

early jurassic normal faulting in a carbonate extensional basin characterization of tectonically driven platform drowning high atlas rift morocco abstract this paper describes a tectonostratigraphic model of the synrift evolution of the early jurassic high atlas rift of morocco the model is constrained by mapping of a set of inverted extensional blocks by facies analysis of carbonate platform and turbiditic to hemipelagic synrift deposits and by high resolution n 100 ka biostratigraphy of the early jurassic succession the chronostratigraphic packages of the high atlas of rich vary significantly in thickness facies and architecture from one tectonic block to another our study shows how synrift strain varied in space and time over a long time interval 14 ma around the high atlas rift initially in sinemurian time the high atlas rift was affected by low strain normal faulting that controlled the growth of an extensive low gradient carbonate platform except in the northern domain towards the rift axis where hemipelagic deposition related to high rate faulting prevailed subsequently in carixian domerian time a rapid increase in accommodation space and block subsidence caused by high strain normal faulting brought about localized drowning of the carbonate platform and the development of calciturbidites and of starved deposits towards the rift axis during this interval high strain upper crustal normal faulting migrated rapidly over a period of 5 ma towards the rift periphery

palaeogeographic significance of mid silurian palaeomagnetic results from southern britain major revision of the apparent polar wander path for eastern avalonia summary wenlock c 430 ma lavas from the east mendips inlier southern briain are characterized by either single component magnetizations type 1 or a more complex system of three magnetization components type 2 during stepwise thermal demagnetization the type of demagnetization behaviour is related to changes in magnetic petrology within the lava pile the lower and intermediate unblocking temperature components of type 2 represent magnetic overprints of tertiary recent and hercynian mid carboniferous origin respectively the primary nature of the high unblocking temperature magnetization component types 1 and 2 dec 095 c inc 24 ag5 8 8 is demonstrated by positive agglomerate tests conversely an overlying upper old red sandstone sequence did not provide primary magnetizations the magnetic signature is governed by syn tectonic hercynian remagnetizations the primary silurian data demonstrate that the east mendips inlier experienced considerable clockwise rotation probably of the order of 80 prior to partial remagnetization during hercynian deformation and folding the silurian palaeopole vgp 13 n 271 e dp dm 5 9 cannot therefore be used for apparent polar wander path construction nevertheless the palaeolatitude estimate 13 f 5s suggests that southern britain eastern avalonia was at comparable mid silurian latitudes to baltica and northern britain 10 20 s both the iapetus ocean and tornquist sea therefore which separated southern britain from laurentia and baltica respectively in ordovician times were closed by wenlock time as ordovician and silurian palaeomagnetic data from baltica and eastern avalonia now record a comparable northwards translation from high southerly to near equatorial latitudes during this time we postulate pre wenlock collision of these blocks to form balonia baltica eastern avalonia a new apparent polar wander path for eastern avalonia constructed on the basis of this palaeogeographic model is presented

extending a thickened crustal bulge toward a new geodynamic evolution model of the paleozoic nw bohemian massif german continental deep drilling site se germany abstract fault bounded tectonic metamorphic complexes assembling the nw bohemian massif around the german continental deep drilling ktb site are seen to be extremely heterogeneous in tectonic and metamorphic histories in current models the different complexes were supposed to reflect a puzzle of small pre devonian microplates and the related collision events supposedly lasted until the carboniferous opposed to these models it will be shown that all the boundaries among the complexes were formed by detachment late in a prolonged overall geodynamic history of a thickened crustal bulge during extensional tectonics and associated thermal events that outlasted the onset of collision in the silurian lower devonian by about 70 80 ma micro structures petrological and geochronological data of individual complexes predominantly preserve the late stages rather than the unbroken record of their tectonometamorphic histories such partial histories strongly different among individual complexes depict diverse snapshots taken at different places in the evolving thickened crustal bulge and at different instants in its overall evolution and do not define different precollisional microplates predominantly p t and deformation episodes after terrane juxtaposition are preserved this article presents an integrated view of the structural geology microscopic fabrics p t data and geochronology of such diverse metamorphic complexes this integrated view provides a new understanding of 1 the tectonic evolution during upper silurian devonian collision of the gondwana derived central european lithosphere with laurussia 2 the postaccretionary events that lasted through the upper carboniferous and 3 the earlier lower ordovician metamorphic and magmatic history which is only locally recorded metamorphic complexes occupying the structurally highest position upper tectonic complexes record devonian and earlier tectonometamorphic and magmatic events after the mid devonian such complexes did not experience any metamorphism the recorded devonian events consist of subduction and exhumation of hp rocks and their exhumation involved thrusting and extensional tectonics upper tectonic complexes comprise fragments of both the plate that was subducted in this period and the overriding plate in the footwall carboniferous extension has brought upper tectonic complexes against metamorphic complexes that essentially record lower and upper carboniferous tectonometamorphic and magmatic events lower tectonic complexes in the lower tectonic complexes such events are 1 consecutive extensional stages that created at least three sets of ductile normal detachment systems intersecting each other and various associated thermal pulses as well as 2 predetachment events that are only recorded as petrologic and structural relics transposed during extension inferred as predetachment events are crustal subduction as well as stacking that outlasted thrusting and exhumation of the upper tectonic complexes in the deeper portion of the thickened crustal bulge represented by the lower tectonic complexes spatial variations of p t t d histories during decompression occurred as a result of continued differential exhumation on the three sets of detachment systems and also from various thermal pulses that have affected different parts of this section during progressive shearing and exhumation to various degrees whereas the lower tectonic complexes of the erzgebirge gneiss dome preserve the record of a lower carboniferous history of hp metamorphism and crustal thickening followed by extension in those of the oberpfalz region ktb site a major upper carboniferous thermal pulse mostly erased the pre upper carboniferous strain fabrics and metamorphic record in the oberpfalz region ongoing extension emplaced mid crustal rocks that pervasively equilibrated in the upper carboniferous at high temperatures and low pressures low p t ratio against rocks not exposed to high temperatures at that time in summary a prolonged postaccretionary history concealed large scale structural relationships linked to crustal subduction and thickening associated with former juxtaposition of gondwana lithosphere to laurussia polyphase crustal extension and strike slip created the variety of different metamorphic tectonic complexes observed in the nw bohemian massif

the armorica microplate fact or fiction critical review of the concept and contradictory palaeobiogeographical data abstract the problem of an armorica microplate detached from gondwana and having had during part of the palaeozoic an independent latitudinal evolution is reconsidered in terms of a critical review of the palaeomagnetic data that were the very roots of this concept and through an alternative approach based on palaeoclimatic and palaeobiogeographical data palaeomagnetic data for the silurian and the devonian of the south european regions supposedly constituting the armorica microplate remain rare and ambiguous those from gondwana are more numerous but contradictory enough to give rise to diverging models regarding the latitudinal evolution of this continent consequently the reality of an armorica microplate cannot be considered as established on the contrary lithological indicators of palaeoclimate and palaeobiogeographical data are in total harmony and indicate that in actual fact the southern european regions remained permanently closely connected with gondwana of which they composed the northern margin although repeatedly maintained for more than 20 years the concept of an armorica microplate can thus be considered a fiction this conclusion should lead to the dismissal of geodynamical models proposed for the variscan belt to which this concept was integrated and are contradicted by inescapable palaeobiogeographical constraints

no evidence for thermogenic methane release in coal from the karoo ferrar large igneous province abstract the early jurassic toarcian oceanic anoxic event t oae and concurrent negative carbon isotope 13 c excursion have recently been attributed to either the release of methane ch 4 clathrates or thermogenic ch 4 gas associated with the karoo ferrar large igneous province lip into coals and organic rich shales 12 c enriched thermogenic ch 4 production associated with the karoo ferrar would result in residual material being 12 c depleted nearer the intrusions in this study geochemical analyses carbon isotopes volatile matter vm vitrinite reflectance r o are reported for two coal transects associated with dykes intruding the no 4l coal in the highveld coalfield karoo basin south africa vm decreases from over 35 to around 15 in one transect and the second transect shows a less pronounced decrease from 25 to 16 accompanying the decrease in vm content is an increase in r o from background levels of around 0 7 to over 4 adjacent to the dyke used as a palaeo geothermometer r o values indicate background temperatures of 100 c increasing to 300 c close to the contact despite changes in vm and r o there are no significant changes in 13 c certainly not of the magnitude that would be expected associated with large scale thermogenic ch 4 generation these and other gondwanan coals have low vitrinite and liptinite contents components more prone to ch 4 generation in part explaining the modest decreases in vm adjacent to the dykes this combined with the relatively narrow metamorphic aureole surrounding the intrusions and the likelihood that at least some of the volatiles generated by the intrusion were trapped as coalbed ch 4 or condensed as pyrolytic carbon suggests only limited ch 4 release in addition based on original estimates of moisture contents in these coals and the depth at time of intrusion 1 000 2 000 m the dykes would have lost most of their energy heating and evaporating water thus having very little remaining energy to generate thermogenic ch 4

carbonate systems along nutrient and temperature gradients some sedimentological and geochemical constraints research over the past several decades has clearly demonstrated that changes in the ocean environment have had major impacts on carbonate systems changes in climate ocean circulation and seafloor spreading rates have influenced temperature and seawater chemistry including carbonate saturation state and nutrient availability and thereby have determined boundary conditions for the biota that form carbonate platforms in turn the biota determine accumulation rates and facies zonations thus controlling platform geometry and facies dynamics in the first section of this paper we examine how nutrient availability influences carbonate facies associations we first discuss the role of temperature and nutrient gradients in the modern ocean and their influence on biotic associations then we discuss how carbonate sedimentation can be characterized along nutrient gradients in the second section we review proxies currently used to reconstruct paleoproductivity in open ocean environments and discuss their applicability to neritic carbonate systems we highlight the variety of existing proxies and their limitations and suggest that multiple lines of evidence are needed for valid interpretations our short review discusses sedimentological biogenic and geochemical proxies that can be used to reconstruct past nutrient fluxes and to constrain paleoceanographic controls over the distribution of carbonate associations however it also reveals that more data and case studies are needed that integrate shallow and deep water carbonate sequences and elucidate the links between temperature vs nutrient supplies changes and facies in ancient carbonate sequences

