calcareous nannofossil zonation of the jurassic cretaceous boundary interval and correlation with the geomagnetic polarity timescale abstract calcareous nannofossil stratigraphy has been investigated in six european land sections and at two deep sea drilling project sites 391c and 534a ranging from the upper kimmeridgian to the lower valanginian most of the sections contain a continuous record of the jurassic cretaceous boundary interval a time of rapid increase in nannofossil abundance and diversity and allow development of a revised nannofossil zonation scheme for this period numerous nannofossil lineages have been studied in detail and the results help to increase potential stratigraphic resolution in this interval the proposed scheme consists of six zones which are units recognizable in all sections regardless of nannofossil preservation and eleven subzones determinable in all but the most poorly preserved material thirty eight additional biohorizons are proposed based on the stratigraphy of the two dsdp sites these can be accurately established only in moderately well preserved material but spotty occurrences of these taxa in other sections may be stratigraphically useful most of the sections studied have been investigated magnetostratigraphically and the results of this study allow correlation between nannofossil zonation and the geomagnetic polarity time scale magnetostratigraphy indicates that many nannofossil events can be quite precisely determined and are not significantly time transgressive finally the magnetostratigraphic definitions of two stage boundaries are refined the kimmeridgian tithonian and berriasian valanginian boundaries are placed within chrons cm22n and cm15n respectively two new genera faviconus and umbria and seven new species are described umbria granulosa rhagodiscus nebulosus cretarhabdus octofenestratus faviconus multicolumnatus nannoconus infans nannoconus compressus and nannoconus wintereri eight subspecies and three taxonomic emendations are also described

middle and late jurassic radiolarian palaeoecology versus carbon isotope stratigraphy abstract jurassic radiolarians are sensitive to palaeoclimatic palaeoceanographic changes recorded by the stable carbon isotope curve carbon isotopic variations have been linked to changes in ocean structure productivity and concentrations of greenhouse gases in the atmosphere in the middle upper jurassic terminilletto section of the umbria marche sabina apennines central italy well preserved radiolarians are present in a sequence of cherty limestones interbedded with platform derived carbonates the 13 c curve in the aalenian kimmeridgian interval indicates three main positive 13 c events in the lower bajocian late bathonian lower callovian and middle oxfordian the positive 13 c shifts are correlated to increases in visible chert in the outcrop and may represent periods of increased biological productivity of surface sea water spumellaria nassellaria ratios and the fluctuations of relative abundance of selected genera tend to correlate with 13 c and chert abundance values and seem to be related to episodes of high nutrient mobilisation from the base of the bajocian the radiolarian associations show a slight tendency towards diversification coincident with increasing 13 c values in the upper part of the middle bajocian last appearances prevail over first appearances associated with the peak of the 13 c event this may indicate maximum eutrophic conditions coincident with the late bathonian early callovian positive 13 c shift the radiolarian associations record a different trend with respect to the bajocian event when the 13 c values are increasing to a maximum radiolarian associations tend to be reduced in species diversity followed by a small diversification trend when the 13 c values are decreasing we speculate that during the callovian a more stressful palaeoenvironment due to high level of eutrophication is indicated by the combination of high productivity of biogenic silica and low carbonate input although varying preservational conditions may have influenced the observed trends in the radiolarian associations the good correlation between changes of sedimentation patterns radiolarian assemblages and 13 c positive anomalies open interesting avenues for further investigations concerning the palaeoecology of mesozoic radiolarians

microseismic signatures of non linear pore fluid pressure diffusion summary most fractured porous rocks show a strong dependence of their fluid transport properties on the effective pressure and thus on the pore fluid pressure we investigate the case where the hydraulic transport properties are exponentially dependent on pore fluid pressure as a consequence the process of pore fluid pressure diffusion is governed by a non linear diffusion equation solutions of this equation that describe fluid injection experiments are analysed and the associated microseismic signatures are explored in particular it is shown that this process of non linear pore fluid pressure diffusion allows us to define two signatures one which is the triggering front corresponding to the triggering of microseismic events without changing the fluid transport properties significantly the other signature is the so called fracturing domain it triggers the majority of microseismic events and alters the fluid transport properties significantly evidence for the microseismic signature of a fracturing domain at the fenton hill hydrofrac experiment is given

selective calcareous nannoplankton survivorship at the cretaceous tertiary boundary calcareous nannoplankton underwent devastating diversity loss at the cretaceous tertiary boundary 65 5 ma but recovered rapidly in the early paleocene from a small number of survivor species an understanding of this survivorship has been hampered by uncertainties introduced by reworking and mixing but new high resolution assemblage data from the northwest pacific shatsky rise ocean drilling program site 1210 allow the unequivocal identification of 10 survivors evidence of shared adaptive strategies among these species provides the first indication that the extinctions were selective with survival limited to a few neritic and or opportunistic species probably facilitated by hardiness and or life cycle escape strategies 2005 geological society of america

evidence for annual records of phytoplankton productivity in the kimmeridge clay formation coccolith stone bands upper jurassic dorset uk the upper jurassic kimmeridge clay formation is a c org rich sequence of mudrocks that was deposited in a dysoxic basin which served unusually to preserve primary coccolithophorid proportions strikingly contrasting within this dark coloured formation are the white coccolith stone bands which contain pristinely preserved coccolithophorid assemblages preserving potentially primary coccolithophorid abundances with minimal diagenetic alteration using methodology described herein scanning electron microscopy sem observation of mum scale laminae within selected stone bands has revealed five distinctive types of microlamina type 1 being barren clay type 2 comprising coccoliths associated with varying amounts of clay or amorphous organic matter type 3 being purely coccolithic type 4 being of putative microbial origin and type 5 containing frequent calcispheres these are all described and illustrated here illustrated sequences of these are discussed with reference to an hypothetical model sequence based on modem records representing a kimmeridgian year of coccolithophorid productivity annual signals are thus interpreted based on the observations presented here and including summaries of the palynomorph records from the stone bands we comment on watznaueriacean palaeoecology and particularly the interpretation of watznaueria fossacincta barnesiae as an opportunistic growth maximising taxon c 2004 elsevier b v all rights reserved

the carbonate signal and calcareous nannofossil distribution in an upper jurassic section balingen tieringen late oxfordian southern germany total abundance assemblage composition and the stratigraphic distribution of the six most abundant species of calcareous nannofossil were analysed in the upper oxfordian succession of balingen tieringen western swabian alb south germany this locality represents a deep shelf less than 200 m environment that is strongly influenced by its proximity to the jura shallow platform nannofossil abundance negatively correlates with carbonate content as well as with accumulation rate calcareous nannofossils thus did not produce the bulk of carbonates and their abundance was essentially controlled by the import of allochthonous carbonate mud gradual increase in carbonate content in the balingen tieringen succession can be related to an intensified carbonate mud export from the jura shallow platform to the swabian alb deep shelf during the late oxfordian changes in the nannofossil assemblage composition along the profile are interpreted as primary although a partial control by diagenesis on the assemblage can not be completely excluded the coccoliths lotharingius hauffii watznaueria britannica cyclagelosphaera margereli watznaueria barnesae watznaueria manivitae and the incertae sedis schizosphaerella spp display their highest relative abundance one after the other in a stratigraphic order this nannofossil succession parallels the gradual increase in allochthonous carbonate mud accumulation on the deep shelf as well as the gradual decrease in the intensity of siliciclastic input recorded on the jura shallow platform this suggests a climatic control by humidity on shallow platform carbonate production and export basinwards and on nutrient availability in deep shelf surface waters that itself controlled the nannofossil assemblage composition therefore the succession of highest relative abundance of l hauffii w britannica c margereli w barnesae w manivitae and schizosphaerella spp can represent a trophic preference continuum from more eutrophic to oligotrophic conditions

size patterns of the coccolith watznaueria barnesiae in the lower cretaceous biotic versus abiotic forcing abstract body size is a major trait allowing the assessment of the respective control of evolution vs palaeo environment on unicellular micro organisms size patterns can be retraced for fossilisable micro organisms especially for coccolithophores over geological time spans here we present coccolith size patterns of watznaueria barnesiae which was a dominant and ubiquitous taxon in the mesozoic measurements were conducted at two sites spanning the upper berriasian to lower hauterivian namely the vergol la charce section in the epicontinental vocontian basin and the odp leg 185 hole 1149b located in the nadezhda basin in the pacific ocean the obtained results were treated using multivariate analysis in order to detect significant trends of the measured parameters size values of coccoliths from both sites are remarkably similar and display comparable patterns namely a statistically significant increase in the lower valanginian until maximum sizes recorded in the aftermath of the weissert event upper valanginian although w barnesiae coccolith size fluctuations in the short term might be related to environmental changes the observed long term size patterns are interpreted as the result of directional selection in which a single phenotype is favored through time comparison with data available in the literature shows a return to a smaller size after the turonian

tectonics and magma dynamics coupling in a dyke swarm of iceland abstract to study the dynamics of dyke and fissure swarms in iceland we analyse the distribution of the dykes across the tertiary alftafjordur dyke swarm and that of open fissures and faults across the active krafla fissure swarm within these narrow 5 to 10 km wide and long length width ratio 10 swarms deformation is accommodated both by tectonics and magmatism the dyke swarm built during the westward regional tilting of the host rock lava pile the populations of dykes suggest that a characteristic thickness at about 3 m is involved during dyking dykes show a quasi periodic distribution across the swarm and form clusters spaced by about 2 5 km the analysis of the fissure distribution in the krafla volcanic system reveals a cluster spacing of about half of that of the dyke clusters we discuss the potential origins of these features as a result of magma dynamics host rock properties and tectonics clustering possibly reflects the intrinsic mechanical response to stretching of the upper levels of the brittle crust we propose a three dimensional model of dyke swarms in which magma likely migrates laterally within the upper crust from igneous centres where high magmatic supply rates locally perturb the regional stress field

