early jurassic late pliensbachian co2 concentrations based on stomatal analysis of fossil conifer leaves from eastern australia the stomatal index a measure of stomatal density of an extinct australian early jurassic araucariacean conifer species allocladus helgei jansson is used to reconstruct the atmospheric carbon dioxide concentration pco 2 in the early jurassic the fossil leaves are preserved in a single bed palynologically dated to late pliensbachian similar to 185 183 mya atmospheric pco 2 is estimated from the ratios between the stomatal index of a helgei and the stomatal indices of three modern analogs nearest living equivalent plants co2 concentration in the range of similar to 750 975 ppm was calibrated from the fossil material with a best estimated mean of similar to 900 ppm the new average pco 2 determined for the late pliensbachian is thus similar to although similar to 10 lower than previously inferred minimum concentrations of similar to 1000 based on data from the northern hemisphere but may help constrain pco 2 during this period our results are the first pco 2 estimates produced using jurassic leaves from the southern hemisphere and show that i paleo atmospheric pco 2 estimates are consistent at a global scale though more investigations of southern hemisphere material are required and ii the stomatal proxy method can now be used without the context of relative change in pco 2 when applying the correct methodology c 2014 the authors published by elsevier b v on behalf of international association for gondwana research less

cenomanian turonian carbon isotope events recorded in terrestrial organic matter from northern japan carbon isotopic fluctuations recorded in terrestrial organic matter from the upper cretaceous cenomanian turonian succession of hokkaido japan show a remarkably similar pattern to isotopic fluctuation in carbonate carbon reported from southern england and the italian apennines chronological correlation for the cenomanian turonian section of japan is based on planktonic foraminiferal biostratigraphy with some additional data from macrofossils allowing a detailed comparison of isotope stratigraphies for japan and europe three distinct features of the isotope profile namely a positive spike near the cenomanian turonian boundary a shoulder in the lower middle turonian and a level segment in the middle upper turonian are observed in japan these features are biostratigraphically comparable and correlative with isotope profiles reported from europe the single factor influencing isotopic composition of both terrestrial organic carbon and marine carbonate carbon is isotopic change in the global co2 reservoir of the ocean atmosphere system if these three carbon isotope features are synchronous they may be inferred as having been caused by global phenomena and as providing global high resolution chemo stratigraphic markers carbon isotope stratigraphy could be an important bridge between terrestrial inland sequences and marine sequences

deciphering methane s fingerprint massive releases of methane from the ocean floor are increasingly being implicated in global climate perturbation in the past the latest example comes from an episode of severe climatic and oceanic change that occurred around 180 million years ago

estimates of the amount and rate of sea level change across the rhaetian hettangian and pliensbachian toarcian boundaries latest triassic to early jurassic an attempt is made to determine the amount and rate of sea level change for two major events in the latest triassic to early jurassic both of which are associated with mass extinctions using the best available facies evidence and time scale the amount of marine deepening across the pliensbachian toarcian boundary based on analysis of the excellent yorkshire coast section is likely to have been a few tens of metres at a rate of between 1cm 1 2 ka 1 and 1cm 0 4 ka 1 the change across the rhaetian hettangian triassic jurassic boundary was different in that the sea level rise was immediately preceded by a sea level fall effectively a 9regression transgression9 couplet the earliest jurassic sea level rise is likely to have been much more rapid than that across the pliensbachian toarcian boundary at a rate of at least 1 cm 0 2 ka 1 whereas the speed of pliensbachian toarcian change is consistent with the growth of oceanic ridges the appreciably more rapid triassic jurassic change is suggestive of a different tectonoeustatic mechanism involving stress induced changes in plate density

spatial variability of watermass conditions within the european epicontinental seaway during the early jurassic pliensbachian toarcian in order to constrain spatial variability in watermass conditions within the european epicontinental seaway prior to during and after the toarcian oceanic anoxic event carbon 13cbel 13ccarb and oxygen 18obel 18ocarb isotope records were obtained from three sections in the grands causses basin southern france these data were then compared with similar records along a north south transect across the european epicontinental seaway as the conclusions reached here strongly depend on the reliability of belemnite calcites as archives of palaeoceanographic changes an attempt was made to improve the understanding of isotope signals recorded in belemnite calcite intra rostral carbon and oxygen isotope data from six belemnite specimens belonging to the genus passaloteuthis were collected intra rostral carbon isotopes are influenced by vital effects whereas oxygen isotopes reflect relative changes in temperature and salinity palaeotemperatures calculated from 18obel isotope records from the grands causses basin confirm relatively low temperatures throughout the late pliensbachian similar cool water conditions have previously been shown in germany england spain and portugal a temperature increase of up to 6 c is observed across the pliensbachian toarcian boundary a pronounced negative shift of at least 3 vienna pee dee belemnite is recorded in bulk carbonate carbon during the lower harpoceras serpentinum zone typical of the toarcian oceanic anoxic event before and after the toarcian oceanic anoxic event a good correlation between 13ccarb and 13cbel exists indicating well ventilated bottom waters and normal marine conditions instead data for the toarcian oceanic anoxic event indicate the development of a strong north south gradient in salinity stratification and surface water productivity for the western tethyan realm this study thus lends further support to a pronounced regional overprint on carbon and oxygen isotope records in epicontinental seaways

tracing seafloor methane emissions with benthic foraminifera in the baiyun sag of the northern south china sea changes in the concentrations of atmospheric greenhouse gases are an important part of the global climate forcing the hypothesis that benthic foraminifera are useful proxies of local methane emission from the seafloor has been verified on sediment cores by numerous studies the calcium carbonate caco3 content and the high resolution carbon and oxygen isotope composition of the benthic foraminifera from the core 08cf7 from the northeastern shenhu gas hydrate drilling area in the baiyun sag of the northern south china sea were analyzed and the benthic foraminifera s evidence for methane release from gas hydrate decomposition are presented here for the first time two rapid obvious carbon isotope negative excursions were observed in the oxygen isotope stage boundaries 5d 5c and 6 5e penultimate deglaciation about 130 ka of the cold to warm climatic transition period the largest negative value of 13c is about 2 95 and the whole change of carbon and oxygen isotope is strikingly similar and is in consonance with the atmospheric methane concentration recorded by the vostok ice core and the carbon isotopic record from lake baikal combining these results with the analysis of the geological conditions of the study area and the fact that gas hydrate exists in the surrounding area it can be concluded that the carbon isotope negative excursions of the benthic foraminifera in the northern south china sea are associated with methane release from gas hydrate decomposition due to deglacial climate warming by recording the episodes of massive gas hydrate decomposition closely linked with the northern hemisphere temperatures during major warming periods the new 13c record from the baiyun sag provides further evidence for the potential impact of gas hydrate reservoir on rapid deglacial rises of atmospheric methane levels