high resolution dynamics of early jurassic marine extinctions the case of pliensbachian toarcian ammonites cephalopoda abstract the pliensbachian toarcian interval was marked by major environmental disturbances and by a second order mass extinction here we reappraise the taxonomic spatiotemporal and selective dynamics of extinctions over the whole interval by analysing a high resolution dataset of 772 ammonite species from nw tethyan and arctic domains on average 40 65 of ammonite species disappeared during each subchronozone but higher extinction pulses reaching 70 90 prevailed from the margaritatus to the dispansum chronozone the main extinctions corresponding to the gibbosus pliensbachian toarcian boundary semicelatum bifrons variabilis and dispansum events differed in their dynamics suggesting episodes of ecological stress related to climate change regression disturbance in the carbon cycle or anoxia the multi pulsed volcanic activity in the karoo ferrar province could well have triggered these ecological changes in addition ammonites experienced a morphological bottleneck during the gibbosus event 1 ma before the early toarcian diversity collapse typically drops in richness were related both to high extinctions and to declines in origination rates this feature could result from strengthened ecological stresses related to the temporal overlap of environmental disturbances after the early toarcian crisis the recovery of ammonites was rapid 2 ma and probably influenced by a coeval marine transgression supplementary material figures showing a comparison of extinction and origination patterns based on different datasets and variations of the morphospace occupation are available at http www geolsoc org uk sup18381

continental weathering and climatic changes inferred from clay mineralogy and paired carbon isotopes across the early to middle toarcian in the paris basin abstract lower toarcian strata lower jurassic have been extensively studied with a view to understanding the oceanographic climatic and biological processes that drove the earth s system into an oceanic anoxic event oae for this time period the evolution of the european marine seaways is now relatively well constrained owing to multiple geochemical studies but investigations regarding climatic trends in the continental realm remain sparse in the present study we test the clay mineralogy as a continental climate sensitive proxy in the well documented sancerre core southern paris basin we compare variations in the kaolinite content with p co 2 fluctuations derived from paired carbon isotopes 13 c 13 c carb 13 c org taking advantage of the detailed chemostratigraphic palaeoenvironmental and sequence stratigraphy frameworks established for this core material the results indicate a substantial decrease in kaolinite abundance at the end of the pliensbachian which is compatible with a long term diminution in continental weathering and an inferred temperature decrease the early toarcian prior to the carbon cycle perturbation and deposition of black shale facies remained relatively cold with minima in both the proportion of kaolinite and reconstructed p co 2 the mineralogical and geochemical responses across the prominent negative carbon isotope excursion cie itself are not univocal the first of four steps that compose the negative limb of the cie at sancerre is associated with decreased kaolinite and p co 2 and increased carbonate oxygen isotope ratios taken together these trends are compatible with a transient cooling phase immediately preceding the onset of black shale deposition conversely the subsequent steps are marked by substantial enrichment in kaolinite that matches increased osmium isotope ratios measured in yorkshire providing compelling evidence for rapid increases in continental weathering and riverine runoff forced by intensification of greenhouse conditions during the cie relaxation in the intensity of continental weathering as suggested by resumed low kaolinite abundance is seen immediately after the cessation of co 2 input after the fourth step of the cie the interval spanning the upper portion of the early toarcian and the middle toarcian records a subsequent long term increase in the proportion of kaolinite synchronous with significant clay enrichment of the sediment continued greenhouse conditions even after the recovery from the carbon isotope perturbation and regional black shale deposition are likely related to sustained co 2 emission by karoo ferrar volcanism through the considered interval

the early toarcian early jurassic and the cenomanian turonian late cretaceous mass extinctions similarities and contrasts abstract the early toarcian eto and cenomanian turonian c t mass extinctions are comparable from a wide range of scales and perspectives from a broad standpoint their similarities include virtually identical extinction intensity at the familial and generic levels widespread basinal facies deposited during sea level highstands an overall greenhouse climate and anoxia as an important causal mechanism the high resolution macroinvertebrate data analyzed here consisting of stratigraphic ranges diversity and abundance point to smaller scale similarities the two events resulted in significant ecological disruption and in both cases the biotic responses were very similar taxa inhabiting the upper water column were unaffected by anoxia and included ammonites and in the eto belemnites in addition epifaunal taxa adapted to low oxygen conditions such as the buchiids posidoniids and inoceramids flourished in the post extinction environment during the survival interval as conditions ameliorated the biota became more diverse and gradually began to resemble pre extinction biotas furthermore the 13c curves predict the end of the survival interval and suggest that the period characterized by carbon isotope excursions represent disrupted environmental conditions this points to the potential application of 13c as a tool for determining the repopulation modes and timing for other mass extinctions

terrane wrecks coupled oroclines and paleomagnetic inclination anomalies abstract three major orogenic belts that are host to coupled oroclines terrane wrecks are likewise characterized by or directly associated with paleomagnetic inclination anomalies 1 the north american cordillera 2 the western european variscan belt and 3 the eastern european alpine system implied paleolatitudinal discrepancies in each orogen are of similar magnitude to the translation required by orogen parallel shortening during oroclinal formation the alaskan coupled oroclines accommodated 3000 km of orogen parallel shortening in the north american cordillera where primary remanent paleomagnetic inclinations in late cretaceous rocks require the bulk of the belt to have been translated 2000 to 3000 km northward relative to cratonic north america the iberian coupled oroclines accommodated 1100 km of orogen parallel shortening in the western european variscan belt within which mid paleozoic paleomagnetic inclinations are shallow with respect to stable gondwana the implied existence of a 3500 km wide paleotethys ocean between the variscan autochthon and the north gondwana margin is seemingly at odds with paleontological constraints the carpathian balkan oroclines accommodated 1000 km of orogen parallel shortening in the eastern alpine system directly north northeast of an eastern mediterranean region where primary remanent inclinations in late oligocene to miocene rocks require northward translation between 500 and 1000 km palinspastic restoration of the alaskan and carpathian balkan oroclines fully accounts for the translations recorded by the cordilleran and eastern mediterranean inclination anomalies respectively restoration of the iberian coupled oroclines confirms the existence of paleotethys but only accounts for one third of the paleolatitudinal discrepancy recorded by the variscan inclination anomaly fully resolving the variscan inclination anomaly requires 1 a paleomagnetic data set of greater temporal and spatial extent 2 refinement of the geometric and geological continuity of the variscan belt across western europe

a devonian palaeomagnetic pole for armorica summary the palaeomagnetic study of siluro devonian redbeds of the san pedro formation at cabrillanes asturias spain reveals a high temperature and pre tectonic magnetization determined by intersection of great circles as well as conventional averaging fisher this magnetization is carried by the haematitic pigment and is therefore of chemical and postdepositional origin the formation mean direction d 113 i 34 ag5 10 is interpreted as representing a record of a devonian palaeofield the corresponding pole after correction for the opening of the bay of biscay and the tectonic closure of the ibero armorican arc is in good agreement with contemporaneous european poles confirming that the arinorica plate had already joined the laurentia baltica assembly at that time

the late frasnian kellwasser horizons of the harz mountains germany two oxygen deficient periods resulting from different mechanisms in the harz mountains germany three late devonian sections aeketal huhnertal and kellwassertal were analyzed for major and trace element concentration the inorganic geochemical data were used to determine the environmental conditions associated with the deposition of the two late frasnian black kellwasser horizons in a submarine rise environment low ti al and zr al values and slight enrichment in nutrient and organic matter related trace elements e g ba cu and ni suggest that the kellwasser horizons were deposited during two periods of minimum detrital input but relatively elevated primary production enrichments in u v and mo in both black kellwasser horizons and values of redox indices including u th v cr and ni co indicate oxygen restricted conditions prevailed during the late frasnian times under oxygen depleted conditions reductive diagenesis caused precipitation of iron sulphides such as pyrite that are observed with sem however variations in major and trace element concentrations are not similar in the two kellwasser horizons particularly regarding redox markers the lower kellwasser horizon seems to be more o2 depleted than the upper kellwasser horizon in some german sections on the basis of these results we propose that the two kellwasser horizons are not caused by identical conditions the lower kellwasser horizon results from increased primary productivity enhanced by land derived nutrient loading and which triggered the anoxic conditions recorded in drowned platform setting in contrast the upper kellwasser horizon results from the onset of oxygen impoverished bottom water in the deepest part of the ocean due to episodic water stratification during maximum frasnian flooding oxygen depleted water may have impinged shallower environment during the linguiformis transgressive pulse nutrients would have been released from organic matter decay under reducing conditions and brought up to the photic zone during episodic water mixing

spatial and temporal distribution of ooids along a jurassic carbonate ramp amellago outcrop transect high atlas morocco carbonate ramp systems are widespread throughout the geological record but very few areas have seismic scale continuous and structurally undeformed outcrops that allow reliable interpretation of facies distributions and stacking patterns the amellago outcrop shows the detailed depositional and stratigraphic relationships of an ooid dominated ramp system that is almost completely exposed along a dip profile 37 km long and 1000 m thick in the lower to middle jurassic of the southern high atlas morocco ammonite and brachiopod fauna provide excellent biostratigraphic control on small scale stacking patterns at amellago the evolution of depositional environments is evident at different scales of space and time during this period of tectonic quiescence dominated by thermal subsidence an important observation is that the amellago ramp system contains micrite rich ooid free intervals that alternate with ooid rich intervals the ooid rich intervals are mainly in the late transgressive and highstand system tracts whereas the ooid free intervals occur in the early transgressive phase more than 25 such alternations were recorded in high frequency cycles and at the scale of one large cycle at the aalenian bajocian transition these compositional changes and the associated different ramp geometries are interpreted to result from the combined effects of eustatic sea level and climatic changes

continental weathering and redox conditions during the early toarcian oceanic anoxic event in the northwestern tethys insight from the posidonia shale section in the swiss jura mountains abstract the geochemistry and mineralogy of organic rich sediments of the rietheim succession in northern switzerland were studied to evaluate the main impacts of the early toarcian oceanic anoxic event t oae 183 ma ago on the depositional environment in the nw tethys ocean the geochemical data obtained notably 13corg 13ccaco3 rock eval pyrolysis toccff support the hypothesis that the negative c isotope excursion at the onset of the t oae mainly reflects a major perturbation in the global carbon cycle rather than local environmental changes e g basinal restriction productivity rock eval pyrolysis and 13corg 15ntotal crossplots indicate that the organic matter was primarily of marine origin during the t oae and deposited in an epicontinental sea setting detrital proxies notably detritus index siliciclastic balance and ln al2o3 na2o suggest that the elevated pco2 levels and greenhouse climate prevailing during the t oae induced significant acceleration of the hydrologic cycle and an increase in continental chemical weathering rates redox sensitive proxies relative pyrite contents organic and sulfide balances v v ni ratios trace elements enrichment factors trace element toc co variation patterns and toc ptotal molar ratios indicate that the rietheim posidonia shale sediments accumulated under oxic to dysoxic bottom water conditions with more reducing conditions during the t oae anoxic and possibly euxinic these changes in redox conditions were probably induced by substantial thermohaline stratification driven by enhanced freshwater input and the recurrent influx of brackish arctic seawater into the nw tethys through the viking corridor in agreement with previous sedimentological and geochemical evidences from the nw tethys ocean in the studied succession such conditions were perhaps also responsible for sequestration of most of the regenerated p in the deeper water column by adsorption and or co precipitation into authigenic phases overall the major palaeoenvironmental changes documented in this study provide a robust example of earth s possible responses to the rapid onset of extreme greenhouse conditions