pyroclast snow interactions and thermally driven slurry formation part 1 theory for monodisperse grain beds lahars are often produced as pyroclastic flows move over snow this phenomenon involves a complicated interplay of mechanical and thermal pro cesses that need to be separated to get at the funda mental physics the thermal physics of pyroclast snow interactions form the focus of this paper a theoretical model is developed of heat and mass transfer at the interface between a layer of uniformly sized pyroclasts and an underlying bed of snow for the case in which there is no relative shear motion between pyroclasts and snow a microscale view of the interface is required to properly specify boundary conditions the physical model leads to the prediction that the upward flux of water vapor which depends upon emplace ment temperature pyroclast grain size pyroclast layer thickness and snow permeability is sometimes suffi cient to fluidize the pyroclasts uniform fluidization is usually unstable to bubble formation which leads to vigorous convection of the pyroclasts themselves thus predicted threshold conditions for fluidization are tantamount to predicted thresholds for particle convection such predictions are quantitatively in good agreement with results of experiments described in part 2 of this paper because particle convection commonly causes scour of the snow bed and transfor mation of the pyroclast layer to a slurry there exists a thermal scour process for generating lahars from pyroclastic flows moving over snow regardless of the possible role of mechanical scour

rift jump process in northern iceland since 10 ma from 40ar 39ar geochronology due to the westward american eurasian plate boundary migration the rift zone location in northern iceland imposed by the stable icelandic hotspot has jumped eastward the present day north volcanic zone of iceland is thus shifted about 100 km to the east with respect to the kolbeinsey ridge a miocene paleo rift location was proposed along the hunafloi skagi axis unconformities inside the lava pile on both sides of the present day rift as a result of the new rift formation have been used to date the rift jump but timing is still controversial whether extinction of the paleo rift was synchronous with initiation of the new rift or the two rifts were active during some time period is unknown using the 40ar 39ar radiochronology method 37 dykes were dated along a 350 km profile parallel to the divergent plate motion direction i e ese wnw crossing the present rift the unconformities and the inferred paleo rift our results reveal that the paleo rift axis was not located along the hunafloi skagi axis but 60 km to the east this previously unknown paleo rift is named the skagafjordur paleo rift it remained active until approximately 3 ma the activity of the present north volcanic zone started about 8 8 5 ma intruding 9 9 5 myr rocks located at least 10 km east of the skagafjordur paleo rift axis consequently the two rifts were simultaneously active for 5 5 5 myr during this time period their accretion rates were nearly equal but with asymmetrical opening the largest rates occurring on external flanks these results are coherent in the icelandic context and permit a first reconstruction of the evolution of northern iceland since 10 ma

palaeoenvironmental control on sponge microbialite reefs and contemporaneous deep shelf marl limestone deposition late oxfordian southern germany abstract the late oxfordian bimammatum zone deep shelf deposits of southern germany are characterised by abundant reefs that laterally pass into marl limestone alternations the bioconstructions are made from three main components sponges microbialites and allochthonous sediment a detailed morphological analysis of sponge microbialite reefs in the plettenberg section swabian alb southern germany is combined with investigation of calcareous nannofossil assemblages in laterally correspondent marl limestone alternations in order to trace the evolution of palaeoenvironmental conditions during reef development fluctuations of caco3 content in deep shelf deposits provides evidence of a succession of small scale lithological units generally formed by five to six marl limestone couplets within a small scale lithological unit a close link is observed between growth patterns of sponge microbialite reefs and the changes in the nannofossil assemblage composition phases of expansion of the build ups correspond to carbonate deposits characterised by a relative dominance of large oligotrophic nannoplankton species large watznaueria britannica watznaueria manivitiae and schizosphaerella spp marly intervals reveal a high relative abundance of small sized taxa mainly lotharingius hauffii small w britannica and subordinated biscutum dorsetensis and bioconstructions show a net reduction in size or a total demise this suggests that the same palaeoenvironmental parameters controlled at the same time marl versus limestone deposition reef growth phases and calcareous nannofossil assemblage composition successive reef growth phases recognized within a bioherm are correlated to the limestone hemi couplets each reef growth phase is composed of several stacked elementary sequences comprising the following succession i sponge ii microbialites and iii carbonate mud in turn the microbialitic crust is formed of three successive microbial layers 1 a fine crust of dense to clotted micrite 2 a centimetre scale to multicentimetric columnar layer of clotted to peloidal micrite 3 a centimetre scale stromatolitic crust the passage from an initial dense microbial layer with numerous microencrusters to a final stromatolitic crust is interpreted as resulting from an increase of the microbial growth rate mainly driven by changes in the accumulation rate deep shelf deposits resulted from changes in platform carbonate production and export basinwards controlled both by sea level and climate changes climate controlled the intensity of carbonate production in shallow platform environments and export towards the deep shelf as well as the trophic conditions of the deep shelf marine waters low trophic conditions favoured platform carbonate production and export basinwards and a large development of the sponge microbialite reefs in the deep shelf however a high carbonate accumulation rate in the deep shelf was also commonly responsible for the demise of sponge microbialite reefs during unfavourable and higher trophic conditions during marl deposition reduced development and frequent demise of the sponge microbialite reefs is observed

depositional sequences in deep shelf environments a response to sea level changes and shallow platform carbonate productivity oxfordian germany and spain abstract oxfordian deep shelf deposits of southern germany and southern spain are characterized by marl limestone alternations that are stacked into small scale medium scale and large scale depositional sequences the german sections contain autochthonous sponge reefs and associated fragments of microbialites whereas the sections studied in spain display tempestites composed of autochthonous deeper water and allochthonous shallow water particles the facies of the german limestones and marl layers has been analyzed in detail condensed intervals implied by glauconitization of particles abundant cephalopods intense bioturbation and generally more marls are also enriched in sponge reefs and associated particles and in nannofossils limestone rich intervals however contain fewer sponges and fewer nannofossils neither bioerosion of sponge reefs nor nannofossil blooms can thus explain the abundance of carbonate mud that forms the limestones consequently it is suggested that most of the carbonate mud is exported from shallow platform areas where carbonate productivity is high the clay fraction was derived from weathering of massifs in the hinterland the observed depositional sequences can be correlated between the studied sections in germany and spain situated in different paleotectonic and paleoclimatic domains and also between deeper water sections and platform sections this suggests that they formed through allocyclical processes comparision with published time scales implies that the small scale and medium scale sequences formed in tune with the 100 kyr and 400 kyr orbital eccentricity cycles respectively however the number of marl limestone alternations is not always consistent with the expected number of 20 kyr precessional cycles 5 per 100 kyr cycle the large scale sequences reflect long term third order sea level changes the observed marl limestone alternations are interpreted to have formed through cyclically varying export of carbonate mud from the platform towards the deep shelf the variations being controlled by climatically induced high frequency sea level fluctuations enhanced marl deposition on the deep shelf can be related to sea level fall causing exposure of the shallow platform and reducing the area of carbonate production or by rapid sea level rise maximum flooding leading to partial or total drowning of the platform and or to retrogradation of facies belts enhanced carbonate deposition occurs during transgression when large production areas are created on the platform or during late highstands when progradation is forced and carbonate mud export enhanced depending on the long term trend of sea level change on which the high frequency fluctuations are superimposed one high frequency 20 kyr sea level cycle can thus create one or two marl limestone alternations or only marly deposits consequently one 100 kyr eccentricity cycle may be formed of a variable number of marl limestone alternations commonly 2 to 8 the studied deeper water depositional sequences thus are strongly linked to the history of the adjacent shallow carbonate platforms

field characteristics of laterally emplaced dikes anatomy of an exhumed miocene dike swarm in reydarfjordur eastern iceland abstract many dikes of the tertiary breiddalur dike swarm in reydarfjordur have a great lateral extent but are vertically upward and or downward discontinuous field relationships indicate that they are not the feeders of the lava pile they intrude their characteristics can be best interpreted as them constituting the fossil expression of lateral magma emplacement analogous to that occurring in the actively spreading krafla volcanic center of the neovolcanic zone average dike thickness increases clearly with depth and the thickness versus depth relationship is different from that described for other tertiary dike swarms in eastern iceland it is suggested that dikes represent the extent of finite spreading which at higher levels in the crust is expressed by fissures and other extensional structures

sr ca variations in cretaceous carbonates relation to productivity and sea level changes abstract we present high resolution sr ca measurements on coccolith dominated pelagic carbonates of berriasian valanginian albian santonian and campanian maastrichtian age from dsdp sites and tethyan land sections carbonate sr ca varies by up to 80 over both long and short timescales and may reflect either temporal variations in seawater sr ca or variable sr partitioning in biogenic carbonates numerical models of seawater sr and ca budgets indicate that while sea level changes can cause comparably large variations in the seawater sr ca ratio the timing and pattern of predicted sr ca variations agrees poorly with the carbonate record reef crises unrelated to sea level changes may also elevate seawater sr ca and in some cases the record of reef crises corresponds to increasing carbonate sr ca changes in sr partitioning likely due to changes in calcareous nannoplankton productivity also appear to control carbonate sr ca in these records cretaceous carbonate sr ca broadly covaries with indicators of productivity derived from microfossil ecology recent studies in sediment core tops and cultures have shown that sr partitioning in coccolith carbonate is strongly influenced by changes in coccolithophorid growth and calcification rate where the effects of changing seawater sr ca can be constrained by independent geological data carbonate sr ca measurements could provide a valuable perspective on changes in the productivity of calcareous nannoplankton and variations in the carbonate and carbon cycle in the cretaceous