high resolution chemo and biostratigraphic records of the early aptian oceanic anoxic event in cantabria n spain palaeoceanographic and palaeoclimatic implications abstract during the early aptian major palaeoenvironmental changes occurred leading to an oceanic anoxic event oae 1a and a perturbation of the global carbon cycle new detailed litho bio and chemostratigraphic toc 13ccarb 13corg records of two superbly exposed and expanded lower aptian sections in cantabria la florida and cuchia allow to recognize the expression of the oae 1a in shallow shelf environments of northern spain the succession consists of shallow platform limestones that include a marly unit patrocinio formation the deposition of which occurred mostly at the onset of the oae 1a 120 5 ma this study presents a new integrated biostratigraphy based on ammonites planktonic foraminifera calcareous nannofossils and palynomorphs that allows an accurate age resolution of the succession the marly unit records an abrupt negative 13c excursion in both bulk organic matter up to 5 and carbonate up to 6 mean 3 as has been already observed at the onset of the oae 1a in other lower aptian deposits worldwide in detail however the negative excursion presents two minima in the studied sections this negative spike is confidently attributed to the upper half of the hayesites irregularis nannofossil zone to the upper part of the blowiella blowi foraminiferal zone and to the middle upper part of the deshayesites weissi ammonite zone a third negative excursion occurs at the base of the rhagodiscus angustus nannofossil zone which may be correlatable with the dufrenoyia furcata ammonite zone this data set refines the age of the oae 1a and reveals the existence of a stratigraphic gap in the westernmost margin of the basque cantabrian basin that covers at least a portion of the upper part of the early aptian sedimentary facies and quantitative analysis of palynomorphs and nannofossils document significant environmental changes associated with the oae 1a compositional changes of neritic carbonates and calcareous nannofossils data indicate the occurrence of a biocalcification crisis inferred to have been related to co2 induced changes in seawater chemistry and palynomorphs identify a thermal maximum followed by a cooling phase the latter show a classopollis maximum during the oae 1a which is followed by a decrease in classopollis and an increase of bisaccate pollen after the event

prediction of modern bauxite occurrence implications for climate reconstruction abstract the formation of bauxite and laterite strongly depends on prevailing climate we use modern climate and bauxite distribution data to investigate quantitatively the role of climate in bauxite formation and to develop a method for testing general circulation models gcms we find that a very satisfactory prediction for potential bauxite formation can be achieved using two different sets of criteria the first requires the mean annual temperature to be greater than 22 c with annual mean precipitation greater than 1200 mm and 6 or fewer months when the rainfall is less than 60 mm the alternative model requires precipitation greater than evaporation for at least 6 months per year and monthly mean temperatures greater than 23 c for at least 8 months these proxy models are also used with simulations of the present day using a general circulation model the results show that the climate models are robust suggesting that bauxitization can be used to evaluate quantitatively such climate models for ancient times we also show that based on climate model predictions the regions favourable for bauxitization have remained stable through the holocene but with the exception of the malay peninsula bauxite formation would have ceased at the last glacial maximum

dissociation of oceanic methane hydrate as a cause of the carbon isotope excursion at the end of the paleocene isotopic records across the latest paleocene thermal maximum lptm indicate that bottom water temperature increased by more than 4 c during a brief time interval 104 years of the latest paleocene 55 6 ma there also was a coeval 2 to 3 excursion in the 13c of the ocean atmosphere inorganic carbon reservoir given the large mass of this reservoir a rapid 13c shift of this magnitude is difficult to explain within the context of conventional hypotheses for changing the mean carbon isotope composition of the ocean and atmosphere however a direct consequence of warming bottom water temperature from 11 to 15 c over 104 years would be a significant change in sediment thermal gradients and dissociation of oceanic ch4 hydrate at locations with intermediate water depths in terms of the present day oceanic ch4 hydrate reservoir thermal dissociation of oceanic ch4 hydrate during the lptm could have released greater than 1 1 to 2 1 1018 g of carbon with a 13c of approximately 60 the release and subsequent oxidation of this amount of carbon is sufficient to explain a 2 to 3 excursion in 13c across the lptm fate of ch4 in oceanic hydrates must be considered in developing models of the climatic and paleoceanographic regimes that operated during the lptm

correlation of early cretaceous carbon isotope stratigraphy and platform drowning events a possible link the early cretaceous carbonate carbon isotope record is marked by three positive high amplitude 1 5 excursions each covering time spans of more than 106 years they are of late valanginian hauterivian and early and late aptian age in a case study along a transect across the western tethys ocean we identified a coincidence between 13c excursions black shale formation and widespread carbonate platform drowning events we conclude that the 13c excursions reflect a change in partitioning of carbon between the organic and carbonate carbon sinks which was triggered by climate induced ecological changes in cretaceous pelagic and neritic environments episodes of intensified greenhouse climate conditions led to an increase in weathering erosion and runoff rates and to elevated nutrient transfer rates from continents into oceans the resulting increase in oceanic nutrient levels favoured marine phytoplankton production and black shale deposition while conditions for carbonate producing biotas became unfavourable partial choking of carbonate production along river influenced coasts resulted in widespread carbonate platform drowning during times of sea level rise in the valanginian and aptian widespread contemporaneous black shale deposits and drowned carbonate platforms therefore reflect the contrasting response of the marine organic and carbonate carbon pumps to nutrient enhanced phytoplankton productivity the change in marine carbon partitioning is mirrored in a shift of the 13c record towards more positive values the transition to the 13c peak values lasted up to several hundred thousand years the peaks of the excursions also covering a time span of up to several hundred thousand years reflect a new stabilisation of the carbon partitioning between carbonate and organic carbon sinks a renewed intensification of carbonate sedimentation under mesotrophic conditions was facilitated by stabilisation of the sea level rise at a high level decreasing 13c values record increasing carbonate carbon burial rates at constant or decreasing organic carbon accumulation rates these changes contributed to the stabilisation of the marine carbon budget and the global carbon cycle up to millions of years after its initial perturbation

numerical paleoceanographic study of the early jurassic transcontinental laurasian seaway the forces governing marine circulation of a meridional transcontinental seaway is explored with the princeton ocean model the jurassic laurasian seaway which connected the low latitude tethys ocean with the arctic sea is modeled quantitatively the global ocean is found to have a profound influence on seaway dynamics a north south density difference and hence sea level difference of the global ocean was probably the main factor in forcing the seaway flow when the tethys waters were the denser water the net seaway flow was southward and conversely it was northward for denser arctic waters marine bioprovincial boundaries and sediment data indicate that the seaway probably was dominated by boreal faunal groups and reduced salinities several times in the jurassic the model results suggest that this can be explained by southward flowing seaway currents which may have been related to an oceanic thermohaline circulation where no northern high latitude deep convection occurred