paleomagnetics of ringerike old red sandstone and related rocks southern norway implications for pre carboniferous separation of baltica and british terranes abstract paleomagnetic study of two stratigraphic sections of the upper silurian lower devonian ringerike sandstone oslo area norway indicate a straightforward multicomponent nrm for these rocks a lower temperature magnetization component gives a southwest and up reversed polarity in situ direction yields a negative fold test i e post folding at the 99 confidence level and is nearly identical to directions obtained from overlying permo carboniferous lavas a high temperature magnetization component exhibits both normal and reversed polarity directions passes a fold test at the 95 confidence level and is supported by a baked contact test which indicates a pre intrusion age of magnetization this characteristic high temperature component was successfully isolated in 79 samples from 19 sites within the ringerike succession results indicate a tilt corrected characteristic magnetic direction of declination 26 inclination 16 which is consistent with earlier work this direction corresponds to a time integrated paleopole position of 19 n latitude 164 longitude d p 4 8 d m 9 3 when compared to previous results from rocks of similar age from the british isles an approximately 20 latitudinal displacement between britain and baltica during late silurian early devonian time is suggested

paleomagnetic constraints on the assembly of the old red continent abstract a model is proposed on the basis of paleomagnetic and geological arguments for the assembly of the old red continent which in the devonian comprised north america the baltic shield and russian platform as well as a third element called armorica the latter is thought to have included most of hercynian europe and southern england and wales as well as possibly the avalonian terranes of northeastern north america and perhaps some parts of the atlas ranges and moroccan meseta the model envisions a late ordovician taconic collision of north america and armorica which are joined in the late silurian by the baltic shield and russian platform not until the late early carboniferous did gondwana join the old red continent in a collision responsible for a large internal megashear parallel to the caledonian orogenic belt as well as the hercynian alleghenian orogeny subduction of the large siluro devonian ocean between gondwana and the old red continent is held responsible for the mid devonian acadian orogeny

biosedimentology of the early jurassic post extinction carbonate depositional system central high atlas rift basin morocco this study documents a liassic example of the long ranging effects of mass extinction on carbonate systems biohistoric constraints inherent in the liassic carbonate depositional system are deciphered from normal marine sub tidal deposits of the central high atlas rift basin morocco through hettangian sinemurian to early toarcian times the integration of results from the analysis of lithofacies depositional geometries microfacies macrobenthos carbonate build ups carbon and oxygen stable isotopes and rare earth element yttrium distribution patterns allows the intrinsic or biohistoric control on the central high atlas deposits to be separated from extrinsic factors such as basin development and palaeoclimate the survival interval in the aftermath of the end triassic mass extinction persisted until the early sinemurian indicated by a severely depleted carbonate system impoverished in skeletal organisms a tectonic pulse at the early to late sinemurian boundary interval caused a basin widening with immigration of a marine fauna however until the latest sinemurian macdonelli subzone of the raricostatum zone the deposits were dominated by filter feeding benthic heterotrophs sponges brachiopods polychaetes and crinoids during this stage primary production within the enlarged basin must have been largely planktonic and there was a net flux of organic matter to the sea floor oxygen minimum zone a regional radiation of organic walled phytoplankton is inferred to explain the selective success of the filter feeding community and the occurrence of sponge mounds thus significant effects of the end triassic mass extinction are still present during the late sinemurian through almost the entire pliensbachian a highly productive shoal rimmed carbonate platform existed it developed subsequent to tectonic reorganization and a marine recirculation event radiolarian facies 13c 1 1 strongly negative ce anomaly photosymbiotic sediment producers mainly large bivalves now state the success of specialists and environmental equilibrium conditions in the latest pliensbachian the climax stage was reached with the development of a coralgal reef rimmed carbonate platform the liassic carbonate depositional system experienced a terminal multicausal early toarcian drowning event during which most of the large bivalves became extinct

evidence for major environmental perturbation prior to and during the toarcian early jurassic oceanic anoxic event from the lusitanian basin portugal the timing and causal relationships between the pronounced negative c isotope excursion and paleoenvironmental perturbations associated with the toarcian oceanic anoxic event early jurassic remain unclear particularly because biotic crises and carbonate production decrease appear to have been initiated earlier than the main c isotope anomaly here we present a new quantification of late pliensbachian early toarcian calcareous nannofossils abundance and size from the peniche reference section portugal together with o and c isotope records of well preserved brachiopod shells from the same section the brachiopod shell 13c curve parallels that of bulk carbonate and records two pronounced negative isotopic excursions close to the pliensbachian toarcian boundary 2 and during the toarcian oceanic anoxic event 3 5 our results indicate that both c isotope negative excursions were characteristic of benthic and shallow water environments suggesting that these two carbon cycle perturbations affected all epioceanic reservoirs coeval shifts toward lower values of brachiopod oxygen isotope compositions and closely correlated northward migrations of mediterranean ammonite fauna suggest that both events coincided with major rises in seawater temperatures probably as a result of increased co2 levels and enhanced greenhouse conditions co2 induced changes in seawater chemistry likely affected the calcification potential of both neritic and pelagic systems as evidenced by synchronous drops of platform derived carbonate accumulation and drastic reductions in size 3 m of the main pelagic carbonate producer schizosphaerella we suggest that the early toarcian paleoenvironmental crisis occurred in two distinct episodes that were most likely related to two successive phases of intense volcanic degassing in the karoo ferrar province

modeling evidences for global warming arctic seawater freshening and sluggish oceanic circulation during the early toarcian anoxic event 1 the paleoecological disturbances recorded during the early toarcian warming event 183 myr ago including marine anoxia sea level rise seawater acidification carbonate production crisis and species extinctions are often regarded as past examples of earth s possible responses to the rapid emergence of super greenhouse conditions however physical mechanisms explaining both the global and local expressions of paleoenvironmental events are still highly debated here we analyze the paleoclimatic and paleoceanographic consequences of increases in atmospheric pco2 levels at a multiscale resolution using a fully coupled ocean atmosphere model foam we show that in association with stronger high latitude precipitation rates and enhanced continental runoff the demise of polar sea ice due to the global warming event involved a regional freshening of arctic surface seawaters these disturbances lead to progressive slowdowns of the global oceanic circulation accountable for widespread ocean stratification and bottom anoxia processes in deep oceanic settings and epicontinental basins in agreement with very negative oxygen isotope values measured on fossil shells from the nw tethys our simulations also show that recurrent discharges of brackish and nutrient rich arctic surface waters through the viking corridor could have led to both vertical and geographical gradients in salinity and seawater 18o in the nw tethyan seas locally contrasted conditions in water mass density and rises in productivity rates due to strong nutrient supplies could partly explain the regional severity of the anoxic event in the restricted euro boreal domains as it has been previously suggested and modeled regionally

upper ordovician palaeogeography of the bohemian massif implications for armorica summary the palaeogeographic position of armorica in the upper ordovician is still rather unclear due mainly to the lack of reliable palaeomagnetic data to help resolve this a palaeomagnetic study of caradocian and ashgillian sediments and volcanics of the barrandian basin in the central bohemian massif czech republic has been carried out a total of 29 sites were sampled including four from a diabase sill intruded into hirnantian age sediments thus providing a baked contact test three components of magnetization labelled a b and c have been resolved in addition to a low temperature component which corresponds to the present day direction of the local geomagnetic field d i 360 67 component a southerly and shallow before bedding correction is only sporadically present and corresponds to a remagnetization event of permo carboniferous age which is well known throughout hercynian europe component b present as stable endpoints in the sill and as a secondary component in the host rock sandstones yields an overall mean direction of 360 39 95 9 3 k 52 3 in situ and 355 21 95 9 3 k 52 3 after bedding correction component c identified as stable endpoints in sedimentary and volcanic rocks passes both a fold test and a baked contact test and is interpreted as being the primary direction of magnetization the overall mean direction for c is 183 59 95 9 5 k 30 3 nine sites after bedding correction which corresponds to a palaeopole position of 80 s 360 e this translates into palaeolatitudes of 40 s for the area studied in latest ordovician times but indicates large amounts of rotation up to 170 of the bohemian massif before final consolidation of hercynian europe although such large amounts of rotation are difficult to accommodate they are consistent with palaeomagnetic results obtained from lower ordovician and upper silurian rocks elsewhere in the barrandian basin if the bohemian massif is interpreted as being an integral part of the armorican microplate then these results indicate that prior to caradoc times armorica rifted away from the northern margin of gondwana and that by ashgillian times it was located at intermediate palaeolatitudes

palaeomagnetism of the ordovician moulin de chateaupanne formation vendee western france summary a palaeomagnetic study of the moulin de chateaupanne formation reveals two characteristic components of magnetization the first one isolated by thermal treatment at intermediate temperatures is strongly resistant to chemical leaching its direction d 217 i 25 ag5 5 5 and the corresponding pole position lat 21 s long 320 e are typical of the hercynian overprints in the armorican massif the second component carried by fine grained haematite presents a steep downward direction d 228 i 81 ag5 6 pole position lat 34 n long 343 e already recognized in other ordovician rock units from the same area it is resistant to high temperature demagnetization treatment and is believed to be of chemical origin as assessed by a conglomerate test a mutual consistency test with the nearby thouars massif suggests that the tilt of the formation has a sedimentary origin and that the thouars massif has not suffered significant tilt during the hercynian orogenic events

statistical significance of the fold test in palaeomagnetism summary the significance of the change in direction dispersion on applying the fold test in palaeomagnetism is used as the criterion for deciding whether or not the fold test has statistical significance a set of tables is compiled which can be used for quick application of the test