calcareous nannofossil assemblage turnover in response to the early bajocian middle jurassic palaeoenvironmental changes in the subbetic basin abstract the calcareous nannofossil assemblages from three lower bajocian pelagic successions from the subbetic basin well dated by ammonites have been investigated biostratigraphically they encompass the njt9 to njt10a nannofossil tethyan zones pca factor analysis performed on percentage abundances of selected taxa suggests that the main factors controlling the early bajocian nannofossil assemblages of the subbetic basin were trophic level nutrient availability and nutricline depth a gradual eutrophication is recorded in the calcareous nannofossil assemblages through an increase in the percentage abundance of watznaueria spp especially in the taxa best adapted to unstable and more eutrophic conditions w britannica gr and w manivitiae gr this eutrophication is concomitant with a trend of increasing carbonate 13 c values over the early bajocian and a rise in biogenic silica production and deposits in the study sections and other basins of the western tethys around the middle part of the laeviuscula subzone a decrease in the absolute and relative abundance of the deep dwelling taxa especially crepidolithus crassus suggests a deterioration of the environmental conditions in the lower photic zone this has been interpreted as caused by a nutricline shallowing and or greater turbidity that could be related in turn to surface water stratification two different episodes have been differentiated in relation to nutricline dynamics 1 from the latest aalenian to earliest early bajocian a weak and or deep nutricline with nutrients rather uniformly spread throughout the photic zone and concomitant with mesotrophic conditions and increasing productivity is suggested 2 a shallowing of the nutricline and or increase in surface water turbidity concomitant with more eutrophic conditions is suggested for the latest early bajocian the observed decrease in the deep dwelling taxa supports the hypothesis of a climatically driven fertilization of the surface waters instead of fertilization by upwelling for the early bajocian eutrophication episode

compressibility of porous rocks four compressibilities are defined for porous rock relating changes in the bulk and pore volumes to changes in the pore and confining pressures using a micromechanical theory based on classical linear elasticity three relations are found between these compressibilities two of these relations are verified experimentally for berea and bandera sandstone bounds are derived for these compressibilities involving only the porosity and the elastic moduli of the rock forming minerals for the strains to be unique functions of the stresses the compressibilities must be functions only of the difference between the confining and pore pressures this dependence is verified for berea bandera and boise sandstone while the strains cannot be expressed as functions of an effective stress regardless of how it is defined the elastic volumetric behavior of porous rocks can be studied through tests conducted at zero pore pressure

secular environmental precursors to early toarcian jurassic extreme climate changes the early toarcian oceanic anoxic event t oae about 183 myr ago was a global event of environmental and carbon cycle perturbations which deeply affected both marine biota and carbonate production nevertheless the long term environmental conditions prevailing prior to the main phase of marine extinction and carbonate production crisis remain poorly understood here we present a similar to 8 myr long record of early pliensbachian middle toarcian environmental changes from the lusitanian basin portugal in order to address the long term paleoclimatic evolution that ultimately led to carbonate production and biotic crises during the t oae paleotemperature estimates derived from the oxygen isotope compositions of well reserved brachiopod shells from two different sections reveal a pronounced similar to 5 degrees c cooling in the late pliensbachian margaritatus spinatum ammonite zones boundary this cooling event is followed by a marked similar to 7 10 degrees c seawater warming in the early toarcian that after a second cooling event in the mid polymorphum zone culminates during the t oae calcium carbonate caco3 contents the amount of nannofossil calcite and the mean size of the major pelagic carbonate producer schizosphaerella all largely covary with paleotemperatures indicating a coupling between climatic conditions and both pelagic and neritic caco3 production furthermore the cooling and warming episodes coincided with major marine regressions and transgressions respectively suggesting that the growth and decay of ice caps may have exerted a strong control on sea level fluctuations throughout the studied time interval this revised chronology of environmental changes shows important similarities with neogene and paleozoic episodes of deglacial black shale formation and thus prompts the reevaluation of ice sheet dynamics as a possible agent of mesozoic events of extinction and organic rich sedimentation c 2010 elsevier b v all rights reserved

timing of sea level tectonics and climate events during the uppermost oxfordian planula zone on the iberian ramp northeast spain the middle oxfordian warming climate and sea level rise initiated the development of vast carbonate platforms in some western european basins at the same time however siliciclastics and siliceous sponges dominated certain marginal areas of the iberian ramp there siliciclastic input was particularly prominent during the latest oxfordian and may have been related to a global sea level fall synsedimentary tectonic activity or humid climatic conditions in the hinterland field analyses and computer modelling have been previously used to determine the factors that controlled sedimentation however it is still unclear if the specific conditions that prevailed during the latest oxfordian were due to eustasy tectonics or climate and when precisely they occurred here we document major changes in sedimentological micropalaeontological and mineralogical records on the iberian ramp during this interval detailed sedimentary facies and palynofacies analyses combined with sequence stratigraphic and cyclostratigraphic analyses of the ricla barranco section enable the establishment of a highresolution time frame based on the quartz and mica percentage fluctuations one large and seven small scale sequences are defined the large scale sequence boundaries correlate with third order sequence boundaries ox 8 and kim 1 defined by hardenbol et al 1998 the large scale maximum flooding surface corresponds to the base of the most calcareous interval and to the maximum abundance of marine phytoplankton and opaque equidimensional phytoclasts the small scale sequences correspond to the 100 kyr orbital eccentricity cycle calcareous nannofossils and clay minerals were used as palaeoclimatic proxies nannofossil abundances and fluxes are lower in the upper part than in the lower part of the interval studied suggesting a decrease in seasurface trophic conditions also shown by an increase in the relative abundance of oligotrophic taxa this upper part is also characterised by an increase in smectite which coincides with the base of the large scale highstand deposit and is interpreted as reflecting the establishment of dry conditions a first increase in smectite occurs in the lower part of the succession and coincides with high percentages of quartz and mica this latter mineralogical assemblage is interpreted as recording the onset of the late jurassic to early cretaceous rifting stage which occurred just before the planula galar ammonite subzone transition the present study points out a return toward optimum conditions for carbonate sedimentation only 300 kyr after the prominent increase in siliciclastic input due to tectonic activity the recovery of carbonate production was accompanied by a global sea level rise and by decreasing rainfall on nearby land

discorhabdus as a key coccolith genus for paleoenvironmental reconstructions middle jurassic lusitanian basin biometry and taxonomic status abstract biometric analyses of discorhabdus coccoliths were performed on a set of 29 samples taken from the marl limestone couplets of the reference section of cabo mondego lusitanian basin west portugal late aalenian to early bajocian in age mixture analysis performed on the whole dataset revealed a bimodal length size distribution for the discorhabus pool composed of d striatus and d ignotus the size limit at 5 m proposed here can be reliably used to distinguish both populations but optical characteristics namely birefringence form an additional criterion essential to discriminate the two species an increase in discorhabdus size occurred from the late aalenian to the early bajocian due to a raise in size of the whole discorhabdus pool and in the abundance of d striatus we propose that an increase in discorhabdus size upward in the section is associated with a positive trend in paleoproductivity as supported by the increase of nannofossil accumulation rates as well as the 13 c values of bulk carbonates sampled in the cabo mondego section however the discorhabdus size pattern may also represent a response to sea surface temperature salinity or an evolutionary trend a relative supra regional warming of sea surface water masses is suggested by 18 o values of biogenic carbonates and apatites that were sampled in several mediterranean tethyan settings rise of sea surface temperature may have triggered an increase in the abundances of d striatus as well as of the size of the discorhabdus pool a hypothetical influence of changing sea surface salinity on discorhabdus size or species specific discorhabdus abundances is not supported by our micropaleontological data or by available geochemical data our results also reveal that the increase in discorhabdus size matches the history of calcareous nannofossil turnover radiation diversity and increasing abundances during the middle jurassic

dike complexes in american samoa abstract coherent dike complexes in which dikes make up more than 50 of the total rock occur in american samoa in two volcanoes on tutuila and in ofu olosega volcano in the manu a islands the dikes are remarkably narrow median width 0 24 0 36 m average width 0 46 0 56 m based on 860 measured dikes significantly narrower than dikes in the koolau volcano oahu hawaii the most closely comparable volcano for which quantitative data are available the vesicularity of these narrow dikes indicates that they were injected at shallow depths below the land surface depths of 200 900 m are inferred from the topography many dikes have prominent bands of vesicles parallel with their margins they tend to form clusters where the weakness of the vesicular rock favored center injection of one dike up the center of another examples are found of center injection repeated up to eight times local breccias often composed largely of dike fragments evidently brecciated in situ occur where irregular dikes were accommodated by deforming the host rocks we see evidence for two dike facies 1 narrow and vesicle banded early formed and shallow injected clustered dikes in the dike complexes which are cut by 2 scattered broad and almost nonvesicular deep injected solitary dikes surface eruptions were fed by solitary dikes the ene trending dikes and elongation of tutuila island contrast with the wnw trend of rift zones and island elongation elsewhere in samoa tutuila is nearly in line with the north fiji transform fracture zone suggesting that at 1 5 1 0 ma when tutuila was formed the volcanism of tutuila was guided by a temporary extension of this fault zone into the pacific plate fault movement offset the samoan hotspot trace by 45 km the en echelon arrangement of dike exposures on tutuila is consistent with left lateral transcurrent motion above this concealed fracture zone

back front of seismicity induced by non linear pore pressure diffusion borehole fluid injections are accompanied by microseismic activity not only during but also after termination of the fluid injection previously this phenomenon has been analysed assuming that the main triggering mechanism is governed by a linear pressure diffusion in a hydraulically isotropic medium in this context the socalled back front of seismicity has been introduced which allows to characterize the hydraulic transport from the spatiotemporal distribution of post injection induced events however rocks are generally anisotropic and in addition fluid injections can strongly enhance permeability in this case permeability becomes a function of pressure for such situations we carry out a comprehensive study about the behaviour and parametrization of the back front based on a model of a factorized anisotropic pressure dependence of permeability we present an approach to reconstruct the principal components of the diffusivity tensor we apply this approach to real microseismic data and show that the back front characterizes the least hydraulic transport to investigate the back front of non linear pore fluid pressure diffusion we numerically consider a power law and an exponential dependent diffusivity to account for a post injection enhanced hydraulic state of the rock we introduce a model of a frozen i e nearly unchanged after the stimulation medium diffusivity and generate synthetic seismicity we find that for a weak non linearity and 3d exponential diffusion the linear diffusion back front is still applicable this finding is in agreement with microseismic data from ogachi and fenton hill however for a strong non linear fluid rock interaction such as hydraulic fracturing the back front can significantly deviate from a time dependence of a linear diffusion back front this is demonstrated for a data set from the horn river basin hence the behaviour of the back front is a strong indicator of a non linear fluid rock interaction