diversity dynamics of early jurassic ostracods of the cordillera iberica spain and the re evaluation of the pliensbachian toarcian mass extinction the extinction and recovery of ostracoda at the pliensbachian toarcian p t boundary are analyzed based on a database of taxonomically revised pliensbachian to toarcian transition ostracod assemblages in contrast to earlier assertions the results of this study indicate that ostracod extinction rates were significant in comparison with other marine invertebrates an extinction rate of 54 has been calculated for upper pliensbachian ostracod species occurring in more than one section diversification took place in the latest pliensbachian spinatum zone and early toarcian tenuicostatum zone whereas diversity decrease occurred in the middle early toarcian strangewaysi subzone serpentinus zone this notable diversity decline in the early toarcian corresponds to a global mass extinction time whose peak has been documented in the tenuicostatum zone meanwhile the ostracod mass extinction occurred within the serpentinus zone and was followed by radiation and recovery in the succeeding bifrons zone s

biogeochemical assessment of sources of organic matter and paleoproductivity during the early aptian oceanic anoxic event at shatsky rise odp leg 198 this study investigates organic rich sedimentary sequences deposited during the early aptian oceanic anoxic event oae1a at sites 1207 and 1213 on shatsky rise odp leg 198 in the west central pacific biomarker analyses provide evidence of the algal and bacterial origin of organic matter om in these sediments where the abundance of steroidal components particularly sterenes and sterones suggests that the om includes major contributions from eukaryotic sources in an environment characterized by high phytoplankton productivity the presence of alkenones at site 1213b is diagnostic of om derived from representatives of haptophyte algae among the calcareous nannoplankton and their 13c values average 31 6 are consistent with those expected during elevated pco2 the occurrence and prominence of 2 methylhopanes and 2 methylhopanones indicates significant contributions to the om from cyanobacteria which are also likely contributors of hopanoids based on their 13c compositions these biomarker data suggest that oceanic conditions perhaps nitrate or iron limited were conducive to cyanobacteria production during oae1a which appears to distinguish this event from other cretaceous oae

carbon isotope stratigraphy of the upper kharaib and shuaiba formations implications for the early cretaceous evolution of the arabian gulf region the carbon isotope profiles of shallow marine carbonates from the barremian aptian kharaib and shuaiba formations of the arabian gulf region range between 0 5 and 7 pmil dgr13c pdb peedee belemnite systematic variations can be correlated with isotope profiles reported from tethyan pelagic limestone sequences the detailed correspondence between the isotopic signature of the relatively well dated pelagic limestones and the poorly dated shallow water limestones from the arabian gulf region suggests that global marine carbon isotope changes apparently affected deep sea and shallow water carbonate sediments similarly and at a similar time resolution although oxygen isotopes have been reset during diagenesis carbon isotopes appear to have maintained their primary marine signature through time no evidence has been found to connect carbon isotope trends to subaerial exposure or later meteoric diagenesis in combination with other data the investigated carbon isotope profiles can be used for basin to platform and regional correlations beyond the current resolution of biostratigraphy in shallow water limestones carbon isotope stratigraphy confirms significant hiatuses in the investigated shallow water carbonate sequences using carbon isotope trends as a proxy for sea level fluctuations the carbon isotope cycles of the late early cretaceous of the arabian gulf region may represent four cycles of rising and falling sea level with a duration corresponding to that of third order sea level fluctuations regional correlations derived from isotope trends provide a scenario for the larger scale stratigraphic evolution of the arabian peninsula during the end of the early cretaceous differential rates of aggradation between realms influenced by siliciclastics and those characterized by siliciclastic poor clean carbonate deposition apparently caused the development of a pronounced basin platform topography during the early aptian shuaiba intrashelf basin and platform

the early toarcian and cenomanian turonian anoxic events in europe comparisons and contrasts two intervals of mesozoic time are demonstrably of sufficient geological brevity to qualify readily for the term anoxic event the earlyfalciferum zone orexaratum subzone of the toarcian and thewhiteinella archaeocretacea interval zone that straddles the cenomanian turonian boundary both periods of time saw regional deposition of anomalously organic rich strata black shales accompanied by a positive 2 pdb carbon isotope excursion in coeval biogenic carbonates and significant faunal change the duration of both events was probably less than half a million years and both were preceded by regional erosion and possible upwelling coincident sea level rise and transgression is likely a european view of these events however spotlights some significant differences whereas thefalciferum zone organic matter is generally more concentrated and more hydrogen rich on the palaeo european shelf than on the tethyan continental margins the reverse holds true for the black shales formed during thewhiteinella archaeocretacea interval zone furthermore the carbon isotope composition offalciferum zone organic matter from both north european and tethyan sites is anomalously negative compared to that developed in thewhiteinella archaeocretacea interval zone differing planktonic organisms may have been involved and their possible diverse ecological requirements may go some way to explaining the nature and distribution of facies deposited during these two events

phanerozoic atmospheric co2 change evaluating geochemical and paleobiological approaches abstract the theory and use of geochemical modeling of the long term carbon cycle and four paleo pco 2 proxies are reviewed and discussed in order to discern the best applications for each method geochemical models provide pco 2 predictions for the entire phanerozoic but most existing models present 5 10 m y means and so often do not resolve short term excursions error estimates based on sensitivity analyses range from 75 200 ppmv for the tertiary to as much as 3000 ppmv during the early paleozoic the 13 c of pedogenic carbonates provide the best proxy based pco 2 estimates for the pre tertiary with error estimates ranging from 500 1000 ppmv pre devonian estimates should be treated cautiously error estimates for tertiary reconstructions via this proxy are higher than other proxies and models 400 500 ppmv and should not be solely relied upon we also show the importance of measuring the 13 c of coexisting organic matter instead of inferring its value from marine carbonates the 13 c of the organic remains of phytoplankton from sediment cores provide high temporal resolution up to 10 3 10 4 year high precision 25 100 ppmv for the tertiary to 150 200 ppmv for the cretaceous pco 2 estimates that can be near continuous for most of the tertiary its high temporal resolution and availability of continuous sequences is advantageous for studies aiming to discern short term excursions this method however must correct for changes in growth rate and oxygen level at elevated pco 2 750 1250 ppmv this proxy loses its sensitivity and is not useful the stomatal density and stomatal index of land plants also provide high temporal resolution 2 year high precision 10 40 ppmv for the tertiary and possibly cretaceous pco 2 estimates and so also is ideal for discerning short term excursions unfortunately this proxy also loses sensitivity at some level of pco 2 above 350 ppmv which currently is largely undetermined our analysis of the recently developed 11 b technique shows that it currently is not yet well constrained most importantly it requires the assumption that the boron isotopic composition of the ocean remains nearly constant through time in addition it assumes that there are no biological or temperature effects and that diagenetic alteration of the boron isotopic composition does not occur a fifth co 2 proxy based on the redox chemistry of marine cerium has several fundamental flaws and is not discussed in detail here