palaeomagnetic constraints upon the palaeogeographic position of the baltic shield in the ordovician the palaeomagnetic analysis of the ordovician limestones in southern scandinavia 75 sites has given the following results 1 in the southern part of the oslo region all ordovician sediments were remagnetized during the lower permian by a thermo chemical mechanism triggered by the emplacement of the permian volcanic and intrusive suite 2 in the northern part of the oslo region the upper caradoc mjosa limestone carries a high temperature component decl 53 incl 40 95 25 4 sites with corresponding palaeopole at 4 s 143 e 3 in southern sweden all lower ordovician orthoceras limestones sampled in dalarna narke scania vaster gotland oland and oster gotland display the same well defined southeast steep high temperature components leading to a palaeopole at 30 n 55 e a95 9 in areas with late palaeozoic volcanic activity scania vaster gotland this high temperature component is partially overprinted by a remagnetization associated with dolerite emplacement according to these results baltica was situated in temperate southern latitudes 50 s in the early ordovician and has drifted northwards at a latitudinal drift rate of 4 to 7 cm year to reach tropical latitudes 20 s by the late ordovician to late silurian this interpretation is in agreement with previous models based on the occurrence of ordovician reef structures and sheds new light on the early palaeozoic position of baltica relative to other continental blocks

new palaeomagnetic constraints on the position of central bohemia during early ordovician times summary apparently conflicting data within the palaeomagnetic database suggest that bohemia was at much shallower palaeolatitudes than the armorican and iberian massifs in ordovician times in order to resolve this controversy a palaeomagnetic study of arenig and llanvirn volcanic and sedimentary rocks from the barrandfan basin central bohemia has been carried out this basin comprises a sequence of unmetamorphosed lower palaeozoic rocks the main folding of which occurred in late devonian to early carboniferous times the samples collected l sites 153 samples were subjected to stepwise thermal and alternating field treatment eleven sites yielded reliable magnetic directions with analysis of the results using standard principal component and great circle analyses up to three directions of magnetiza tion can be identified in these rocks the first termed a is generally removed below 200 c although in some cases it persists up to 450 oc in the volcanic rocks it is approximately parallel to the present day earth s magnetic field in the study area dec inc 360 67 and is thus thought to be of recent origin the second direction identified termed b is isolated at intermediate blocking temperatures 150 350 c and yields an overall in situ mean direction of 195 8 k 22 3 a 95 13 1 seven sites these directions fail the fold test of mcfadden 1990 and correspond to a palaeopole position of 34 s 356 e this coincides with the late carboniferous sector of the european apparent polar wander path and thus the b direction is interpreted as being a secondary overprint of this age the highest blocking temperature direction termed c is identified at temperatures between 350 and 450 600 c it passes the fold test resulting in an overall mean direction of 312 83 k 21 9 a 95 14 6 six sites after bedding correction this direction is interpreted as being representative of the early ordovician palaeomagnetic field direction in the bohemian massif and yields a palaeopole position of 58 n 355 e this translates into palaeolatitudes of 76 s for bohemia thus demonstrating that the bohemian massif was at similar peri polar latitudes to the rest of armorica during early mid ordovician times and formed part of the northern margin of gondwana

palaeomagnetism of the ibero armorican arc and the hercynian orogeny in western europe van der voo1 has pointed out that further advances in the understanding of the hercynian orogeny require palaeomagnetic data from well studied orogenic areas we report here palaeomagnetic results of palaeozoic rocks taken in brittany portugal and spain in the ibero armorican arc iaa a large geological feature considered to be a key area for the hercynian orogeny in western europe they show evidence that the iaa formed in two stages extending results obtained in spain by ries et al 2 giving precise constraints for the former shape of the arc we can use pre hercynian palaeomagnetic directions found in spain and portugal to follow its evolution during palaeozoic times this megastructure linking brittany to iberia squeezed between gondwana and the northern continents during the carboniferous did break away from the former block during ordovician times

carbonate platform evidence of ocean acidification at the onset of the early toarcian oceanic anoxic event abstract the early toarcian oceanic anoxic event early jurassic 183 myr ago is associated with one of the largest negative carbon isotope excursion cie in the whole phanerozoic 3 7 estimates of the magnitude and rate of co 2 injection in the ocean atmosphere system are compatible with a scenario of ocean acidification many carbonate platforms drowned in the pliensbachian well before the early toarcian event in this paper we test the hypothesis of surface water ocean acidification by presenting data from a resilient carbonate platform the apennine carbonate platform of southern italy the studied sections document a dramatic shift of the carbonate factory from massive biocalcification to chemical precipitation lithiotis bivalves and calcareous algae palaeodasycladus mediterraneus which were the most prolific carbonate producers of pliensbachian carbonate platforms disappear during the first phase of the early toarcian cie before the most depleted values are reached we discuss the local versus supraregional significance of this shift and propose a scenario involving abrupt decline of carbonate saturation forced by co 2 release at the beginning of the early toarcian cie followed by a calcification overshoot driven by the recovery of ocean alkalinity attribution of the demise of carbonate platform hypercalcifiers to ocean acidification is supported by palaeophysiology and reinforced by experimental data on the detrimental effects of ocean acidification on recent shellfishes and calcareous algae

massive dissociation of gas hydrate during a jurassic oceanic anoxic event in the jurassic period the early toarcian oceanic anoxic event about 183 million years ago is associated with exceptionally high rates of organic carbon burial high palaeotemperatures and significant mass extinction heavy carbon isotope compositions in rocks and fossils of this age have been linked to the global burial of organic carbon which is isotopically light in contrast examples of light carbon isotope values from marine organic matter of early toarcian age have been explained principally in terms of localized upwelling of bottom water enriched in 12c versus 13c refs 1 2 5 6 here however we report carbon isotope analyses of fossil wood which demonstrate that isotopically light carbon dominated all the upper oceanic biospheric and atmospheric carbon reservoirs and that this occurred despite the enhanced burial of organic carbon we propose that as has been suggested for the late palaeocene thermal maximum some 55 million years ago the observed patterns were produced by voluminous and extremely rapid release of methane from gas hydrate contained in marine continental margin sediments

tectonic and paleogeographic interpretation of the paleomagnetism of variscan and pre variscan formations of the bohemian massif with special reference to the barrandian terrane abstract paleomagnetic data from variscan and pre variscan formations of the bohemian massif bm have been interpreted tectonically and paleogeographically evaluated with respect to the european paleomagnetic results the interpretation of data from the bm was preceded by the evaluation of the effect of the trans european suture zone tesz on the dispersion of the european paleomagnetic pole positions during the variscan orogeny stability of the european lithospheric plate is well documented by paleomagnetic results for the early permian to the quaternary rocks of the regions between the ural mountains and great britain north of the alpine tectonic belt for the late carboniferous and earlier times the stable plate is defined in the regions ne of the tesz in the east european craton only late carboniferous and older rocks from the regions sw of the tesz largely occupied by the west european variscides show tectonic deformations controlled by prominent horizontal rotations these rotations can be studied on a theoretical model simulating the distribution of pole positions controlled by rotation of rock formations about a vertical axis in the bm horizontal paleotectonic rotations are well documented for variscan and pre variscan rocks they reach several tens of degrees up to 180 in extreme cases of mostly clockwise sense the study of the tectonics and paleogeography of cambrian to devonian formations of the barrandian area which is considered a peri gondwana terrane with affinities to armorica may serve as a case history of the study of a terrane incorporated into a stable lithospheric plate here the barrandian terrane became a part of the european plate as a component of the emerging pangea supercontinent in the final phase of the variscan orogeny high almost peri polar paleolatitudes for the cambrian rocks of the barrandian area are within the limits of data scatter in agreement with the presumed position of gondwana and armorica rocks of ordovician age did not produce consistent results they will require further paleomagnetic and paleoclimatologic studies in relation to the data for the late cambrian and early silurian rocks of the barrandian area the analysis of data presented here shows that paleomagnetic declinations of variscan and pre variscan formations reflect paleotectonic rotations following the formation of the respective rocks including rotations induced by the variscan orogeny and rotation of the european plate to which the rocks became incorporated therefore paleomagnetic declination of none of the pre variscan formations can be identified with paleotectonic rotation either of the barrandian terrane or even of the whole bm magnitudes of horizontal paleotectonic rotations in the bm affected by variscan tectonism are notably similar to those in the western carpathians affected by alpine deformation senses of rotation are however different with clockwise rotation prevailing in the bm and counterclockwise rotations prevailing in the western carpathians recognition of the effect of horizontal rotations of rock formations if located in tectonically mobile belts and incorporated into a stable plate during their later history should be respected while deriving the apparent polar wandering path apwp

calcareous nannoplankton changes across the early toarcian oceanic anoxic event in the western tethys 1 calcareous nannoplankton were profoundly affected by environmental perturbations coincident with the early toarcian oceanic anoxic event t oae we quantify the abundance of nannofossils across the t oae at three locations in western europe where the event is marked by a prominent negative carbon isotope excursion cie data were treated by statistical analysis and the shannon diversity index was applied in order to describe nannofossil assemblage changes related to paleoenvironmental evolution in the basal toarcian before the t oae high proportions of taxa with an affinity for low temperature e g p liasicus t patulus bussonius occurred this observation is consistent with interpretations of published oxygen isotope records during the t oae the lowest abundance of nannofossils is observed but there is a peak of a coccolith calyculus from an organism that probably thrived in low saline surface waters depleted of nitrate at the end of the perturbation the lowest diversities of nannofossils occurred and assemblages are dominated by crepidolithus crassus a deep dweller this interval corresponded to progressive reoxygenation of deep water and the reoccupation of the deep photic zone by nannoplankton the highest abundance of nannofossils is recorded above the cie and testifies to the recovery of the entire nannoplankton community the t oae was widespread perhaps global and probably linked to major changes in the atmosphere and hydrosphere however the record of this event varies in the basins surrounding the western tethys suggesting regional imprints on the global signal the regional variability may attest to establishment of effective connections between the arctic and tethys oceans which allowed cool low saline water formed at high latitudes to stream toward the western tethys