dynamics of volcanic systems in iceland example of tectonism and volcanism at juxtaposed hot spot and mid ocean ridge systems volcanic systems are swarms of tectonic fractures and basalt volcanoes formed as a result of plate pull as the plates are pulled apart associated with the mid ocean ridges and the magma dynamics of the iceland mantle plume most systems are 40 150 km long 5 20 km wide and develop a central volcano they supply magma to all eruptions in iceland data obtained in the last few years have greatly improved our knowledge of their volcanotectonic environment as a result the geometry of the plume is better constrained and the crust previously considered thin 10 km is now modeled as thick 20 40 km depending on the location of the volcanic systems their activity either decreases or increases faulting in the two main seismic zones from this we can infer that emplacement of the feeder dike to the largest historical eruption in iceland that of laki in 1783 increased shear stress in the south iceland seismic zone and almost certainly triggered the largest m 7 1 in 1784 historical earthquake in ic

isotopic evidence for late jurassic early cretaceous climate change abstract strontium oxygen and carbon isotope ratios have been determined from late jurassic early cretaceous belemnites from the volga basin russia and kawhia harbour new zealand 87sr 86sr ratios derived from well preserved belemnites from the volga basin support a middle tithonian age derived from the analysis of the endemic ammonite fauna the kawhia harbour section records a gradual rise in 87sr 86sr values and in comparison with the published 87sr 86sr curve suggests that the lower part of the section is latest oxfordian in age whilst the upper part of the section correlates well with the biostratigraphic correlation suggestion of an early middle tithonian age although the published strontium calibration curve shows a degree of scatter our new data confirm the uniform rise in 87sr 86sr values from the late jurassic into the early cretaceous such an increase may result from either a decrease in mid oceanic ridge spreading and or an increase in weathering rates and flux of radiogenic strontium although a eustatic drop in sea level and concurrent western cordillera uplift suggests that weathering may have been the controlling factor of late jurassic seawater strontium isotope composition palaeotemperatures derived from the well preserved belemnite 18ocarb values from the volga basin indicate that the middle volgian late kimmeridgian was warm 14 20 c followed by a slight cooling and a subsequent gradual increase to the jurassic cretaceous boundary the 18ocarb values from new zealand located at a palaeolatitude of 80 s if interpreted in terms of palaeotemperature indicate a high degree of variability such variability may not be related to palaeotemperature but to changes in oceanic chemistry resulting from the formation and dissolution of an ice sheet and or snow during the oxfordian kimmeridgian carbon isotope trends for the late jurassic show a fall in values from the oxfordian with lowest values occurring in the early middle tithonian before rising but without reaching values obtained in the oxfordian the overall low 13ccarb may be related to a global increase in continental weathering and or upwelling of cooler oceanic water enriched in oxidised organic carbon 12c enriched

handbook of environmental isotope geochemistry vol 1 the terrestrial environment a p fritz and j ch fontes editors elsevier amsterdam 1980 xii 532 pp u s 90 25 dfl 185 00

palaeoceanographic differences of early late aptian black shale events in the vocontian basin se france abstract a high resolution study focussing on the distribution of calcareous nannofossils and carbon isotopes was carried out to improve the understanding of mid cretaceous black shale formation the studied interval of the early late aptian is characterized by two major black shale couplets the niveau noire 4 nn4 and niveau noire calcaire 2 nnc2 of the serre chaitieu section in the vocontian basin se france breheret 1997 this interval occurs within a long term negative carbon isotope excursion of 1 5 following the early aptian oceanic anoxic event 1a oae1a in contrast to the local nn4 black shales the black shales of nnc2 are of regional significance and occur at the end of the major negative carbon isotope excursion of the early late aptian time equivalent black shales are suggested to be coeval with black shales found in the western tethys and atlantic ocean herrle et al 2004 calcareous nannofossil analyses and carbon isotopes indicate higher surface water productivity mesotrophic warmer surface water and higher sea level during the formation of the nn4 black shales in contrast the formation of the nnc2 black shales took place during a cooler phase lower surface water productivity and lower sea level a sea level fall may cause a restriction of water mass exchange between the open marine western tethys and the vocontian basin this resulted in a longer residence time of the bottom water decreased ventilation and less mixing of surface waters and thus enhanced preservation of organic matter at the sea floor our results indicate that the black shale formation of nn4 and nnc2 was caused by different processes such as increased surface water productivity and enhanced preservation of organic matter at the sea floor thus we emphasize the role of different forcing factors which control the formation of local and regional black shales the most important factors are sea level fluctuations increasing productivity and changes in precipitation and evaporation rates

pore pressure profiles in fractured and compliant rocks1 fluid permeability in fractured rocks is sensitive to pore pressure changes this dependence can have large effects on the flow of fluids through rocks the authors define the permeability compliance gamma 1 k partial derivative k partial derivative p sub p sub pc which is the sensitivity of the permeability k to the pore pressure p sub p at a constant confining pressure p sub c and solve the specific problems of constant pressure at the boundary of a half space a cylindrical cavity and a spherical cavity the results show that when the magnitude of permeability compliance is large relative to other compliances diffusion is masked by a piston like pressure profile the authors expect this phenomenon to occur in highly fractured and compliant rock systems where gamma may be large the pressure profile moves rapidly when fluids are pumped into the rock and very slowly when fluids are pumped out consequently fluid pressure its history and distribution around injection and production wells may be significantly different from pressures predicted by the linear diffusion equation the propagation speed of the pressure profile marked by the point where partial derivative p sub p partial derivative x is a maximum decreases with time approximately as radical t and the amplitude of the profile also dissipates with timemore or distance the effect of permeability compliance can be important for fluid injection into and withdrawal from reservoirs for example excessive drawdown could cause near wellbore flow suffocation also estimates of the storage capacity of reservoirs may be greatly modified when gamma is large the large near wellbore pressure gradients caused during withdrawal by large gamma can cause sanding and wellbore collapse due to excessive production rates less

biomineralization within vesicles the calcite of coccoliths coccolithophores are a group of unicellular plant plankton which produce exoskeletons of minute calcite plates called coccoliths despite their small size 1 10 m across coccoliths are remarkably elaborate biomineral structures characterized by precise control of both nucleation and growth of calcite by the organic system they are also of considerable environmental and applied geological importance coccolithophores occur in enormous abundances forming a significant proportion of total marine primary production and carbon fixation especially in open ocean environments their biogeochemical impact is magnified by export of coccoliths to the ocean floor where coccoliths are the largest single component of deep sea sediments forming vast accumulations of calcareous oozes and chalks including the late cretaceous chalks of northwestern europe moreover coccoliths are extensively employed by geologists as marker fossils to determine the ages of sediments especially from drill cores more recently satellite imaging of immense coccolithophore blooms has led to extensive multi disciplinary research into their biology and impact on the marine ecosystem and global carbon cycle e g westbroek et al 1993 paasche 2002 thierstein and young in press biological affinities coccolithophores belong to the algal division or phylum haptophyta other haptophytes are non calcifying and include the well known genera phaeocystis prymnesium pavlova and chrysochromulina they are all characterized by possessing golden brown chloroplasts two smooth flagella and a third flagellum like organelle the haptonema the haptonema shows distinctive coiling behavior and has a quite different microtubular sub structure from flagella molecular genetic data have shown that the haptophytes are a group discrete from other algal protists that probably originated during the precambrian 600 ma protist radiation and acquired chloroplasts subsequently possibly in the late palaeozoic ca 300 400 ma as a result of secondary endosymbiosis medlin et al 1997 molecular genetic phylogenies of the haptophytes are still being developed but all results to date edvardsen

global calibration of gephyrocapsa coccolith abundance in holocene sediments for paleotemperature assessment 1 a global sea surface temperature calibration based on the relative abundance of different morphotypes within the coccolithophore genus gephyrocapsa in holocene deep sea sediments is presented there is evidence suggesting that absolute sea surface temperature for a given location can be calculated from the relative abundance of gephyrocapsa morphotypes in sediment samples with a standard error comparable to temperature estimates derived from other temperature proxies such as planktic foraminifera transfer functions a total of 110 holocene sediment samples were selected from the pacific indian and atlantic oceans covering a mean sea surface temperature gradient from 13 6 to 29 3 c standard multiple linear regression analyses were applied to this data set linking the relative abundance of gephyrocapsa morphotypes to sea surface temperature the best model revealed an r2 of 0 83 with a standard residual error of 1 78 c for the estimation of mean sea surface temperature this new proxy provides a unique opportunity for the reconstruction of paleotemperatures with a very small amount of sample material due to the minute size of coccoliths permitting examination of thinly laminated sediments e g a pinhead of material from laminated sediments for the reconstruction of annual sea surface temperature variations such fine scale resolution is currently not possible with any other proxy application of this new paleotemperature proxy may allow new paleoenvironmental interpretations in the late quaternary period and discrepancies between the different currently used paleotemperature proxies might be resolved

biometry of emiliania huxleyi and its biostratigraphic significance in the eastern north atlantic ocean and western mediterranean sea in the last 20 000 years abstract a detailed biometric study of coccoliths of emiliania huxleyi has been performed on 34 samples from three sediment cores of the north atlantic ocean and the western mediterranean sea su90 08 m39029 7 and md95 2043 all three cores contain the last glacial interglacial transition marine isotopic stages 1 2 enabling us to study in detail the morphology of this taxon during this period of change one hundred coccoliths of e huxleyi were randomly chosen in each sample distal shield length and width measurements were performed on each of the individuals selected the data show that e huxleyi specimens larger than 4 m are frequent in glacial samples and that these larger forms decreased sharply in abundance during the deglaciation and holocene smaller forms are more abundant in this latter group of samples the decrease in larger forms seems to be time transgressive since it is recorded between 12 and 11 kyr cal bp in southern locations alboran sea and gulf of cadiz and around 8 4 kyr cal bp in the central north atlantic scanning electron microscope analyses indicated that the two forms have the same degree of calcification and hence this parameter should not be used in taxonomic classifications we suggest that the larger coccoliths belong to a cold water variety of e huxleyi which can be distinguished from the small coccolith variety in light microscope analyses by its distal shield length the decrease in the abundance of this larger variety during the deglaciation period could be used as a biostratigraphic event in the north atlantic and mediterranean areas