biostratigraphy and paleoceanography of the cretaceous seaway between norway and greenland abstract the narrow seaway between greenland and norway in cretaceous time was over 1500 km long and 300 km wide and partly of bathyal water depth during aptian through campanian it received a large volume of fine grained siliciclastic sediments with intercalated gravity flow sandstone wedges as a conduit for heat transport between the low latitudes and polar region atlantic watermasses particularly affected the eastern norwegian margin of this seaway despite its high latitude setting calcareous and few siliceous planktonic microfossils thus play an important role in regional stratigraphy and facies analysis and a majority of fossil events correlate to nw european basins the eastern margin sedimentary succession may be subdivided in several broad lithologic units 1 thin multicolored marly sediments of hauterivian barremian age with a shallow marine falsogaudryina nodosariid ostracod assemblage 2 dark mudstones and minor sands aptian early cenomanian in age with an upper bathyal agglutinated assemblage and monotypic hedbergella floods 3 thick mudstone facies with thin slope apron turbidite sands and an impoverished benthic planktonic assemblage of late cenomanian coniacian age deposited in an upper bathyal oxic dysaerobic environment where turonian sedimentation rates are low a planktonic foraminiferal assemblage with whiteinella hedbergella dicarinella and marginotruncana occurs 4 grayish laminated mudstones santonian campanian in age with local sands in the north a low diversity middle to upper bathyal benthic planktonic foraminiferal assemblage and an inoceramus prisms and radiolarian diatom flood a campanian agglutinated foraminiferal bloom also is known from the atlantic oceanic realm 5 more marly sediments of maastrichtian age with a low diversity planktonic benthic foraminiferal assemblage using the distribution of 1755 foraminiferal and dinoflagellate microfossil events in over 30 exploration wells a rasc ranking and scaling probabilistic zonation served as a template to build a cretaceous zonal model with 19 assemblage and interval zones including over 100 events variance analysis ranks 72 events according to reliability in correlation three new index taxa include uvigerinammina una and ammoanita globorotaliaeformis albian and fenestrella bellii campanian widespread planktonic flood events occur in late albian through early cenomanian early mid turonian late santonian earliest campanian and mid maastrichtian the result of northwards shifts of warmer water masses and disruptions in water stratification in the dysaerobic basins an earliest cretaceous hiatus separates jurassic from cretaceous strata ongoing block faulting coupled to thermal subsidence and global sealevel rise increased water depth in aptian albian time from neritic to bathyal and created sand accommodation space in dysaerobic restricted settings in cenomanian coniacian time sedimentation rates in the central basin increased 10 fold whereas paleo waterdepth did not deepen this relatively brief less than 10 m y tectonic episode resulting in deposition of deep water sands is tentatively linked to stress re orientation in the rockall area a widespread upper maastrichtian danian hiatus the result of shoulder uplift reflects break up prior to the onset of paleogene seafloor spreading in the norwegian sea

lower jurassic epicontinental carbonates and mudstones from england and wales chemostratigraphic signals and the early toarcian anoxic event sections through lower jurassic epicontinental carbonates from southern britain junction bed and equivalent show a positive carbon isotope excursion 13ccarbonate detectable in bulk rock in the falciferum zone of the lower toarcian isotopic data from organic matter in more clay rich sections from wales and north east england together with determinations on belemnite calcite indicate that highest 13c values are localized in the upper exaratum subzone of the falciferum zone levels of particular enrichment in organic carbon were developed in the early to mid exaratum subzone and hence pre date this 13c maximum these phenomena reflect the impact of the early toarcian oceanic anoxic event in the british area similar isotopic trends have been recorded in other toarcian sections from tethyan europe and are interpreted as reflecting the chemistry of sea water on the assumption of isotopic correlation between the english and tethyan sections the 13c maximum would be everywhere dated as latest exaratum subzone in terms of the north european ammonite scheme absolute oxygen isotope values in carbonates probably reflect both early diagenetic cementation and later temperature related burial diagenesis although a palaeotemperature maximum is tentatively identified as characterizing the early falciferum zone subsequent climatic deterioration may have been triggered by drawdown of co2 related to regional excess carbon burial during the oceanic anoxic event using the positive 13c excursion as a correlative level in sections from different faunal provinces britain italy and spain implies that lower toarcian zonal stratigraphy is diachronous between northern and southern europe there is evidence for partitioning of water masses between the north european shelf and the tethyan continental margin during the early jurassic

ice age terrestrial carbon changes revisited n shackleton 1977 first proposed that changes in the marine 13 c record 13 c could be used to infer ice age changes in carbon storage on land the previously published best estimate from the marine record is equivalent to about 490 gt 0 32 13 c however adams et al 1990 utilized a pollen database to estimate a 1350 gt change in carbon storage which would cause a 13 c of about 0 90 the nearly trillion ton difference in estimates amounts to almost half of the total carbon stored on land to address the nature of this discrepancy i have reexamined the terrestrial carbon record based on a new pollen database compiled by r webb and the cooperative holocene mapping project cohmap group i estimate about 750 1050 gt glacial interglacial change in terrestrial carbon storage with the range reflecting uncertainties in carbon storage values for different biomes additional uncertainties apply to rainforest and wetland extent and presence of c4 plants which have a significantly different isotopic signature than c3 plants although some scenarios overlap a new estimate of carbon storage based on the oceanic 13 c record revised to 0 40 and 610 gt most estimates seem to fall outside the envelope of uncertainty in the marine record several possible explanations for this gap involve 1 a missing sink may be involved in land sea carbon exchange e g continental slopes 2 the geochemistry of the exchange process is not understood 3 carbon storage by biome may have changed under ice age boundary conditions or 4 the standard interpretation of whole ocean changes in the marine 13 c record requires reevaluation this latter conclusion receives some support from comparison of the 13 c records for 18 o stages 2 and 6 for the stage 6 glacial the 13 c changes are 50 60 larger than for the stage 2 glacial yet implications of increased aridity are not supported by longterm trends in atmospheric dust loading to summarize the above analysis implies that despite the uncertainties remaining in estimates of terrestrial carbon storage changes a case can be made that our understanding of the transfer process is incomplete and that the eventual explanation may require clarification of factors affecting the marine 13 c record