apatite fission track analyses on basement granites from south western meseta morocco paleogeographic implications and interpretation of aft age discrepancies abstract this work is based on apatite fission track analysis of samples mostly granites from the basement of the cretaceous tertiary phosphate and ganntour plateaus exposed in the jebilet and rehamna massifs western meseta morocco this basement belongs to the carboniferous early permian variscan belt and the earlier marine onlap is late triassic in age however the aft ages are post triassic and different in the jebilet 186 203 ma and rehamna 148 153 ma track length modelling support the occurrence of moderate heating events during the jurassic and the eocene respectively with cooling during the permian and cretaceous intervals these results are partly accounted for by considering a moderate subsidence during the late triassic liassic which is a noticeable change in the regional paleogeographic concept of west moroccan arch however the discrepancies between the aft results from the studied massifs make necessary to explore further explanation we interpret the observed discrepancies by the difference in age and depth of crystallization of the sampled granites in the variscan orogen i e 330 ma 5 6 km in the jebilet versus 300 ma 8 10 km in the rehamna the importance of the late jurassic early cretaceous uplift and erosion of the entire meseta and that of its late eocene burial are emphasized

ultrapotassic dykes in the moldanubian zone and their significance for understanding of the post collisional mantle dynamics during variscan orogeny in the bohemian massif abstract we report mineral textures geochemistry and age relations of two ultrapotassic dykes from a dyke swarm in the western part of the moldanubian zone at contact to the tepla barrandian block the dykes have orientation almost perpendicular to the nne ssw trending central bohemian plutonic complex and cross cut metamorphic foliation in basement gneisses and migmatites based on mineral compositions and geochemistry the dykes show close relations to mg k syenite plutons in the moldanubian zone the two dykes are vaugnerite and syenite in compositions and contain talc pseudomorphs after olivine within a fine grained matrix that consists of k feldspar phlogopite with small amounts of clinopyroxene and accessory quartz apatite titanite and sulphides of fe cu ni the syenite porphyry dyke cross cuts the vaugnerite it contains quartz phenocrysts and comparing to vaugnerite has lower modal content of talc pseudomorphs the vaugnerite and syenite porphyry have high k 2 o 6 to 7 wt and mg number 0 66 0 74 but low cao and na 2 o contents the vaugnerite is markedly rich in p 2 o 5 2 wt and comparing to syenite porphyry has higher amount of mantle incompatible elements e g rb cs ba pb th u v and cr id tims analyses on titanite in vaugnerite and on zircon in syenite porphyry yielded 338 59 0 68 and 337 87 0 21 ma respectively mineral and bulk rock chemistry of the dykes indicates that the source magma was formed by a low degree of partial melting of a highly anomalous domain in the upper mantle which subsequently fractionated and was contaminated with crustal material during its ascent formation of ultrapotassic magma is explained by transcurrent shear zones in the mantle lithosphere that occurred due to block rotation and post collisional mantle dynamics initiated by slab break off and asthenosphere upwelling into the moldanubian accretionary complex during the variscan orogeny

first record of the early toarcian oceanic anoxic event from the southern hemisphere neuquen basin argentina abstract the first record of the early toarcian oceanic anoxic event c 183 ma from the southern hemisphere is described from the neuquen basin argentina identified chemostratigraphically on the basis of a relative increase in marine organic carbon and a characteristic negative carbon isotope excursion 13 c org in bulk rock and fossil wood the negative excursion of 6 in bulk organic carbon falling to 31 3 crosses the boundary of the tenuicostatum hoelderi andean ammonite zones equivalent to the tenuicostatum falciferum serpentinum zones of europe these data indicate that the early toarcian oceanic anoxic event was a global phenomenon supplementary material a detailed stratigraphic log chemostratigraphic data and nannofossil data are available at http www geolsoc org uk sup18411

the timing of paleoenvironmental change and cause and effect relationships during the early jurassic mass extinction in europe the early jurassic marine mass extinction is one of several crises thought to coincide with anoxia transgression and warming caused by catastrophic release of gas hydrates however high resolution study of expanded sections in yorkshire england reveal that only the first of these factors is truly coincidental with extinction the well known transgression and large negative carbon isotope perturbation attributed to massive release of methane from gas hydrates occur after these events the anoxic event is developed diachronously in the european area with anoxia developing and fading away earlier in the mediterranean region compared with the nw european record however for a brief interval in the mid semicelatum subzone anoxia was simultaneously developed throughout the european region and it is this time that is marked by extinction of both benthic and pelagic marine fossils the sea level curve is also more complex than hitherto assumed with a minor regression occurring late in the semicelatum subzone shortly after the extinction the toarcian crisis occurred during a phase of global warming but the postulated release of methane from gas hydrates is too late to be implicated in the extinction mechanism as indeed is a recently reported cooling event from within the warming trend

palaeomagnetic and rock magnetic study of devonian olistoliths set in early carboniferous flysch west sudetes poland summary the upper devonian greywacke mudstone and limestone olistoliths from the zdanow bardo mlynow zbm allochthonous complex of the gory bardzkie gb west sudetes have been sampled for palaeomagnetic investigations 46 hand samples and 30 drill cores from seven sites the random distribution of the olistolithic blocks within early carboniferous flysch provides an opportunity for the megaconglomerate test microscopic and rock magnetic studies showed that pyrrhotite and magnetite are the carriers of magnetic remanence thermal and af demagnetization revealed four components of characteristic remanence the c component found in one site only yields an in situ pole position of 19 s 340 e close to early carboniferous data for baltica the p component also found in one site yields an in situ pole position of 42 s 339 e which coincides with late permian poles for baltica the r component of moderate inclinations and northwesterly and northeasterly declinations found in all but one site is probably partial remagnetization of a devonian component during carboniferous and permian times the d component found in all sites shows significant dispersion of declination of site mean directions in geographical and stratigraphic coordinates but inclinations after tilt correction are consistent with inclinations expected for the middle late devonian calculated for the sudetes from the palaeopoles for baltica significant dispersion of declination is expected for primary magnetization of olistoliths distributed at random within flysch the fold test for inclination only applied to the site mean directions of the d component for the zbm complex is positive the d component is therefore interpreted as representative of the middle late devonian local field inclination the calculated palaeolatitudes 15 s 7 s indicate that the west sudetes were positioned in that time at the same latitude as the southwestern margin of baltica

platform collapse and pelagic seamount facies jurassic development of central tunisia abstract in central tunisia the lower liassic facies consists of peritidal carbonates deposited on a wide platform which has been broken into irregular tilted blocks in the early pliensbachian black shale was deposited in some of the down faulted blocks during the early toarcian oceanic anoxic event the bajocian bathonian deposits are represented by ironstones and hemipelagic ammonite and zoophycos rich marl limestones the upper jurassic is composed by pelagic limestones and debris flow deposits the middle jurassic deep marine sedimentation was frequently associated with condensed sections submarine erosion leading to stratigraphic hiatuses the latter results from the interaction between global sea level fluctuations and regional tectonic activity generating an uplifted block in central tunisia in relation with early mesozoic rifting and associated distensional drifting of the north african tethyan margin this horst block has acted as a starved pelagic shoal on which sedimentation has been recorded especially during retrogradational and the aggradational phases

toarcian extreme warmth led to tropical cyclone intensification abstract models predict that tropical cyclone intensity should increase on a warming planet observing this relationship remains however a difficult task since no clear trend is yet emerging from the anthropogenic record the geological past offers the opportunity to study this relationship by looking at episodes of extreme warmth such as the toarcian oceanic anoxic event t oae early jurassic ca 181 ma in this study we document an increase in the occurrence of storm related deposits during the onset of the t oae in the tide dominated high atlas basin of morocco associated with a drastic deepening of the mean storm weather wave base the palaeolatitude of the high atlas basin 18 north during the early jurassic rules out winter storms as the driving mechanism behind the formation of these deposits and points to a dramatic intensification of tropical cyclone intensity during the t oae global warming these new results combined with the previously reported ubiquitous appearance of storm deposits during the t oae in tropical seas of the western tethyan area support the concept that the globally averaged intensity of tropical cyclones will increase in the coming century due to the anthropogenic climate change

post variscan tectonics in eastern anti atlas morocco new field data integrated by fission track ft analysis unravel an innovative scenario for the post variscan evolution of the eastern anti atlas this area unaffected by meso cenozoic tectonics according to most workers is crosscut by crustal faults bearing evidence of a polyphase deformation history apatite ft ages ranging between 284 and 88 ma point to fast neogene exhumation and unravel contrasting cooling paths across major faults results show that the study area was buried beneath 2 km of allochthonous variscan units now eroded the eastern anti atlas acted as the southern shoulder of the atlasic rift in the mesozoic and underwent a dextral transpressional structuring of neogene age followed by sub meridian shortening the southern front of atlasic deformation is therefore located inside the anti atlas region and it is still active

hydrothermal venting of greenhouse gases triggering early jurassic global warming abstract the climate change in the toarcian early jurassic was characterized by a major perturbation of the global carbon cycle the event lasted for approximately 200 000 years and was manifested by a global warming of 6 c anoxic conditions in the oceans and extinction of marine species the triggering mechanisms for the perturbation and environmental change are however strongly debated here we present evidence for a rapid formation and transport of greenhouse gases from the deep sedimentary reservoirs in the karoo basin south africa magmatic sills were emplaced during the initial stages of formation of the early jurassic karoo large igneous province and had a profound influence on the fate of light elements in the organic rich sedimentary host rocks total organic carbon contents and vitrinite reflectivity data from contact aureoles around the sills show that organic carbon was lost from the country rocks during heating we present data from a new type of geological structures termed breccia pipes rooted in the aureoles within the shale of the western karoo basin the breccia pipes are cylindrical structures up to 150 meters in diameter and are mainly comprised of brecciated and baked black shale thousands of breccia pipes were formed due to gas pressure build up during metamorphism of the shales resulting in venting of greenhouse gases to the toarcian atmosphere mass balance calculations constrained by new aureole data show that up to 1800 gt of co2 was formed from organic material in the western karoo basin about 15 times this amount of co2 27 400 gt may have formed in the entire basin during the intrusive event u pb dating of zircons from a sill related to many of the pipes demonstrates that the magma was emplaced 182 5 0 4 million years ago this supports a causal relationship between the intrusive volcanism the gas venting and the toarcian global warming