gravity and height variations connected with the current rifting episode in northern iceland abstract following a first gravity survey in 1938 high precision gravity measurements with lacoste romberg gravimeters have been carried out in northern iceland in 1965 1970 1971 and 1975 in order to detect eventual secular gravity variations at that supramarine part of a constructive plate boundary since 1975 when an active rifting episode started there gravity and height variations have been observed in annual intervals these surveys concentrated on the approx 100 km long and 5 km wide fissure zone associated with the krafla central volcano which is the center of the present activity the observations which have been performed at an accuracy level of 0 01 mgal resp 0 03 m give significant information about the long term behaviour of the vertical mass shifts occuring during the rifting process a steep gravity variation gradient observed south of the krafla caldera between 1965 and 1975 may be interpreted as some kind of precursor of the current process since 1975 the rifting event is characterized by shifting activity areas with gravity decrease and elevation at the flanks and gravity increase and subsidence at narrow central parts of the krafla fissure zone the annual variations reaching 0 1 mgal resp 0 5 m and more since 1978 the activity has concentrated on the gjastikki region north of the krafla volcano with decreasing variations in the southern and northern parts of the fissure swarm the average gravity height variation ratio calculated for two profiles south and north of krafla over the observation periods between 1975 and 1979 amounts to 0 2 mgal m but extreme values are found at recently activated areas

biscutum constans coccolith size patterns across the mid cretaceous in the western tethys paleoecological implications abstract the morphometric record of the calcareous nannofossil species biscutum constans from the western tethys indicates that this species underwent marked size changes across the latest barremian cenomanian interval 28 myrs significant size reduction occurred during the core of the early aptian oceanic anoxic event oae 1a 120 ma and soon after oae 1a followed by a 9 myr long interval characterized by small specimens larger coccoliths marked the middle to late albian and particularly the lower part of the nannofossil subzone nc10a was characterized by abundant large specimens a minor size decrease coincided with the late albian oae 1d onset 100 ma but very small size was reached again during the latest cenomanian oae 2 94 4 ma the sequence of relatively long millions of years intervals marked by different average sizes is interpreted to reflect adaptation to changing paleoecological conditions temperature and fertility were not directly influencing the size of b constans on shorter time scale abrupt changes in ocean chemistry related to large igneous provinces submarine volcanism played a central role in secretion of small b constans coccoliths to face ocean acidification during oae 1a and oae 2 contrarily during the late early aptian early albian time interval smaller b constans size was related to cooler temperature which possibly also promoted co2 sequestration in cooler surface waters forcing b constans to produce small coccoliths to survive lowered ph in addition pulses of subaerial volcanism of the southern kerguelen plateau may have further affected the ocean chemistry the termination of intense volcanism and warmer temperature in the middle albian early cenomanian coincided with restoration of bigger b constans specimens

size variations of coccoliths in cretaceous oceans a result of preservation genetics and ecology abstract biometric studies of coccoliths the remains of coccolithophores offer the opportunity to survey single species instead of entire assemblages we obtained and analyzed size data of three common species biscutum constans zeugrhabdotus erectus and watznaueria barnesiae in a stratigraphically very well defined interval of early aptian age 126 ma cretaceous material is derived from four sites lower saxony basin north sea western tethys mid pacific covering nearshore to open oceanic paleosettings length and width measurements of 1986 specimens were evaluated the recorded size patterns show a larger data spread for b constans and w barnesiae in the western tethys and the mid pacific than in the north sea and the lower saxony basin the latter two sites are dominated by small coccoliths of b constans while coccoliths of w barnesiae show similar sizes at all four sites solely small specimens of z erectus characterize the samples from the north sea and the lower saxony basin while only large ones are present in the samples of the western tethys and mid pacific for explaining the recorded size patterns three theories are discussed in detail these include 1 preservation of nannofossils 2 genetics and 3 paleoecology 1 intense dissolution or overgrowth of the nannofossils may have altered the original coccolith sizes particularly when biometric data from different sites with potentially varying states of preservation are compared due to its delicate morphology z erectus appears most prone to dissolution probably explaining its size pattern 2 if the recorded size data of the remaining two species represent original patterns these can be interpreted by new findings in recent coccolithophore genetics it has been shown that size variations within a single cryptic species are only minor shifts of coccolith sizes both in recent and fossil taxa can be related to genotypic variation the varying size ranges recorded for b constans and w barnesiae may therefore reflect diversity changes of cryptic species at the different sites 3 these cryptic species may have preferred different depth habitats depending on geographically controlled factors such as sea surface temperatures light availability or trophic load

morphometry of the eocene nannofossil reticulofenestra umbilicus lineage and its biochronological consequences abstract morphometruc measurements of eocene and early oligocene reticulofenestrids from dsdp site 523 in the south atlantic show that a 90 size increased led to the appearance of reticulofenestra umbilicus this increase occurred between about 45 2 ma and 42 9 ma the attempts made to recognize r umbilicus during this period of change as a distinct subpopulation among the reticulofenestrid assemblages did not yield any tenable conclusions however using a lower size limit of 14 m for recognition of the r umbilicus morphovariant resulted in an easily identified first appearance event which occurred during chron 19r at an age of about 44 4 44 5 ma from a morphometric point of view the two morphovariants r hillae and r umbilicus could not be distinguished as two distinct subpopulations the oligocene assemblages show markedly smaller 10 central opening placolith length ratios as compared to the eocene forms but for several reasons it seems difficult to use this decrease biostratigraphically

spatial distribution patterns of calcareous nannofossils across the callovian oxfordian transition in the french subalpine basin abstract callovian oxfordian calcareous nannofossils are documented from four sections distributed along a proximal distal slope to central basin transect in the french subalpine basin south eastern france the sections were dated and correlated using integrated ammonite dinoflagellate and nannofossil biostratigraphies the nannofossil assemblages are constantly dominated by watznaueria britannica however significant spatial changes in nannofossil assemblage composition are recorded nannofossil assemblages recovered in slope and slope to basin transition sections are characterized by high abundances lowest diversities highest percentages of smallest sized morphotypes of w britannica subordinated w britannica manivitiae a morphotype described in this study and are indicative of turbulent unstable and or eutrophic conditions towards the central part of the basin as the nutrient content and turbulence declined and paleoenvironmental stability increased the nannofossil assemblages become more diverse and characterized by lower relative abundances of w britannica but higher contribution of larger morphotypes and higher relative abundances of w barnesiae fossacincta biscutum dubium and zeugrhabdotus erectus these latter small coccoliths markers of high surface waters fertility for the cretaceous were probably not competitive with respect to smaller sized w britannica in unstable and eutrophic surface waters for the jurassic an increase in the surface water productivity is recorded both in proximal and distal sections at the callovian oxfordian transition and is demonstrated both by an increase in nannofossil total abundances and in the relative abundances of taxa adapted to high trophic levels thermal minimum in surface waters as indicated by an increase in the percentages of the cool water nannofossil taxa is associated to increased productivity further studies will be necessary to demonstrate whether surface water productivity changes already recorded in the eastern paris basin were global

faroe iceland ridge experiment 2 crustal structure of the krafla central volcano the seismic velocity structure of the krafla central volcano is characterized by large variations in compressional velocity a 40 km wide high velocity dome extends from the lower crust 11 14 km depth beneath the volcano narrowing upward a magma chamber sits at its top near 3 km depth it is defined by both 0 2 0 3 s compressional wave delays and shear wave shadowing to be 2 3 km n s 8 10 km e w and 0 7 1 8 km thick the near surface structure uppermost 2 5 km of the krafla caldera is approximately flat lying with only minor lateral heterogeneities the crust beneath the magma chamber has low shear wave attenuation and anomalously high compressional and shear wave velocities shear waves reflected from a 19 km deep moho are clearly visible for some paths through the crustal volume below the magma chamber even though the more shallow diving s waves are severely attenuated the midcrust beneath the shallow magma chamber cannot contain partial melt or even be at near solidus temperatures the krafla central volcano plays a major role in crustal genesis along the plate boundary the high velocity dome in our view represents crust generated in and around the magma chamber which has subsequently been advected to greater depths

crustal spreading due to dikes and faults in southwest iceland abstract a 30 km long profile extending from 3 million year old pleistocene rocks to the holocene rocks of the rift zone in southwest iceland is dissected by 443 dikes and 156 normal faults nearly all the dikes are basaltic with an average strike of n26 e which is subparallel to the trend of fractures in the nearby rift zone their average dip is 69 and average thickness 1 37 m both values much lower than averages for regional tertiary dike swarms in iceland many dikes are seen to change into sills the great majority of the faults are pure dip slip with an average throw of 10 2 m their average strike n37 e is different from the average strike of the dikes in this area and the fractures in the nearby rift zone their average dip is 75 2 which is somewhat steeper than in the tertiary areas but the dips are less in the interbedded sedimentary layers than in the lava flows growth faults are very rare in the area and most of the faults appear to have completed their growth in less than 10 000 years in subsections crustal dilation due to dikes is 0 4 3 7 that due to faults is 0 6 5 3 and that due to both dikes and faults is 1 04 6 44 in these subsections average fault spacing is one fault per 170 563 m spacing decreases toward the active rift zone it is concluded that in typical fissure swarms of the rift zone the dilation at the surface is almost entirely due to extensional fractures and gaping vertical normal faults in the depth range of a couple of hundred metres to perhaps 1 km the non vertical faults contribute most to the dilation at deeper crustal levels however dilation due to dikes becomes dominant