the effects of refractor topography and overburden anisotropy on time term solutions of refractor anisotropy summary it is generally held that a non iterative time term solution gives a satisfactory model if refractor dips are everywhere less than 10 mo although this is a reasonable assumption for a refractor with a constant dip for structures with strong directional characteristics such as anticlines and syn clines the solution can be seriously degraded and will exhibit a refractor velocity that could be erroneously interpreted as anisotropic by ray tracing through a continental model of an isotropic crust and upper mantle separated by an anticlinal moho nowhere dipping more than 8 mo it is demonstrated that the pntravel time can be well fitted by a non iterative time term solution with a 4 4 per cent upper mantle anisotropy the effect is smaller for oceanic cases these studies illustrate the importance of carrying out an iterative sequence of solutions and indicate that convergence at the correct solution will occur when a good azimuthal distribution of observations is available it is further demonstrated that an anisotropic crust can perturb the travel times in a similar way to an anisotropic upper mantle the correct separation of crustal and upper mantle effects requires the independent utilization of pn and crustal phases

abrupt deep sea warming palaeoceanographic changes and benthic extinctions at the end of the palaeocene a remarkable oxygen and carbon isotope excursion occurred in antarctic waters near the end of the palaeocene 57 33 myr ago indicating rapid global warming and oceanographic changes that caused one of the largest deep sea benthic extinctions of the past 90 million years in contrast the oceanic plankton were largely unaffected implying a decoupling of the deep and shallow ecosystems the data suggest that for a few thousand years ocean circulation underwent fundamental changes producing a transient state that although brief had long term effects on environmental and biotic evolution

evidence of recycling of isotopically light co2 aq in stratified black shale basins contrasts between the whitby mudstone and kimmeridge clay formations united kingdom calculation of co 2 aq and p co 2 atm based on proxy temperature and salinity data 18 o and 13 c from belemnite rostra and 13 c toc toc total organic carbon from toarcian marine organic rich shales yields unreasonably large intraformational p co 2 atm variations we postulate that the low 13 c toc values in the shales can instead be explained by episodic release of isotopically light co 2 that had built up below a relatively deep pycnocline halocline the lowest 13 c toc values would thus be associated with mixing and erosion of the pycnocline following extended periods of stable stratification in contrast the studied tithonian oil shales were probably deposited during periods of only slight perturbation of a photic zone chemocline which probably led to only modest release of isotopically light co 2 in addition the higher productivity associated with the nutrient release would have resulted in depletion of the mixed layer co 2 leading to lower fractionation and higher 13 c toc values

a blast of gas in the latest paleocene simulating first order effects of massive dissociation of oceanic methane hydrate carbonate and organic matter deposited during the latest paleocene thermal maximum is characterized by a remarkable 2 5 excursion in delta 13c that occurred over approximately 10 4 yr and returned to near initial values in an exponential pattern over approximately 2 x 10 5 yr it has been hypothesized that this excursion signifies transfer of 1 4 to 2 8 x 10 18 g of ch4 from oceanic hydrates to the combined ocean atmosphere inorganic carbon reservoir a scenario with 1 12 x 10 18 g of ch4 is numerically simulated here within the framework of the present day global carbon cycle to test the plausibility of the hypothesis we find that 1 the delta 13c of the deep ocean shallow ocean and atmosphere decreases by 2 3 over 10 4 yr and returns to initial values in an exponential pattern over approximately 2 x 10 5 yr 2 the depth of the lysocline shoals by up to 400 m over 10 4 yr and this rise is most pronounced in one ocean region and 3 global surface temperature increases by approximately 2 degrees c over 10 4 yr and returns to initial values over approximately 2 x 10 6 yr the first effect is quantitatively consistent with the geologic record the latter two effects are qualitatively consistent with observations thus significant ch4 release from oceanic hydrates is a plausible explanation for observed carbon cycle perturbations during the thermal maximum this conclusion is of broad interest because the flux of ch4 invoked during the maximum is of similar magnitude to that released to the atmosphere from present day anthropogenic ch4 sources

biogeochemical constraints on the triassic jurassic boundary carbon cycle event 1 the end triassic mass extinctions represent one of the five most severe biotic crises in earth history yet remain one of the most enigmatic ongoing debate concerns the environmental effects of the central atlantic magmatic province camp eruptions and their linkage with the mass extinction event across the triassic jurassic boundary there is conflicting paleo evidence for changes in atmospheric pco2 during the extrusion of the camp basalts studies on sediments from european and pacific localities have however identified a substantial negative isotopic anomaly up to 3 5 across the tr j boundary providing an important indicator of changes in the operation of the ancient global carbon cycle we sought to explain the paleo evidence by utilizing a carbon cycle model for the hothouse world of the end triassic that emphasizes the chemical weathering of silicate and carbonate rocks and the ocean carbonate chemistry we find that volcanic co2 outgassing fails to fully account for either a sufficient rise in atmospheric pco2 indicated by the stomata of fossil leaves or the sedimentary isotopic fingerprint instead the scenario that best fits all of the geologic evidence is a positive feedback loop in which warming due to a buildup of volcanically derived co2 triggers destabilization of seafloor methane hydrates and the catastrophic release of ch4 palfy et al 2001 we calculate that this carbon cycle perturbation was huge involving the release of 8000 9000 gt c as co2 during the camp basaltic eruptions and 5000 gt c as ch4 in the model the initial isotopic excursion is assumed to take place over 70 kyr while complete reequilibration of the ocean atmosphere system with respect to co2 is accomplished over 700 1000 kyr our results thus provide a preliminary theoretical explanation for the bioevents estimated pco2 changes and isotopic excursions observed in marine and continental sediments at this time

the early toarcian anoxic event in the westernmost tethys subbetic paleogeographic and paleobiogeographic significance a positive delta 13 c excursion reveals an oceanic anoxic event in the subbetic corresponding to the westernmost end of the tethys during the early jurassic this positive delta 13 c excursion exists in five stratigraphic sections consisting of lower and middle toarcian pelagic sediments the positive delta 13 c excursion in the subbetic is located in the upper part of the early toarcian serpentinus approx falciferum biozone while in other tethyan regions alps apennines and dinarides it has been found in the lower part of the same biozone the most significant positive delta 13 c excursion detected in the subbetic is therefore about half a million years younger although other relative peaks had been detected this isotopic anomaly can be correlated with a maximum transgression that occurred earlier in the regions closer to the open sea of the tethys ocean other features of the anoxic event faunal crises and black shale deposits appear at different ages related t

lateral and vertical variability in crustal velocity structure in a small area of the north atlantic abstract the detailed velocity structure of a 70 by 35 km area of 6 10 ma old crust on the flank of the mid atlantic ridge at 24 n was studied using 358 explosive charges and several hundred 16 4 1 airgun shots fired into an array of eight ocean bottom hydrophones inversion of the first arrival refracted travel times shows that the crust comprises a normal oceanic section about 5 km thick with a steep velocity gradient in the upper crust increasing from about 3 5 km sec at the seafloor overlying a typical oceanic layer 3 and a probably anisotropic mantle delay time function mapping using two datasets containing arrivals from layer 3 and from the mantle show that lateral variability is generally low over most of the survey area with a small region of high delay times in the northwest corner caused by the presence of abnormal crust probably associated with a minor fracture zone we find that the topography of the base of layer 2 is similar to that of the top indicating that the normal faulting which occurs along the margins of the median valley extends down at least into layer 3 our observations from mantle arrivals are consistent with a much flatter moho which constrains possible models of crustal formation at the spreading centre