spatial and temporal distribution of calcareous nannofossils along a proximal distal transect in the lower jurassic of the umbria marche basin central italy abstract three upper pliensbachian lower toarcian successions of the umbria marche basin were analysed for sedimentology and calcareous nannofossil content across a 120 km long transect from proximal to distal areas with respect to the shallow latium abruzzi platform the uppermost part of the corniola unit uppermost pliensbachian and the marne di monte serrone formation p p lower toarcian polymorphum to levisoni ammonite zones that contains organic matter rich levels were studied in the three localities the analysis of the sedimentary structures in the studied successions indicates that the basin was a shallow water system above storm wave base during the late pliensbachian early toarcian the stratigraphic evolution of the sedimentary structures recorded in the studied localities shows that the upper pliensbachian corresponded to a regressive sedimentary interval whereas a complete transgressive regressive relative sea level cycle occurred during the lower toarcian polymorphum zone a further transgression occurred in the lowermost levisoni zone lower toarcian changes from carbonate dominated to marl dominated sediments form decimetre to metre scale lithological units that can be correlated from succession to succession and may have resulted from short term sea level and or palaeoenvironmental variations the mean duration of these lithological units suggests that they formed in tune with the first orbital eccentricity cycle 100 ka the analysis of calcareous nannofossil quantities abundance per mm2 of the smear slide percentage mean abundance shows both spatial and temporal changes in the community structure schizosphaerella spp and crepidolithus crassus were more abundant in proximity to the shallow latium abruzzi platform these two taxa were likely affiliated to oligotrophic conditions in marine surface waters explaining why they were generally more abundant in carbonate rich intervals than in marls mitrolithus jansae is interpreted as being a deep dwelling species which was more abundant during rising sea level in the earliest toarcian species of the genera lotharingius biscutum and calyculus were more abundant when higher trophic conditions occurred and were thus more common during periods of marl deposition the stratigraphic distribution of calcareous nannoplankton and its interpretation in terms of palaeoecological affinities allow three main phases in the environmental evolution during the early toarcian anoxic event to be recognised the first phase corresponds to a long term transgression and high nannoplankton abundance in distal areas mainly lotharingius biscutum and calyculus species suggests high productivity that may have resulted from upwelling the beginning of accumulation and preservation of organic matter in the sediments occurred during this phase the scarcity of nannoplankton during the second phase may indicate oligotrophic conditions in surface waters possibly due to water stratification during the highest sea level stand a high abundance of schizosphaerella spp and c crassus is recorded in proximal areas probably resulting from sporadic nutrient input from continental or shallow platform areas the maximum of organic matter preservation is recorded during this phase during the third phase relative sea level fall and efficient water mixing by frequent storm events led to enhanced nannoplankton productivity in proximal and distal areas and to a decrease in organic matter accumulation in the umbria marche basin

drowning of a carbonate platform as a precursor stage of the early toarcian global anoxic event southern provence sub basin south east france the toarcian oceanic anoxic event is well known as coinciding with a carbonate crisis coupled with organic matter accumulation and perturbation of the carbon cycle expressed by carbon isotope excursions in this palaeoenvironmental setting the present research attempts to better constrain the palaeoenvironmental conditions leading to the drowning of a carbonate platform during late pliensbachian to early toarcian times this study is based on the integrated sedimentological diagenetic and geochemical stable isotopes and rock eval pyrolysis analysis of several stratigraphic successions located in the southern provence sub basin south east france eodiagenetic ferroan calcite cements below two hardground surfaces at the pliensbachian toarcian transition record episodic eutrophication events such events were precursor stages of a global palaeoenvironmental perturbation i e a global carbon cycle perturbation that led to the demise of the carbonate platform and occurred prior to the toarcian oceanic anoxic event subsequently the acme of the palaeoenvironmental deterioration that coincided with the toarcian oceanic anoxic event is recorded by a pronounced negative carbon isotope excursion 3 followed by a positive one 2 87 organic matter preservation and inferred high temperature of surface sea water ca 25 c as given by oxygen isotope values from chondrichthyan tooth enamel sedimentological comparisons as well as chemostratigraphic and biostratigraphical correlations between the early toarcian sedimentary series of the southern provence and eastern dauphinois sub basins indicate that tectonic tilting occurred earlier in the southern provence sub basin this resulted in the preservation of organic rich deposits in the first sub basin and depositional hiatus in the second sub basin thus local conditions such as contrasting structural setting could markedly modify the sedimentary signature of a global climatic event

shrimp dating of zircons from high grade metasediments of the schwarzwald sw germany and implications for the evolution of the moldanubian basement four metasedimentary zircon populations from different tectonometamorphic units of the central and the northern schwarzwald variscan belt sw germany were investigated using sem cathodoluminescence and shrimp dating despite partially strong modifications of primary internal morphologies during variscan metamorphism at amphibolite 750 c 0 4 0 6 gpa and granulite facies conditions 950 1 000 c 1 4 1 8 gpa many grains show well preserved protolith ages the detritus indicates a northern gondwana origin and different palaeozoic episodes of sediment deposition and consolidation two of the studied sediments were deposited in cambrian early ordovician times and consolidated in positions close to northern gondwana late ordovician and rare devonian detritus from sediments of two other tectonometamorphic units indicates much later sedimentation close to the leading edge of gondwana or a terrane assemblage during northern drift towards laurussia subsolidus growth of new zircon due to variscan granulite facies metamorphism of one of the tectonometamorphic units is precisely dated at 335 2 ma

effect of a jurassic oceanic anoxic event on belemnite ecology and evolution the toarcian oceanic anoxic event t oae 183 million y ago is possibly the most extreme episode of widespread ocean oxygen deficiency in the phanerozoic coinciding with rapid atmospheric pco2 increase and significant loss of biodiversity in marine faunas the event is a unique past tipping point in the earth system where rapid and massive release of isotopically light carbon led to a major perturbation in the global carbon cycle as recorded in organic and inorganic c isotope records modern marine ecosystems are projected to experience major loss in biodiversity in response to enhanced ocean anoxia driven by anthropogenic release of greenhouse gases potential consequences of this anthropogenic forcing can be approximated by studying analog environmental perturbations in the past such as the t oae here we present to our knowledge the first organic carbon isotope record derived from the organic matrix in the calcite rostra of early toarcian belemnites we combine both organic and calcite carbon isotope analyses of individual specimens of these marine predators to obtain a refined reconstruction of the early toarcian global exogenic carbon cycle perturbation and belemnite paleoecology the organic carbon isotope data combined with measurements of oxygen isotope values from the same specimens allow for a more robust interpretation of the interplay between the global carbon cycle perturbation environmental change and biotic response during the t oae we infer that belemnites adapted to environmental change by shifting their habitat from cold bottom waters to warm surface waters in response to expanded seafloor anoxia

40ar 39ar ages of the sill complex of the karoo large igneous province implications for the pliensbachian toarcian climate change 1 reliable geochronological results gathered so far n 76 have considerably constrained the timing of the emplacement of the karoo large igneous province lip yet strikingly missing from this dating effort is the huge southern sill complex cropping out in the 0 6 106 km2 main karoo sedimentary basin we present 16 new 40ar 39ar analyses carried out on fresh plagioclase and biotite separates from 15 sill samples collected along a n s trend in the eastern part of the basin the results show a large range of plateau and miniplateau ages 176 2 1 3 to 183 8 2 4 ma with most dates suggesting a 3 ma 181 184 ma duration for the main sill events the available age database allows correlation of the karoo lip emplacement with the pliensbachian toarcian second order biotic extinction the global warming and the toarcian anoxic event provided that adequate calibration between the 40k and 238u decay constant is made the mass extinction and the isotopic excursions recorded at the base of the toarcian appear to be synchronous with both the increase of magma emission of the karoo lip and the emplacement of the sills the co2 and so2 derived from both volcanic emissions as well as carbon rich sedimentary layers intruded by sills might be the main culprits of the pliensbachian toarcian climate perturbations we propose that the relatively low eruption rate of the karoo lip is one of the main reasons explaining why its impact on the biosphere is relatively low contrary to e g the camp triassic jurassic and siberia permo triassic lips

a mechanism for brief glacial episodes in the mesozoic greenhouse 1 the mesozoic perhaps the longest period of warmth during the phanerozoic earth history has been repeatedly affected by short lived cold interludes lasting about one million years while the origin of these cold snaps has been classically attributed to a temporary atmospheric co2 drawdown quantified mechanisms explaining these instabilities of the carbon cycle are still lacking based on a climate carbon cycle model we show that the general demise of carbonate platforms accompanying these short lived cold interludes is a powerful mechanism capable of generating a fast atmospheric co2 decrease and a moderate sea level drop associated with ice sheet buildup the temporary nature of the carbonate production decline explains the relative short time of these cold events but makes it possible to account for ice sheet waxing and waning

a carbon isotope perturbation at the pliensbachian toarcian boundary evidence from the lias group ne england a perturbation in the carbon isotope record at the time of the pliensbachian toarcian boundary 184 ma in the early jurassic is reported based on new data from yorkshire england two sharp 13 c org negative excursions each with a magnitude of 2 5 and reaching minimum values of 28 5 are recorded in the bulk organic matter record in sediments of latest pliensbachian to earliest toarcian age a similar pattern of negative carbon isotope excursions has been observed at the stage boundary in the sw european section at peniche portugal in 13 c carbonate 13 c wood and 13 c brachiopod records the isotopic excursion is of interest when considering the genesis and development of the later toarcian oceanic anoxic event oae as well as the second order global extinction event that spans the stage boundary furthermore the isotope excursion potentially provides a chemostratigraphic marker for recognition of the stage boundary which is currently achieved on the basis of different ammonite faunas in the nw european and tethyan realms

deciphering the message of early cretaceous drowning surfaces from the helvetic alps what can be learnt from platform to basin correlations drowning unconformities are a frequent feature of carbonate platforms and generally express the incapacity of shallow marine ecosystems to adapt to abrupt sea level rise and or palaeoenvironmental change during the late hauterivian and early barremian the helvetic carbonate platform experienced a major drowning phase documented by the phosphate and glauconite rich altmann member it has been shown that a drowning unconformity in the form of a hardground at the base of the altmann member on the pilatus summit central switzerland results from a complex polyphased diagenetic history that includes two main phases of phosphogenesis using carbon isotope stratigraphy biostratigraphy and sequence stratigraphy the correlation of this drowning unconformity from the helvetic domain with more distal and more complete sedimentary archives allows reconstruction of the sequence stratigraphic context of the drowning surface and also an estimation of the amount of time represented by the episodes of condensation unconformities associated with the altmann member drowning phase developed during transgressive episodes when strong currents arrived onto the previously subaerially exposed platform and periods of phosphogenesis may have lasted up to 2 8 myr variations in trophic levels and hitherto less well known intervening emersion phases played an essential triggering role for the altmann drowning phase high resolution studies of drowning unconformities unravel the diversity of palaeoenvironmental parameters involved in the unfolding of such crises during the evolution of carbonate platforms