paleoclimatic control of biogeographic and sedimentary events in tethyan and peri tethyan areas during the oxfordian late jurassic abstract the paleobiogeographical distribution of oxfordian ammonites and coral reefs in northern and central europe the mediterranean area north and east africa and the middle east and central asia is compared with the distribution in time and space of the most important lithofacies interest in the oxfordian is focused on changes in facies and in biogeographical patterns that can be interpreted as the results of climatic events paleotemperature trends inferred from oxygen isotopes and paleoclimatic simulations are tested against fossil and facies data a late callovian early oxfordian crisis in carbonate production is indicated by the widespread absence of lower oxfordian reefal formations there is a gap hiatus in deposition on epicontinental platforms with middle oxfordian deposits resting paraconformably on upper callovian while shales accumulated in adjacent intracratonic basins simultaneously in mediterranean tethys radiolarites accumulated in deep troughs while rosso ammonitico facies formed on pelagic swells however deposition on swells was also discontinuous with numerous gaps hiatuses and sequences that are much reduced in thickness middle callovian deposits are generally overlain by middle oxfordian limestones the dearth of carbonates is consistent with a cooling event lasting about 1 my by the middle oxfordian a warming leading to greenhouse type conditions is suggested on the basis of both biogeographical mostly coral reef distribution and geochemical data carbonates spread onto an extensive european platform while radiolarites reached a maximum development in the mediterranean tethys two distinct latitudinal belts with seemingly different accumulation regimes are therefore inferred similar latitudinal belts were also present in the late oxfordian when carbonates were widespread the distribution of reefal facies in the late oxfordian early kimmeridgian fits relatively well with gcms simulations that imply low rainfall in the tethyan mediterranean area and slightly higher precipitation in central and northern europe local salinity variations reflecting more arid or humid conditions may bias the paleotemperature signal inferred from 18o values biogeographical and facies distributions combined with 18o values unravel the ambiguity and support a late callovian early oxfordian cooling followed by warming in the later oxfordian

effects of pore pressure in pyroclastic flows numerical simulation and experimental validation pyroclastic flows are mixtures of volcanic gases and particles that can be very hazardous owing to their fluid like behavior one possible mechanism to explain this behavior is the reduction of particles friction due to the internal gas pore pressure to verify this hypothesis we present a numerical model of a granular flow with high initial pore pressure that decreases with time as the gas particle mixture propagates first we validate the model by reproducing laboratory experiments then the numerical code is applied to pyroclastic flows of lascar volcano 1993 eruption chile the simulation reproduces the runout and the morphological features of the deposits with lateral levees a central channel and a lobate front our results support the hypothesis of the role of gas pore pressure in pyroclastic flows and explain both the fluid like behavior of the flows and the formation of lateral levees

distribution of sedimentary organic matter in a mixed carbonate siliciclastic platform environment oxfordian of the swiss jura mountains outcrop sections from the swiss jura consisting of carbonate siliciclastic deposits spanning the middle late oxfordian boundary provide a palaeogeographical cross section ranging from coastal to shallow platform and intra platform basin environments using a sedimentological and a 3rd order sequence stratigraphic framework based on those sections the distribution of sedimentary organic matter palynofacies has been spatially studied in relation to sequence stratigraphy the main factors influencing the spatial variations of sedimentary organic matter are the proximity of land organic productivity level of biodegradation and hydrodynamic conditions of the palaeoenvironment these factors determine the distribution of the land derived relatively allochthonous organic constituents phytoclasts pollen and spores and marine relatively autochthonous constituents phytoplankton and foraminifera linings five main palynofacies parameters appear as good indicators of proximal distal trends the relative proportion of total phytoclasts the proportion of fresh translucent fragments among these phytoclasts the ratio of marine to continental palynomorphs and the relative amount and species diversity of dinoflagellate cysts in lowstand deposits high energy hydrodynamic conditions may affect these trends on the shallow platform where storms tend to increase the proportion of the marine organic fraction landwards these five palynofacies parameters are largely related to water depth therefore when compared with the established 3rd order sequence stratigraphic framework their stratigraphic variations indicate relative sea level changes or alternatively proximality changes parameters indicative of more distal conditions increase in the transgressive and early highstand and decrease in the late highstand the opposite occurs for parameters indicative of more proximal conditions foraminifera linings seem to be particularly concentrated near the shelf edge and on the slope finally bisaccate pollen may display cyclicity associated with alternating humid and dry periods this palaeoclimatic signature may be related to orbital cyclicity

the impact of calcareous nannofossils on the pelagic carbonate accumulation across the jurassic cretaceous boundary abstract calcareous nannofossils were important producers of pelagic carbonates in mesozoic oceans in order to better understand the origin of mesozoic pelagic carbonates we studied upper jurassic to lower cretaceous sediments in the central atlantic ocean dsdp sites 105 367 534a with respect to their content of calcareous nannofossils the interval under investigation is characterized by a significant increase in the deposition of carbonate rich sediments a trend that goes along with the rapid radiation of calcareous nannofossils the assemblage composition and the size variation of common taxa were analyzed and subsequently the contribution of this phytoplankton group to the pelagic carbonate accumulation was calculated results reveal two long term changes of the nannofossil assemblage composition 1 the early tithonian coccolith assemblages are of low diversity watznaueria spp cyclagelosphaera spp zeugrhabdotus spp whilst the mid to late tithonian assemblage is dominated by nannoliths conusphaera mexicana polycostella beckmannii nannoconus spp and large sized watznaueria 2 the early berriasian is characterized by a shift from the late tithonian nannolith rich assemblage to a highly diverse coccolith assemblage morphometric studies of the placolith genus watznaueria show for all three dsdp sites large sized forms of this genus in the mid and late tithonian followed by a decrease of up to 2 m for the mean size in the earliest berriasian at dsdp site 105 the studied nannolith taxa show an increase in size during the mid tithonian to berriasian interval the records of both nannofossil carbonate estimates and the measured bulk rock carbonate reveal two periods of increase in the nannofossil carbonate record of dsdp site 105 a first significant increase of the carbonate accumulation is observed in the mid and late tithonian probably caused by mass occurrences of strongly calcified taxa c mexicana p beckmannii nannoconus spp watznaueria cf manivitae this interval is here named nannofossil calcification event nce the second carbonate maximum in the late berriasian is related to a rise in absolute abundances of nannofossils this peak is amplified by an overall increase of the sedimentation rate the calculated accumulation rates of nannofossils nannofossil carbonate and bulk rock carbonate for the late berriasian are on the same scale as values from recent ocean surface sediments a comparison of nannofossil carbonate values with the bulk rock carbonate content shows that on average only 27 of the total carbonate can be explained by our nannofossil carbonate estimates this discrepancy is most likely caused by the high amount of unidentifiable micrite and fragments of calcareous nannofossils other factors contributing to the error are possible inaccuracies in the determination of absolute abundances and nannofossil volume calculations the nce occurs during a long term sea level fall dry climate and presumed low pco 2 levels a decline in abundance of strongly calcified nannofossils coincides with the opening of the pacific atlantic seaway via central america which may have had a substantial impact on the palaeoceanographic situation in the central atlantic ocean these changes are here considered to have at least partly caused the shifts in abundance size and assemblage composition of calcareous nannofossils observed across the jurassic cretaceous boundary interval

variation in the form and distribution of dykes in the mull swarm scotland the dykes of the mull swarm part of the british tertiary igneous province have been examined in a series of well exposed coastal sections traverses through the dyke swarm show systematic changes in the thickness and spacing of dykes both of which are typically log normally distributed both the geometric mean thickness and spacing of dykes increase with distance from the mull centre concordant with a decrease in crustal extension variations in the thickness and spacing result from changes in character of the magma flow it is inferred that the magma flux was fairly evenly distributed near the mull centre but farther away from the source it becomes focused into larger dykes the form orientation and opening direction are also observed to vary with distance away from the volcanic centre on mull the dykes intrude a range of pre existing fractures the range of fractures used decreases with distance from the centre this together with other features of the form of the dykes have been interpreted in terms of the varying magma pressure and regional stress which is expressed as the r ratio analysis of the variable strikes of fracture filled dykes is consistent with decreasing magma pressure with distance from the mull centre assuming that regional stresses were relatively invariant across the same distance

magma chambers modeled as cavities explain the formation of rift zone central volcanoes and their eruption and intrusion statistics the volcanic rift zone in iceland is characterized by elongate volcanic systems consisting of tension fractures normal faults and volcanic fissures where most of the volcanotectonic activity takes place most volcanic systems develop central volcanoes the formation of which is still poorly understood there is great difference in the eruption and intrusion statistics of the volcanic systems inside and outside the central volcanoes in the central volcanoes eruptions are frequent typically one every several hundred years of small volume normally less than 0 1 km3 and fed mainly by thin average thickness of 0 5 m inclined sheets outside central volcanoes the eruptions are rare typically one every several thousand years but of large volume typically more than 1 km3 and fed mainly by thick average thickness of 4 5 m subvertical dikes using the results of a boundary element study of magma chambers modeled as a cavities or holes in a semi infinite plate these empirical relations as well as the formation of specific central volcanoes in the volcanic systems can be explained once a cavity like magma chamber has formed its existence in the rift zone concentrates tensile stress this stress concentration causes the segment containing the magma chamber to rupture much more frequently than the other parts of the volcanic system which partly explains the formation of central volcanoes and their high eruption frequencies this concentration also gives rise to a local stress field that encourages injection of small volume sheets in all directions and at various dips from their source magma chamber the magma chamber acts as a trap for upward propagating dikes from mantle reservoirs and channels magma through inclined sheets toward a limited area at the surface where the central volcano gradually forms the inverse relationship between eruption frequency and eruption volume when the central volcano is compared with other parts of the volcanic system is also partly due to the trap like nature of the magma chamber the chamber is normally much smaller than its source mantle reservoir so that a single magma flow through a dike from the reservoir lasting perhaps many years may trigger tens of magma flows through sheets from the chamber many of which would reach the surface in the central volcano