higher temperatures and lower oceanic pco2 a climate enigma at the end of the paleocene epoch one of the largest and most abrupt climatic warming events documented in the geologic record occurred at the end of the paleocene epoch oceanic deep waters warmed to 10 c and high latitude surface waters warmed from 10 c to 20 c within several thousand years this coincided with weakened atmospheric circulation and the extinction of 50 of deep sea benthic foraminiferal species it has been suggested that this warm excursion was forced by higher atmospheric pco2 and greenhouse effects caused by a pulse of hydrothermal activity and or volcanism stable isotopic evidence is presented from two widely separated locations that suggest this warming was associated with a drop in oceanic pco2 rather than an increase oceanic pco2 change across this event was estimated using a model of 13c fractionation in photosynthate organic carbon versus co2aq with solubility constants for co2 and stable isotopic paleotemperature estimates to derive a well preserved record for surface ocean 13c change the organic carbon bound within the calcite lattice of well preserved planktonic foraminifera was extracted for isotopic analysis with allowance for uncertainty in the isotopic differences between phytoplankton and foraminiferal organic matter the initial results indicate a drop in surface ocean pco2 at high and low latitudes from 600 700 parts per million ppm to 200 ppm lower pco2 persisted for at least 10 000 years the duration of the pco2 excursion was long enough for the ocean and atmosphere to have reached a new steady state condition there is no evidence of increased organic carbon burial in the deep sea during this period two alternative explanations are presented to account for such a rapid drop in oceanic pco2 one involves reduced upwelling induced by diminished wind stress as atmospheric circulation weakened in response to climate warming this would have reduced the rate of metabolic co2 recycling into the surface ocean it will be necessary to obtain data from regions outside potential upwelling zones in order to evaluate this the second involves a readjustment of carbonate equilibria in the ocean to higher co3 in the surface ocean particularly at high latitudes where surface waters warmed to approximately 20 c such a shift in carbonate equilibria would have lowered the ocean s capacity to take in co2 if the initial results presented here do accurately reflect a change in the global ocean co2aq the paleocene eocene boundary event may provide clues about the ocean s physical and biological response to rapid large scale perturbations in atmospheric pco2 and to global warming

significant increases in global weathering during oceanic anoxic events 1a and 2 indicated by calcium isotopes abstract calcium isotope ratios 44 42ca were measured in carbonate rich sedimentary sections deposited during oceanic anoxic events 1a early aptian and 2 cenomanian turonian in sections from resolution guyot mid pacific mountains coppitella italy and the english chalk at eastbourne and south ferriby uk a negative excursion in 44 42ca of 0 20 and 0 10 is observed for the two events these 44 42ca excursions occur at the same stratigraphic level as the carbon isotope excursions that define the events but do not correlate with evidence for carbonate dissolution or lithological changes diagenetic and temperature effects on the calcium isotope ratios can be discounted leaving changes in global seawater composition as the most probable explanation for 44 42ca changes in four different carbonate sections an oceanic box model with coupled strontium and calcium isotope systems indicates that a global weathering increase is likely to be the dominant driver of transient excursions in calcium isotope ratios the model suggests that contributions from hydrothermal activity and carbonate dissolution are too small and short lived to affect the oceanic calcium reservoir measurably a modelled increase in weathering flux on the order of three times the modern flux combined with increased hydrothermal activity due to formation of the ontong java plateau oae1a and caribbean plateau oae2 can produce trends in both calcium and strontium isotopes that match the signals recorded in the carbonate sections this study presents the first major element record of a weathering response to oceanic anoxic events

contrasting watermass conditions during deposition of the whitby mudstone lower jurassic and kimmeridge clay upper jurassic formations uk abstract the whitby mudstone formation wmf and kimmeridge clay formation kcf are both jurassic organic rich black shale units containing well preserved marine organic matter despite such gross similarities they exhibit opposite trends in 13 c toc versus wt decarbonated total organic carbon toc r 0 76 for wmf and 0 71 for kcf the 13 c toc versus toc trend also varies geographically within both units only the kcf data exhibit a positive correlation between 13 c toc and 15 n org r 0 76 and the presence of aryl isoprenoids probable indicators of photic zone anoxia maturity differences are insufficient to explain such contrasts bulk and compound specific isotopic variations in the kcf are interpreted largely as a reflection of primary productivity during deposition rather than a dominantly diagenetic signal due to incorporation of sulphurised carbohydrate the contrasts between the formations and the geographic variation in coeval shelf sediments are explained by a model stressing the relative positions and stabilities of the chemocline pycnocline nutricline and euphotic zone and the effect these have on productivity co 2 recycling and bacterial floras

early aptian paleoenvironmental evolution of the bab basin at the southern neo tethys margin response to global carbon cycle perturbations across ocean anoxic event 1a 1 lower aptian carbonates in the bab basin at the southern neo tethys margin record significant environmental changes across oceanic anoxic event 1a oae1a a long lasting negative shift of carbon isotope ratios 13c associated with a distinct decrease in oxygen isotope ratios 18o in orbitolinid rich carbonates characterizes the onset of oae1a livello selli supporting a hypothesis that a long lasting volcanic co2 emission is the main cause of oae1a inducing global warming a bloom of microencrusters lithocodium and bacinella across the proto bab basin occurred synchronously at the beginning of the subsequent positive 13c excursion responding to the global carbon cycle perturbations the carbonates formed during the oae1a show higher strontium isotope ratios 87sr 86sr compared with those of global seawater this was likely caused by a local influx of isotopically heavier strontium along with nutrients into the proto bab basin these biotic proliferations were triggered by an increased nutrient supply induced by intensified continental weathering due to the global warming suggested by the increase in 18o values spatial variations in the 13c values among sites in the bab basin and its surrounding platform are related to local environmental factors such as the degree of mixing of basin water with ocean water and local removal of 12c by metabolic activity at the platform top the 13c profile of the studied core indicates global removal of organic carbon of oae1a began during the early stage of the second order transgression and lasted until the early stage of the highstand after the oae1a the livello selli corresponds to the early stage of this transgression