from central atlantic continental rift to neogene uplift western anti atlas morocco terra nova 23 35 41 2011 abstract to put constraints on the mesozoic to recent growth of the anti atlas system we investigated the temperature time history of rocks by applying extensive low temperature thermochronological analysis to three precambrian inliers along the coast and 250 km into the interior bedrocks yield old u th he ages on zircon 248 193 ma and apatite 150 50 ma and also fission track ages of 173 121 ma on apatite these datasets are interpreted as recording passive margin upward movements from central atlantic rifting until the early cretaceous a phase of sedimentary burial was evidenced for the cretaceous eocene the extension of this thin 1 5 km basin is loosely constrained but can be extended to the western regions of northern africa effects of the existing thermal perturbation of lithospheric origin 100 km below the atlas show that the 120 60 c isotherms are not much deflected large scale uplift has possibly occurred in the western anti atlas since c 30 ma and is associated with a mean denudation rate of 0 08 km ma 1

silurian paleogeography of armorica new paleomagnetic data from central bohemia due to the lack of reliable paleomagnetic data from armorica for mid ordovician to late devonian times the drift history for this tectonic element is poorly understood in order to help rectify this problem a paleomagnetic study of upper silurian volcanic intrusive and sedimentary rocks from the barrandian basin of the central bohemian massif czech republic has been carried out this basin comprises a sequence of unmetamorphosed lower paleozoic rocks the folding of which has an uppermost age limit of early carboniferous stepwise thermal demagnetization of the samples collected 182 samples from 21 sites and principal component analysis of both great circle and stable endpoint data reveal a total of three directions of magnetization the first labelled a is in the direction of the present day local earth s magnetic field d i 360 67 and is thus interpreted as being of recent origin the other two directions termed b and c comprise the intermediate and high blocking temperature directions respectively the overall in situ mean direction obtained for b identified between 150 and 320 c is 195 8 d i k 18 5 95 13 2 eight sites it fails the fold test and yields an in situ paleo south pole position of 42 s 354 e this coincides with the late carboniferous segment of the apparent polar wander apw path for stable europe and is considered to be a secondary overprint of this age the third magnetization termed c is identified between 320 and 600 c gives an overall mean direction of 208 40 k 96 8 95 5 7 eight sites after bedding correction it passes fold and reversal tests on both local and regional levels and is interpreted as representative of the late silurian paleofield direction in the bohemian massif it yields a paleo pole position of 55 s 325 e assuming a normal polarity the inclination value obtained translates into paleolatitudes of 23 s for this part of armorica and implies a position adjacent to the southern margin of baltica this therefore indicates closure of the intervening ocean by late silurian times however adopting this polarity also implies large scale up to 140 anticlockwise rotations of the bohemian massif prior to deformation in the early carboniferous

failed silurian continental rifting at the nw margin of gondwana evidence from basaltic volcanism of the prague basin tepla barrandian unit bohemian massif the silurian volcanic rocks of the prague basin represent within plate transitional alkali to tholeiitic basalts which erupted in a continental rift setting through the thick cadomian crust of the tepla barrandian unit bohemian massif despite the variable often intense alteration resulting in post magmatic replacement of the basalt mass due to carbonatization the geochemical signatures of silurian basalts are still sufficiently preserved to constrain primary magmatic processes and geotectonic setting the studied interval of silurian volcanic activity ranges from wenlock homerian 431 ma to late ludlow gorstian 425 ma with a distinct peak at the wenlock ludlow boundary 428 ma trace element characteristics unambiguously indicate partial melting of a garnet peridotite mantle source wenlock basalts are similar to alkaline oib with depleted radiogenic nd signature compared to ludlow basalts which are rather tholeiitic emorb like with enriched radiogenic nd signature the correlation of petrogenetically significant trace element ratios with nd isotopic compositions points to a mixing of partial melts of an isotopically heterogeneous possibly two component mantle source during the wenlock ludlow melting lava eruptions were accompanied by intrusions of doleritic basalt and meimechite sills the latter represent olivine rich cumulates of basaltic magmas of probably predominantly ludlow age meimechites with dolerites and to a lesser extent some lavas were subject to alteration due to wall rock fluid interaction the trigger for the wenlock to ludlow 431 425 ma extension and related volcanism in the prague basin is related to far field forces namely slab pull regime due to progressive closure of the iapetus ocean the main stage of the baltica laurentia collision then caused the prague basin rift failure at ca 425 ma that has never reached an oceanic stage

the significance of an early jurassic toarcian carbon isotope excursion in haida gwaii queen charlotte islands british columbia canada abstract during the early toarcian there was a significant disruption in the short term active carbon reservoir as revealed by carbon isotope records which show a broad positive shift that is interrupted by a large 5 7 negative excursion 13 c org carbon isotope excursion co occurs with the deposition of organic rich shales in many areas this perturbation in carbon isotopes is thought to be indicative of severe climate change and marine anoxia the two leading hypotheses as to the cause of this event invoke either global or regional controls here we present carbon isotope data from haida gwaii british columbia canada showing a significant perturbation within a temporally constrained early toarcian succession that was deposited in the northeastern paleo pacific ocean these data reinforce the concept that the short term active carbon reservoir was affected globally and assist with the correlation of ammonite zonal schemes between western north america and europe the 13 c org data show a broad positive shift that is interrupted by a sharp and pronounced negative excursion of 7 8 5 in 13 c wood in the early toarcian kanense zone this negative excursion also coincides with increasing total organic carbon toc from 0 4 to 1 2 these data suggest that the early toarcian carbon isotope perturbation was indeed global and imprinted itself on all active global reservoirs of the exchangeable carbon cycle deep marine shallow marine atmospheric

astronomical forcing and chronology of the early toarcian early jurassic oceanic anoxic event in yorkshire uk 1 during the early toarcian 183 ma ago a high rate of organic carbon burial globally over a brief interval of time has led to the recognition of a major oceanic anoxic event oae a pronounced negative excursion in the carbon isotope composition of marine organic matter marine carbonate and terrestrial plant material is a key feature of this event but the precise timescale and cause s of this isotopic anomaly are debated associated with the negative carbon isotope excursion is evidence for a coeval rise in seawater palaeotemperature an increase in continental weathering rates and the mass extinction of marine invertebrate species the early toarcian oae provides evidence for the earth s response during rapid climate change and critical to our understanding of the event is a high resolution timescale that allows us to quantify the rates duration and lead lag times of environmental processes in this study we present 2743 new high resolution organic carbon sulphur and carbonate concentration data from samples of well preserved organic rich mudrocks spanning the early toarcian oae in yorkshire uk we have used these data to document the geochemical changes and significantly extend and refine the astronomical timescale across this event our detailed analysis of the relationship between astronomical forcing and carbon isotope changes in both yorkshire and a section from peniche portugal indicates that astronomical forcing paced the timing of major shifts in 13c and hence climate in both sections our analyses also demonstrate that there was a marked increase in the relative strength of astronomical forcing recorded at the onset of the oae and that the recorded nature of astronomical forcing changed during the event both the yorkshire and peniche cyclostratigraphies suggest that one astronomical forcing parameter paced environmental change through the 13c event and that this parameter was obliquity or precession

paleomagnetism of siluro devonian sequences ne spain the paleogeography of northeastern spain is addressed in a detailed paleomagnetic study of siluro devonian boundary sediments from the catalan coastal ranges ccr at santa creu d olorda barcelona and the eastern pyrenees at baen the sequences studied are all well dated biostratigraphically and have undergone only very minor deformation mainly during the late carboniferous and extremely low grades of metamorphism anchizone or less after stepwise thermal demagnetisation of samples and principal component analysis of the data up to two paleomagnetic directions can be clearly identified after removal of a present day magnetic overprint the first component labeled b is identified only in samples from the ccr with intermediate unblocking temperature spectra 300 400 c an overall mean direction of 187 03 semi angle of cone of 95 confidence 95 9 0 and precision parameter k 43 1 in situ and 195 03 95 24 4 and k 7 1 51 samples at seven sites after bedding correction is obtained for b the exact age of this secondary component is uncertain but when compared to published paleomagnetic data a late carboniferous to early triassic age could be inferred a stable endpoint direction labeled c can be identified between 400 and 600 c in samples from both regions for the ccr it is southwesterly directed with intermediate inclination values in situ and yields a mean direction of 208 48 95 12 8 and k 28 4 six sites after bedding correction it passes the fold test is considered to be primary in origin and is thus indicative of the siluro devonian paleofield direction in the ccr in the pyrenean samples the c direction is northwesterly directed in situ with a mean direction of 240 51 95 6 5 and k 86 5 seven sites after bedding correction although there is a clear improvement in grouping of these site mean directions this is not significant at the 95 level of confidence combining the site mean directions obtained for component c from both regions yields an overall mean direction of 224 51 95 8 5 and k 24 6 13 sites after bedding correction the improvement in grouping is significant at the 99 level of confidence at 90 unfolding reinforcing the interpretation that this is a primary magnetisation adopting the reversed polarity option the inclination value obtained translates into paleolatitudes of 30 s placing ne spain in the same latitudinal belt as the armorican massif and the barrandian terrane of the bohemian massif i e the armorican terrane assemblage in latest silurian earliest devonian times

post variscan exhumation of the central anti atlas morocco constrained by zircon and apatite fission track thermochronology the origin of the anti atlas relief is one of the currently debated issues of moroccan geology to constrain the post variscan evolution of the central anti atlas we collected nine samples from the precambrian basement of the bou azzer el graara inlier for zircon and apatite fission track thermochronology zircon ages cluster between 340 20 and 306 20 ma whereas apatite ages range from 171 7 ma to 133 5 ma zircon ages reflect the thermal effect of the variscan orogeny tectonic thickening of the ca 7 km thick paleozoic series likely enhanced by fluid advection apatite ages record a complex mesozoic cenozoic exhumation history track length modelling yields evidence that i the precambrian basement was still buried at ca 5 km depth by permian times ii the central anti atlas was subjected to erosional exhumation during the triassic early cretaceous then buried beneath ca 1 5 km thick cretaceous paleogene deposits iii final exhumation took place during the neogene contemporaneously with that of the high atlas