shallow platform palaeoenvironmental conditions recorded in deep shelf sediments c and o stable isotopes in upper jurassic sections of southern germany oxfordian kimmeridgian a high resolution carbon and oxygen isotope analysis of late oxfordian early kimmeridgian deep shelf sediments of southern germany is combined with investigation of nannofossil assemblage composition and sedimentological interpretations in order to evaluate the impact of regional palaeoenvironmental conditions on isotopic composition of carbonates this study suggests that carbonate mud was essentially derived from the jura shallow platform environments and also that the isotopic signature of carbonates deposited in the swabian alb deep shelf indirectly expresses the palaeoenvironmental evolution of the platform short term fluctuations in d 13 c and d 18 o are probably controlled by changes in salinity fresh water input versus evaporation in platform environments long term fluctuations in carbon and oxygen isotope record throughout the late oxfordian early kimmeridgian result from the interplay of increasing temperature and decreasing humidity which both control the trophic level changes from mesotrophic to oligotrophic conditions in platform environments and in the deep shelf surface waters are inferred during the late oxfordian bimammatum subzone to planula zone the d 13 c curve displays a positive shift of about 1x which is comparable in intensity to global perturbations of the carbon cycle this evident isotopic shift has not been documented yet in other basinal settings it can be reasonably explained by local palaeoenvironmental changes on the jura platform salinity temperature and nutrient availability that controlled platform carbonate production and the geochemistry of overlying waters however increasing carbonate production on the jura platform and related positive d 13 c shifts recorded in the

infrastructure and mechanics of volcanic systems in iceland the divergent plate boundary in iceland consists of more than two dozen systems where most of the volcano tectonic activity takes place at the surface the volcanic systems are characterised by 5 20 km wide and 40 100 km long swarms of tension fractures 102 m long normal faults 103 m long and volcanic fissures the holocene fissure swarms are confined to less than 10 000 year old basaltic lava flows mostly pahoehoe occurring near the centre of the active rift zone in any particular swarm the number of tension fractures exceeds that of normal faults all tension fractures and normal faults are vertical at the surface indicating that the surface parts were generated by an absolute tension in addition to a fissure swarm many volcanic systems have a central volcano some of which have developed collapse calderas in the late tertiary and pleistocene lava pile of iceland extinct volcanic systems are represented by local sheet swarms and regional dyke swarms the sheet swarms are normally circular or slightly elliptical several kilometres in radius and are confined to the extinct central volcanoes many swarms are associated with large plutons exposed at 1 2 km depth beneath the initial top of the rift zone and presumably the uppermost parts of extinct crustal magma chambers and in short traverses up to 90 of the rock may consist of sheets the sheets have a very variable strike dip on average 45 65 mostly towards the centre of the associated volcano and have an average thickness of about 1 m the regional dykes occur outside central volcanoes in swarms that are commonly 50 km long and 5 10 km wide in several kilometre long traverses commonly 1 5 of the rock consist of dykes but occasionally as much as 15 28 most regional dykes are subparallel and subvertical the average dyke thickness in the pleistocene swarms is less than 2 m but 4 6 m in the tertiary swarms while active the volcanic systems in the rift zone are supplied with magma from reservoirs located at the depth of 8 12 km at the boundary between the crust and upper mantle the reservoirs are partially molten with totally molten top regions and of cross sectional areas similar to those of the volcanic systems that they feed some active volcanic systems especially those that develop central volcanoes high temperature areas and calderas have in addition to the deep seated magma reservoirs shallow crustal magma chambers located at 1 3 km depth which in turn are fed by the deeper magma reservoirs it is proposed that the regional dyke swarms are supplied with magma largely from the deep seated reservoirs whereas the local sheet swarms are mainly fed from the associated crustal magma chambers because the volume of a crustal chamber is less than that of its deeper source reservoir a single magma flow dyke intrusion from the reservoir may trigger tens of magma flows sheet intrusions from the chamber which is one explanation for the enormous number of sheets associated with many central volcanoes the sheets follow the stress trajectories of the local stress fields around the source chambers whereas the regional stress field associated with the divergent plate movements controls the emplacement of the regional dykes it is suggested that many dykes develop as self affine some as self similar structures when applied to the krafla volcanic system in northern iceland model calculations suggest that a magma flow from the krafla reservoir with regional dyke formation should occur on average once every several hundred years and that tens of sheets might be injected from the shallow krafla chamber and triggered by a single magma flow from the krafla reservoir these results are in broad agreement with the available data

thermal evolution of tethyan surface waters during the middle late jurassic evidence from δ18o values of marine fish teeth 1 oxygen isotope compositions of phosphate from vertebrate tooth enamel were measured to determine the evolution of tropical sea surface 0 owing to limited growth of continental ice during the early middle oxfordian the resulting sea level fall is estimated to be at least 50 m and is compatible with a global regression stage the middle oxfordian thermal minimum is followed by a new warming stage of 3 4 c from the middle to the late oxfordian

western tethys early and middle jurassic calcareous nannofossil biostratigraphy an exhaustive biostratigraphic study based on the calcareous nannofossil content from lower and middle jurassic lusitanian basin sections in portugal is here presented located at the strategic intersection of important jurassic water masses during a period of drastic environmental changes the lusitanian basin conveyed mixed phytoplanktonic communities in its surface waters approximately 800 m of exceptionally continuous lower and middle jurassic series bearing detailed ammonite biostratigraphy and two global stratotype section and point are studied vertical and lateral ubiquity of calcareous nannofossils in marine sediments and rapid evolutionary rate and radiation make this fossil group a remarkable biostratigraphic tool nearly 100 bioevents are acknowledged in 24 myr time span this work refers to bridging 11 nannofossil zones and 29 subzones a thorough revision of the existing literature referring to early and middle jurassic calcareous nannofossil biostratigraphy is presented and causes and implications for zonal marker discrepancies discussed under a palaeoceanographic and jurassic nannofossil evolution angle in order to create a solid biostratigraphic frame and to compare it with c and o stable isotopic trends brachiopod calcite shells were analysed for geochemistry this work yields for the first time a very detailed and inclusive biostratigraphic scheme for the early and middle jurassic epochs

paleoenvironmental controls on the morphology and abundance of the coccolith watznaueria britannica late jurassic southern germany the coccolithophore species watznaueria britannica is dominant in middle upper jurassic calcareous nannofossil assemblages and presents morphological variation including different coccolith size shape and length of the central area and of the bridge six morphotypes can be recognized in the polarizing light microscope the aim of this work is to better understand the morphological variability of w britannica and determine if this variability is controlled by paleoecological factors in order to investigate the potential paleoecological controls on w britannica morphology and abundance we carried out a biometric study on a restricted temporal interval the late oxfordian in the swabian alb southern germany characterized by increasing carbonate production linked to climatic changes the balingen tieringen section where previous works on sedimentology nannofossil assemblage composition and 18o and 13c analyses were performed was selected for this study the variations in morphology and abundances ofw britannicawere studied on 40 samples of the balingen tieringen section presenting variable lithologies and calcium carbonate contents for each level seven biometric parameters coccolith length width and ellipticity central area length width and ellipticity and central area proportion with respect to the coccolith were measured or calculated on digitally captured images of the first 100 w britannica coccoliths observed in the light microscope the relationships between the different biometric variables were described using bivariate and principal component analyses biometric parameters and principal component factors extracted from nannofossil assemblages as well as other paleoenvironmental proxies were investigated using regression and their stratigraphic trends were compared principal component analysis of the six biometric variables 3938 measurements on w britannica coccoliths shows a reduced morphological variability compared to a significant size gradient an allometric trend recognized on the total placolith and on the central area within thew britannica assemblages suggests that the different morphotypesmay represent intra specific variability rather than different species the general trend through late oxfordian shows an increase in size ofw britannica coccoliths mainly driven by an increase in the contribution of the largemorphotypes increasing placolith size is associated with drier and warmer climatic conditions during the latest oxfordian

pliensbachian early jurassic calcareous nannofossils from the peniche section lusitanian basin portugal a clue for palaeoenvironmental reconstructions abstract quantitative analysis of early jurassic calcareous nannofossil assemblages from the peniche section in portugal have been performed in order to interpret palaeoenvironmental changes occurring in the lusitanian basin during the late pliensbachian davoei margaritatus and spinatum ammonite zones nannofossil data are compared to already published carbon and oxygen stable isotope data organic matter content wt total organic carbon toc and biomarker analysis a significant change in calcareous nannofossil assemblages and species diversity at the transition between the margaritatus and spinatum ammonite zones matches with the pattern shown by geochemical data this suggests that a profound change in environmental conditions occurred at that time in the davoei and margaritatus ammonite zones in a context of general sea level rise the lusitanian basin was characterized by water column stratification that favoured the sedimentation and preservation of organic matter biomarkers and oxygen isotope trends suggest that stratification of water masses occurred because of low salinity in surface waters the shallowest part of the water column characterized by oligotrophic conditions was inhabited by the probable calcareous dinocyst schizosphaerella spp while the deep dweller crepidolithus crassus flourished in the lower photic zone layers in the spinatum ammonite zone a regressive trend occurred and a salinity increase is inferred on the basis of oxygen isotope values water masses were probably less stratified at that time the upper photic zone nannofossil assemblages were still dominated by schizosphaerella spp whilst in the deep photic zone mitrolithus jansae a mediterranean taxon replaced c crassus a taxon with nw european affinity this pattern may indicate a change in palaeoceanographic conditions related to surface current circulation the sea level fall occurring during the spinatum ammonite zone may have resulted in the partial isolation of the lusitanian basin from the nw europe basins because of the creation of a threshold alternatively tectonic drowning of north african carbonate platforms in the late pliensbachian may have promoted better exchanges of nannoplankton between the lusitanian basin and the mediterranean tethys via the creation of new seaway connections