numerical evaluation of mechanisms driving early jurassic changes in global carbon cycling the early jurassic early toarcian ca 183 ma carbon cycle perturbation is characterized by aabout 5 parts per thousand delta sup 13 c excursion in the exogenic carbon reservoirs a 1000 ppm rise in atmospheric co sub 2 and a 6 7 degrees warming two proposed explanations for this presumed global carbon cycle perturbation are the liberation of massive amounts of isotopically light ch4 from 1 gondwanan coals by heating during the intrusive eruption of the karoo ferrar large igneous province lip or 2 the thermal dissociation of gas hydrates carbon cycle modeling indicates that the release of ch4 from gondwanan coals synchronous with the eruption of the karoo ferrar lip fails to reproduce the magnitude or timing of the co sub 2 and delta sup 13 c excursions however sensitivity analyses constrained by a marine cyclostratigraphically dated delta sup 13 c record indicate that both features of geologic record can be explained with the huge input of about 15 340 24 750 gt c over about 220 k y a result possibly pointing to the involvement of hydrothermal vent complexes in the karoo basin the simulated release of 6000 gt c from gas hydrates also reproduces aspects of the early toarcian rock record but the large mass involved raises fundamental questionsmore about its formation storage and release less

structure of the earth oceanic crust and uppermost mantle the past four years have witnessed the introduction of a variety of new instruments and methods for the study of the seismic structure of the oceanic crust and lithosphere the application of these and existing tools has led to the discovery of a number of new phenomena and to a fuller understanding of the genesis and evolution of the oceanic ithosphere borehole seismic instrumentation has become more widely employed ocean bottom seismographs while generally decreasing in number have become significantly more reliable and useful and multichannel seismic systems have been employed in innovative experiments ranging from studies of fracture zones to the regular detection of magma chambers beneath rise axes the techniques available for the analysis of seismic data have become more sophisticated waveforms collected in seismic experiments can now be used directly in constructing and evaluating seismic velocity models and travel time data are regularly inverted directly for structure trial and error modeling has become increasingly unimportant marine seismologists are becoming increasingly involved in understanding the coupling between the ocean and the underlying oceanic lithosphere this has led to a more complete understanding of seafloor noise processes and the partitioning of energy between acoustic and elastic waves

water mass exchange and variations in seawater temperature in the nw tethys during the early jurassic evidence from neodymium and oxygen isotopes of fish teeth and belemnites article i nfo oxygen and neodymium isotope analyses performed on biostratigraphically well dated fish remains recovered from the hettangian to toarcian of the paris basin were used to reconstruct variations of early jurassic seawater temperature and to track oceanographic changes in the nw tethys our results indicate a strong correlation between 18 o trends recorded by fish remains and belemnites confirming the paleoenvironmental origin of oxygen isotope variations interestingly temperatures recorded by pelagic fishes and nektobenthic belemnites and bottom dwelling fishes are comparable during the late pliensbachian sea level lowstand but gradually differ during the early toarcian transgressive episode recording a difference in water temperatures of 6 c during the bifrons zone this could suggest that the surface bottom water temperature difference was not large enough during regressive phases to be recorded by organisms living near the lower and upper part of the water column the globally unradiogenic nd budget of euro boreal waters through the early jurassic suggests that these waters were strongly affected by continental neodymium input from surrounding emerged areas and that exchange with more radiogenic waters from the tethys and panthalassa oceans remained limited this supports the existence of a southward directed current in the euro boreal area for most of the early jurassic the only exception is observed at the early late pliensbachian transition where a positive end excursion is recorded suggesting northward influx of low latitude tethyan or panthalassan waters which may have contributed to the warming of nw tethyan seawater recorded at this time the absence of a marked negative excursion in end concomitant with a negative 18 o shift recorded during the falciferum zone exaratum subzone argues against the influence of less radiogenic arctic water influxes with low 18 o values during this interval instead we suggest that

the influence of vegetation cover on soil organic matter preservation in antarctica during the mesozoic a possible biological component of the explanation for the lack of substantial coal formation on antarctica between the late jurassic and late cretaceous is hypothesised forcing a process based terrestrial biogeochemistry model with climate model simulations for three successive intervals of the mesozoic late jurassic 150 ma mid cretaceous 100 ma and late cretaceous 65 ma predicts the establishment of productive terrestrial ecosystems with substantial soil carbon pools throughout the land surface of antarctica between 150 and 65 million years ago sensitivity analyses indicate the annual land surface hydrological budget of antarctica 150 65 ma was largely balanced due to the effects of canopy transpiration on soil moisture post depositional erosion and tectonic setting must have played an important role in determining coal preservation on the continent however the model results indicate that an active vegetated land surface precluded the preservation of the substantial soil organic carbon inputs by minimising the potential for anaerobic preservation i e water logging

early jurassic schizosphaerellid crisis in cantabria spain implications for calcification rates and phytoplankton evolution across the toarcian oceanic anoxic event 1 the toarcian oceanic anoxic event 183 myr ago represents a global perturbation marked by increasing organic carbon burial and a general decrease in calcium carbonate production likely triggered by elevated carbon dioxide levels in the atmosphere here we present quantitative analyses of calcareous nannofossil diversity and abundance from the castillo de pedroso section in cantabria northern spain we compare these data with geochemical data c and o isotopes obtained from biogenic and bulk carbonate records in order to highlight the response of calcareous phytoplankton to major climatic and paleoceanographic changes the pliensbachian toarcian boundary is characterized by an abrupt decrease in abundance of schizosphaerella punctulata the most important lithogenic contributor to hemi pelagic carbonates in the early jurassic the early toarcian nannofloral assemblages show an increase in abundance of mitrolithus jansae and small sized r selected taxa and a progressive decrease in s punctulata percentages the deep dwellers m jansae and s punctulata experienced a major crisis slightly prior to the deposition of the toarcian black shales that are characterized by high abundances of eutrophic taxa such as lotharingius spp and biscutum spp the return of s punctulata associated with lower percentages of eutrophic taxa was observed just above the toarcian black shales the toarcian episode reveals that high co2 levels and increasing primary productivity probably triggered a shift in abundance from highly calcified nannoliths such as s punctulata and m jansae to small sized r selected coccoliths that overall record a biocalcification crisis at the onset and during the toarcian episode

sedimentological evaluation of general circulation model simulations for the greenhouse earth cretaceous and jurassic case studies abstract conceptual climate models based on the workings of the present day climate system provided a first order approach to ancient climate systems they are potentially very subjective in character their main drawback was that they involved the relocation of continents beneath a stable atmospheric circulation modelled upon that of the present general circulation models gcms use the laws of physics and an understanding of past geography to simulate climatic responses they are objective in character however they require super computers to handle vast numbers of calculations nonetheless it is now possible to compare results from different gcms for a range of times and over a wide range of parameterisations gcms are currently producing simulated climate predictions which compare favourably with the distributions of climatically sensitive facies e g coals evaporites and palaeosols they have been used effectively in the prediction of oceanic upwelling sites and the distribution of petroleum source rocks and phosphorites parameterisation is the main weakness in gcms e g sea surface temperature orography cloud behaviour sensitivity experiments can be run on gcms which simulate the effects of milankovitch forcing and thus provide insights into possible patterns of climate change both globally and locally i e provide predictions that can be evaluated against the rock record future use of gcms could be in the forward modelling of sequence stratigraphic evolution and in the prediction of the diagenetic characteristics of reservoir units in frontier exploration areas the sedimentary record provides the only way that gcms may themselves be evaluated and this is important because these same gcms are being used currently to predict possible changes in future climate