two phased collapse of the shallow water carbonate factory during the late pliensbachian toarcian driven by changing climate and enhanced continental weathering in the northwestern gondwana margin abstract the end pliensbachian toarcian is characterized by several carbon cycle perturbations and faunal turnovers e g ammonites and foraminifera which are most likely triggered by pulses of the karoo ferrar chon aike large igneous province the majority of information about these events is based on detailed studies of sites deposited in deep water settings which leaves vast uncertainties about the expression of and response to these events in shallow marine ecosystems here we present a comprehensive assessment of paleoclimatic impacts on neritic depositional environments from the latest pliensbachian through the middle toarcian in the central high atlas basin morocco and compare those with changes observed in coeval neritic environments within the western tethyan realm a total of four new stratigraphic sections were investigated in the southern part of central high atlas basin and these new sections are synthesized with six previously published sections distributed over eight localities correlations between sections are based on biostratigraphy chemostratigraphy and lithostratigraphy in morocco two episodes of carbonate factory shutdown are observed spanning the pliensbachian toarcian boundary and the polymorphum levisoni transition each carbonate factory collapse correlates to well documented environmental disturbances during the latest pliensbachian middle toarcian interval including the toarcian oceanic anoxic event t oae moreover each episode of carbonate factory shutdown coincides with an interval characterized by a significant increase of coarse siliciclastic input in the basin further demonstrating the link between global warming increased continental weathering and ecosystem turnovers furthermore these two episodes of carbonate factory shutdown are each followed by episodes of renewed carbonate production showing the resilience of the neritic carbonate factory in this region the first recovery interval occurring during the late polymorphum zone is associated with a mixed siliciclastic carbonate system the second episode of carbonate recovery quickly follows the shutdown associated with the onset of the t oae it is associated with an abiotic dominated carbonate production mode resulting in an elevated ooid production a full recovery of biotic carbonate production only occurs in the late stage of the t oae although biotic turnover occurs at both events from a shallow marine perspective the major biotic and abiotic crisis occurred at the pliensbachian toarcian boundary and not during the t oae this is in contrast to the deep marine record where the t oae is often inferred to be the most significant event an enhanced hydrological cycle and the subsequent increase of continental nutrient shedding might have triggered the most severe changes of the carbonate productivity at the pliensbachian toarcian transition whereas ocean acidification and increased storm activity likely played a significant role at the onset of the t oae

interactions between environmental change and shallow water carbonate buildup along the northern tethyan margin and their impact on the early cretaceous carbon isotope record 1 the evolution of the early cretaceous northern tethyan carbonate platform was not only influenced by changes in sea level detrital influx and surface water temperature but also by changes in trophic levels we distinguish between phases of carbonate production dominated by oligotrophic photozoan communities and by mesotrophic and eventually colder water heterozoan communities superimposed on this bimodal trend in platform evolution were phases of platform demise for which we provide improved age control based on ammonite biostratigraphy the initial phase of these episodes of platform demise corresponds in time to episodes of oceanic anoxic events and environmental change in general on the basis of a comparison between the temporal changes in an early cretaceous ammonite calibrated d 13 c record from southeastern france and coeval changes in the platform record we suggest that the history of carbon fractionation along the northern tethyan margin was not only influenced by changes in the oceanic carbon cycle such as in the rate of production and preservation of organic and carbonate carbon and in the size of the oceanic dissolved inorganic carbon reservoir but it was also influenced by the above mentioned changes in the ecology and geometry of the adjacent carbonate platform phases of photozoan carbonate production induced positive trends in the hemipelagic carbonate d 13 c record phases of heterozoan carbonate production pushed the d 13 c system toward more negative values platform drowning episodes implied an initial increase in d 13 c values followed by a longerterm decrease in d 13 c values

new early devonian paleomagnetic data from nw france paleogeography and implications for the armorican microplate hypothesis to constrain the early devonian paleogeography of the armorican massif and determine the tectonic relationships between the armorican and bohemian massifs the armorican microplate hypothesis a paleomagnetic study of lochkovian pragian sediments from the pointe de l armorique brittany nw france has been carried out after removal of a present day overprint a higher unblocking temperature component of magnetization is identified the b direction which yields an overall mean direction of 222 35 k 121 6 95 4 2 after bedding correction 11 sites no fold test is possible but the presence of a thin sill allowed sampling for a contact test the results of which are positive thus the b direction of magnetization is considered to be primary in origin and closely bracketed with the rock age the resulting paleopole 315 e 13 s corresponds to a paleolatitude of 19 s for the armorican massif placing it adjacent to the southern margin of baltica and avalonia this clearly demonstrates for the first time that this sector of the rheic ocean had closed by early devonian times and that the armorican massif was in its present day orientation with respect to baltica these results are significantly different to existing paleomagnetic data from bohemia which indicate large scale 140 rotation between the earliest and latest devonian and thus provide important information for the pre and syncollisional tectonic relationships of these massifs as it is unlikely that such large scale differential crustal rotations could have occurred in an intraplate setting it is proposed that the armorican and bohemian massifs did not form a single coherent microplate but belonged to a fairly loose mosaic of terranes prior to collision with avalonia thus the concept of an armorican microplate is no longer tenable and the term armorican terrane assemblage ata is proposed in reference to the gondwana derived paleozoic terranes now situated within the european variscan fold belt south of the rhenohercynian zone avalonia

organic geochemistry of the early toarcian oceanic anoxic event in hawsker bottoms yorkshire england a comprehensive organic geochemical investigation of the hawsker bottoms outcrop section in yorkshire england has provided new insights about environmental conditions leading into and during the toarcian oceanic anoxic event t oae 183 ma rock eval and molecular analyses demonstrate that the section is uniformly within the early oil window hydrogen index hi organic petrography polycyclic aromatic hydrocarbon pah distributions and tricyclic terpane ratios mark a shift to a lower relative abundance of terrigenous organic matter supplied to the sampling locality during the onset of the t oae and across a lithological transition unlike other ancient intervals of anoxia and extinction biomarker indices of planktonic community structure do not display major changes or anomalous values depositional environment and redox indicators support a shift towards more reducing conditions in the sediment porewaters and the development of a seasonally stratified water column during the t oae in addition to carotenoid biomarkers for green sulfur bacteria gsb we report the first occurrence of okenane a marker of purple sulfur bacteria psb in marine samples younger than 1 64 ga based on modern observations a planktonic source of okenane s precursor okenone would require extremely shallow photic zone euxinia pze and a highly restricted depositional environment however due to coastal vertical mixing the lack of planktonic okenone production in modern marine sulfidic environments and building evidence of okenone production in mat dwelling chromatiaceae we propose a sedimentary source of okenone as an alternative lastly we report the first parallel compound specific c13 record in marine and terrestrial derived biomarkers across the t oae the c13 records of short chain n alkanes acyclic isoprenoids and long chain n alkanes all encode negative carbon isotope excursions cies and together they support an injection of isotopically light carbon that impacted both the atmospheric and marine carbon reservoirs to date molecular c13 records of the t oae display a negative cie that is smaller in magnitude compared to the bulk organic c13 excursion although multiple mechanisms could explain this observation our molecular petrographic and rock eval data suggest that variable mixing of terrigenous and marine organic matter is an important factor affecting the bulk organic c13 records of the t oae

palaeozoic palaeomagnetic studies in the welsh basin recent advances in the last two years new palaeomagnetic data fro m wale s have resulted i n radical revision of the ordovician palaeogeography of eastern avalonia part of the southern margin of the iapetus ocean combined with palaeozoic palaeomagnetic data from laurentia and gondwana these data suggest that eastern avalonia was a peri gondwanide high latitude continental fragment during at least part of ordovician time with a palaeolatitude of about 62 s and 51 s in arenig and llanvirn time respectively this implies a latitudinal width of the early ordovician iapetus ocean between eastern avalonia and laurentia of at least 30 geological evidence for the proximity of eastern avalonia and laurentia suggests that the intervening iapetus ocean closed during silurian time from late llandovery to early ludlow recent palaeolatitude data from the iapetus bordering continents are consistent wit h closure b y middle to late silurian time new pre acadian early devonian palaeomagnetic data from the old red sandstone places the welsh basin at about 17 s consistent with a palaeogeography in which laurentia baltica avalonia armorica and possibly gondwana wer e part o f a singl e supercontinent pervasiv e lat e carboniferous early permia n remagnetization affect s the welsh basin the remagnetization i s probably associated with fluids emanating from th e variscan thrust front w e do not observe remagnetization associate d with acadian orogen y and the remagnetizations which have been studied in more detail in north america appear to be a unique feature of the variscan hercynian alleghenian orogeny

paleozoic paleogeography of north america gondwana and intervening displaced terranes comparisons of paleomagnetism with paleoclimatology and biogeographical patterns new paleomagnetic data have become available in the past 5 yr that require modifications in previously published paleogeographic reconstructions for the silurian and devonian in this paper the new paleopoles are compared to published paleogeographic models based on paleoclimatologic and biogeographic data the data from the three fields of paleomagnetism paleoclimatology and biogeography are generally in excellent agreement and an internally consistent paleogeographic evolutionary picture of the interactions between north america gondwana and intervening displaced terranes is emerging during the interval of the ordovician silurian and devonian north america stayed in equatorial paleoposition while rotating counter clockwise the northwest african part of gondwana was in high southerly latitudes during the late ordovician and was fringed by peri gondwanide terranes such as southern europe armorica and avalonian basement blocks now found in eastern newfoundland nova scotia the boston basin the appalachian piedmont and northern florida subsequently gondwana and the peri gondwanide terranes displayed rapid drift with respect to the pole this drift translates into the following pattern of movement for northwest africa during the latest ordovician early silurian this area moved rapidly northward from polar to subtropical latitudes followed by equally rapid southward motion from subtropical to intermediate about 50 s paleolatitudes during the late silurian middle devonian it is likely that significant east to west motion accompanied the latter shift in paleolatitudes with the caledonian acadian orogeny the result of silurian to early devonian convergence and collision between gondwana and north america this collision sandwiched several of the intervening displaced terranes between gondwana and north america subsequent to this collision gondwana was separated in the late devonian by a medium width ocean from north america and the avalonian and southern european blocks which were left behind adjacent to north america this new ocean closed during the carboniferous and the resulting convergence and collision were the cause of the hercynian alleghanian orogenic belt problems remaining for future research besides the further gathering of reliable paleopoles involve the uncertain pre devonian position of the southern british isles in this scenario and the very rapid velocity with respect to the pole that results from the rapid late ordovician silurian apparent polar wander for gondwana