fertility changes in surface waters during the aalenian mid jurassic of the western tethys as revealed by calcareous nannofossils and carbon cycle perturbations abstract in aalenian times the south iberian palaeomargin was part of the westernmost tethys ocean the median subbetic palaeogeographic domain of the betic cordillera was a relatively deep trough in the south iberian palaeomargin during the early jurassic late cretaceous interval where mainly pelagic and hemipelagic limestones and marls were deposited a semiquantitative study of nannofossil assemblages was performed in sediments from the upper toarcian lowest bajocian from two median subbetic sections agua larga and cerro de mahoma nannofossil assemblages are composed mainly of cosmopolitan and tethyan taxa the nj8a nj8b and nj9 zones as well as other useful biohorizons fos of triscutum tiziense and carinolithus magharensis and lo of similiscutum finchii were identified and directly correlated to ammonite zones the analysis of the relative abundances of some common to abundant taxa including biscutum carinolithus superbus crepidolithus crassus lotharingius schizosphaerella and watznaueria display noticeable fluctuations that can be correlated between the two sections the comparison of these fluctuations with the 13ccarb curves and the interpretation of the palaeoecologic significance of some of these taxa provided an outline of the palaeoceanographic trophic regime throughout the interval studied during the latest toarcian early aalenian the high proportions of oligotrophic schizosphaerella moderately high proportions of c crassus and low proportions of eutrophic biscutum correlate with low to moderate values in the 13ccarb curves radiolarians display low abundance throughout this interval this is interpreted as an interval where mesotrophic to oligotrophic and stable conditions occurred in surface waters the middle aalenian characterized by high proportions of schizosphaerella and c crassus and low proportions of biscutum correlates with low values in the 13ccarb curves and was interpreted to correspond to an episode when stable oligotrophic conditions occurred in surface waters radiolarians moderately increased throughout this interval finally the late aalenian earliest bajocian interval with lower proportions of schizosphaerella and c crassus and higher proportions of biscutum also correlates with a significant positive excursion in the 13ccarb curve suggesting a shift from oligo to eutrophic conditions in surface waters this change in productivity is also revealed by a conspicuous increase in radiolarian abundance at the same time as a quasi complete replacement of early jurassic radiolarian fauna took place the analysis of faunal flora turnovers reveals a causal link between the global carbon cycle and the pelagic response this noticeable faunal flora change throughout the late aalenian bajocian can be interpreted as a major biological response to the drastic modification in the western tethys palaeogeography as consequence of the atlantic opening which in turn caused a new pattern in the oceanic circulation

palaeoenvironmental vs evolutionary control on size variation of coccoliths across the lower middle jurassic size is an easily quantifiable trait probing the respective influence of evolution and palaeoenvironment on organisms through time after the stressed period that spanned across western tethys epicontinental seas during the early toarcian the following rebuilding of a restored water column ecosystem is coeval with the size increase of the lotharingius genus coccoliths one of the main pelagic producers at that time in order to quantitatively assess the coccolith size evolution during a time span of 10 myr measurements of seven morphospecies of lotharingius across the toarcian and early aalenian were performed this study was carried out on a total of 5500 specimens from samples retrieved from two distinctive palaeoenvironmental settings in the western mediterranean tethys three sections from the lusitanian basin in portugal and one section from the causses basin in southern france for each specimen coccolith length width and central area length and width were measured using specific measurement software statistical and data analysis tools were used in order to evaluate significant resemblances and or differences between different size groups and samples two main clusters are acknowledged from this study small sized lotharingius is 4 m in addition to common evolutionary processes changes in palaeoenvironmental conditions were taken into account to explain such a size distribution although background evolution pushed the lotharingius genus toward larger sizes palaeoenvironmental conditions seemed to play an important role in the toarcian early aalenian morphological evolution of this taxon the production of larger coccoliths profited from stable environmental conditions whereas smaller morphotypes dominated during periods marked by stress the transition from small to large lotharingius was synchronous in the two investigated western tethys basins at the bifrons gradata or variabilis ammonite zone boundary middle toarcian thus representing an important event for the lower jurassic calcareous nannofossil biostratigraphy

use of vegetable oil and silica powder for scale modelling of magmatic intrusion in a deforming brittle crust in the upper crust intrusive bodies adopt different configurations depending on the tectonic setting this paper describes a new technique for analogue modelling of such intrusions specifically of low viscosity magma into a deforming brittle crust proper dynamic scaling is an important consideration if the crust has a coulomb failure envelope the model material should have a cohesion c of 40 pa to 7500 pa if the intruding magma is highly mobile the model fluid should have a viscosity of 4 10 9 pa s to 75 pa s for the model crust we have used crystalline silica powder si crystal siliceous microspheres si sphere of grain size 30 m and a mixture si mix of both materials the mechanical properties of these powders have been obtained by shear and tension shear tests si crystal powder is cohesive c 300 pa angle of internal friction 45 and represents competent rock si sphere powder is much less cohesive c 10 pa 25 and represents incompetent rock si mix has intermediate properties the model magma is a vegetable oil which solidifies at room temperature its viscosity when molten is 2 10 2 pa s at 50 c using these materials we have done some preliminary experiments to investigate the intrusion of low viscosity magma into sedimentary basins in non deformed settings intrusions were saucer shaped cone sheets in horizontal extension they were steep dykes finally in horizontal shortening intrusions were basal sills that branched into thrust faults

detailed oxygen and carbon isotope stratigraphy of the oxfordian in central poland a collection of 73 belemnites and brachiopods from detailed stratigraphically documented sections in the polish jura chain of central poland representing almost the whole oxfordian stage was studied for oxygen and carbon isotopes the 18o and 13c v pdb values range from 3 6 to 0 5 and from 1 6 to 2 for belemnite rostra and from 2 5 to 0 4 and from 1 6 to 4 3 for brachiopod shells respectively the 13c temporal trends of belemnite rostra and brachiopod shells show a constant isotopic difference ca 2 5 3 which is interpreted as a vital effect of belemnites or as a result of different life environments 18o values are similar in both groups of fossils the 18o values in the lower and middle oxfordian oscillate around 0 and decrease in the upper oxfordian this decrease results mainly from diagenetic overprint as proved by the cathodoluminescence studies the lower and middle oxfordian 18o derived temperatures were constant in consequence of significant ocean depth for this reason there are no established links with distribution of ammonite faunas the 13c temporal trend displays a gentle slope through the oxfordian and reaches maximum values at the turn of the plicatilis transversarium zones of the middle oxfordian which is commonly considered as a global event it can be related to the sea level highstand and growth crisis of carbonate platforms the decrease in 13c values starting in the upper middle oxfordian is interpreted to be due to the acceleration of carbonate sedimentation rate during this time period

palaeobiogeography of early cretaceous berriasian barremian calcareous nannoplankton in order to assess early cretaceous nannoplankton biogeography we studied a series of sites which provide a north south transect across the atlantic ocean supplemented by sections from the north sea basin barents sea falkland plateau weddell sea antarctica argo abyssal plain nw of australia and neuquen basin argentina quantitative assemblage data were gathered from each site for seven time slices within the berriasian to barremian interval each horizon being determined by a nannofossil datum trends in species relative abundance and measures of diversity evenness and richness provide revealing biogeographic information a broad low to mid palaeolatitude zone 50 degrees n 50 degrees s is flanked in both northern and southern hemispheres by distinct high palaeolatitude zones major changes in assemblage abundance and composition occur across a sharp biogeographic front at around 50 degrees n and s palaeolatitude high palaeolatitude assemblages are lower in species richness and diversity and characterised by the presence of abundant typically bipolar taxa e g crucibiscutum salebrosum a less distinct biogeographic boundary at similar to 40 degrees n is distinguished by the presence absence of rarer but biogeographically significant taxa many of which have previously been assigned to boreal or tethyan provinces continental shelf sites are characterised by lower diversity assemblages with common to dominant diagnostic taxa which vary with palaeolatitude nannoconus and micrantholithus at low palaeolatitudes and biscutum constans and zeugrhabdotus spp at higher palaeolatitudes the latter two taxa are considered to be indicative of elevated surface water fertility and the former two may have been similarly adapted the genus watznoueria is ubiquitously dominant giving the populations an unevenness which appears to be a common feature throughout coccolithophore history c 2000 published by elsevier science b v all rights reserved

tectonic and eustatic control on a mixed siliciclastic carbonate platform during the late oxfordian kimmeridgian la rochelle platform western france boreal and tethyan realms of western europe present significant sedimentological paleontological and stratigraphic differences the purpose of this study is to constrain regional versus global controls on the dynamics of a sedimentary system located at the interface of these two realms in order to better understand the origin of their differences detailed sedimentological palynofacies and calcareous nannofossil analyses were performed on two sections from the la rochelle platform western france the pas section includes part of the late oxfordian and early kimmeridgian and the rocher d yves section is assigned to the late kimmeridgian they correspond to monotonous marl argillaceous limestone alternations limestones are essentially mudstones with echinoderms bivalves and foraminifera that suggest low energy open marine conditions highly bioclastic and or peloidal deposits occur commonly and show wackestones to wacke pack grainstones textures these deposits indicate frequent high energy events and are interpreted as storm deposits marls dominate in the most proximal depositional environments while calcareous deposits are more important in more distal environments the rocher d yves section is globally more marly than the pas section suggesting a more proximal setting palynofacies are dominated by woody particles suggesting shallow water proximal depositional environments calcareous nannofossils are ascidian spicules coccoliths and schizospheres watznaueria britannica dominate calcareous nannofossil assemblages in the pas section the rocher d yves assemblages are quasi exclusively composed of cyclagelosphaera margerelii and indicate more proximal paleoenvironments than those of the pas section different orders of depositional sequences are defined with sequence boundaries corresponding to the most rapid relative sea level falls they are hierarchically stacked and correlate on the basis of ammonite zones with the sequences of contemporaneous sections from tethyan and boreal realms the stacking pattern of these sequences suggests an orbital control on sedimentation small medium and large scale sequences correspond to precession 20 ky cycles and to 100 ky and 400 ky eccentricity cycles respectively the elementary sequences have durations shorter than 20 ky the kimmeridgian was a period of global sea level rise that ended in the late kimmeridgian more proximal depositional environments in the rocher d yves section late kimmeridgian than in the pas section early kimmeridgian imply a progradation of the la rochelle platform during the kimmeridgian this progradation resulted from a slowdown of the subsidence in the aquitaine basin during the kimmeridgian corresponding to the first steps of atlantic ocean opening high frequency cycles on the la rochelle platform formed in sync with milankovitch orbital cycles while tectonics controlled the formation of the low frequency cycles