the cenomanian turonian boundary at eastbourne sussex uk a proposed european reference section the cenomanian turonian boundary ctb section at eastbourne sussex england is the thickest at outcrop in the anglo paris basin is rhythmic throughout rich in macro micro and nanno fossils and shows less diagenetic alteration than other sections in nw europe geochemical and biostratigraphic data are all related to detailed rhythmic lithostratigraphy allowing correlation with the global boundary stratotype section to within 20 000 years and accurate estimates of rates of changes very detailed stable isotope curves for oxygen and carbon are presented and show that the build up phase of the ctb carbon excursion had two portions separated by a trough patterns of occurrence of coarse fraction benthic foraminifera 250 mm show two peaks of extinction corresponding to the two build up phases of the carbon excursion benthic and planktic foraminifera ostracods and calcareous nannofossils all show declines in diversity and abundance across the ctb foraminifera also decline significantly in size key biomarker events for the ctb include first occurrences of the ammonites fagesia catinus and mammites nodosoides the bivalve mytiloides the planktic foraminifera helvetoglobotruncana praehelveticaand marginotruncana renzi and the calcareous nannofossils quadrum intermedium q gartneri and eprolithus octopetalus and last occurrences of the ammonite sciponoceras the planktic foraminifera rotalipora greenhornensis and r cushmani the calcareous nannofossils corolithion kennedyi axopodorhabdus albianus lithraphidites acutus rhagodiscus asperand microstaurus chiastius cyclostratigraphy allows very precise correlation of isotopic and bioevents with other sections in europe and north america eastbourne is recommended as a nw european standard section 1999 elsevier science b v all rights reserved

osmium isotope evidence for the regulation of atmospheric co2 by continental weathering the long term stability of earth9s climate throughout the phanerozoic stands in marked contrast to the dramatic fluctuations that have taken place on time scales as short as a few years reflecting the high efficiency of longer term climate regulation through negative feedbacks a fundamental mechanism is thought to involve control of co 2 in the ocean atmosphere system through continental weathering although unambiguous high resolution data supporting this hypothesis have hitherto not been available organic rich mud rocks from yorkshire england which were deposited during the toarcian oceanic anoxic event ca 181 ma early jurassic contain evidence of an exceptionally large excursion in the 187 os 188 os ratio of contemporaneous seawater from 0 4 to 1 0 the most likely explanation for this excursion is that it resulted from a transient increase in global continental weathering rates of 400 800 the os isotope excursion coincided with a well documented global 13 c excursion of 6 that affected all the major biospheric reservoirs of the time higher mean global temperatures caused global chemical weathering rates to increase substantially while in turn chemical weathering was very effective in reducing the elevated levels of atmospheric co 2 and the high temperatures to preexcursion levels

influence of oceanic anoxic events on the evolution of mid cretaceous radiolaria in the north atlantic and western tethys abstract mid cretaceous aptian turonian extinction radiation events of radiolaria from the north atlantic and western tethys are summarized in relation to oceanic anoxic events oae from investigations of radiolarian faunas black shales and stable carbon isotopes extinction and radiation events of radiolaria can be observed from the late early aptian the earliest albian the latest albian and the cenomanian turonian boundary these events correlate well with organic carbon rich sediments that contain type ii kerogen and show positive 13c excursions they correspond to the oceanic anoxic events oae in the early aptian oae 1a selli level early albian oae 1b late albian oae 1d and latest cenomanian oae 2 bonarelli level detailed investigations of the two most significant events the selli and the bonarelli level in the umbria marche basin of central italy show that extinctions coincide with the first occurrence of marine black shales type ii kerogen and the most prominent rise of 13corg values stepwise extinctions and continuous radiations precede and follow the events similar patterns are evident for the oae 1b and 1d black shales dominated by terrestrial organic matter type iii kerogen e g oae 1c are not characterized by isotopic excursions or by radiolarian extinction radiation events contemporaneous drowning events documented from carbonate platforms around the investigated basins provide a link to a model where flooding of lowlands and leaching of nutrients during a relative rise of sea level lead to an increased productivity and an expansion of the oxygen minimum zone omz because of the loss of deep habitats deeper dwelling forms become extinct shallower dwelling radiolarians survive a narrowing of the omz during highstands and relative sea level falls together with a coupled decrease of the nutrient supply leads to the development of new niches and therefore to radiation

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

nitrogen isotope evidence for water mass denitrification during the early toarcian jurassic oceanic anoxic event bulk sedimentary nitrogen isotope 15ntot data have been generated from lower jurassic black carbon rich shales in the british isles and northern italy deposited during the early toarcian oceanic anoxic event a pronounced positive 15ntot excursion through the exaratum subzone of the falciferum zone defined by characteristic ammonites in the british isles broadly correlates with a relative maximum in weight percent total organic carbon and in some sections with a negative 13corg excursion upwelling of a deoxygenated water mass that had undergone partial denitrification is the likely explanation for relative enrichment of 15ntot and parallels may be drawn with quaternary sediments of the arabian sea gulf of california and northwest mexican margin the development of early toarcian suboxic water masses and consequent partial denitrification is attributed to increases in organic productivity approximately coincident phenomena include the following a relative climatic optimum realignment of major oceanic current systems and a possible release of methane gas hydrates from continental margin sediments early in the history of the oceanic anoxic event

δ13c depth profiles from paleosols across the permian triassic boundary evidence for methane release stable carbon isotopic analyses of organic carbon 13 c in individual paleosol profiles from permian triassic sequences of antarctica reveal systematic isotopic variations with profile depth these variations are in many cases analogous to those in modern soils which are functions of redox conditions soil development and degree and type of microbial decay in modern soils these isotopic depth functions develop independently from vegetation changes c 3 versus c 4 vegetation and can be diagnostic of soil orders this study shows that soil intrinsic functions can be preserved in the 13 c values of paleosols as old as 260 ma and constitute valuable data for paleoecological interpretations a large carbon isotopic offset of as much as 10 in whole paleosol profiles across the permian triassic boundary indicates significant changes in the soil biogeochemistry and the soil atmosphere system early triassic paleosols are distinctive in their extremely low 13 c values to 42 and often show an anomalous 13 c depth distribution compared to both permian paleosols and modern soils highly depleted 13 c values as the ones in early triassic paleosols are suggested to be associated with microbial methane oxidation methanotrophy this hypothesis implies increased methane concentrations in the early triassic soil atmosphere system increased atmospheric methane was probably partly responsible for the global carbon isotopic shift documented in marine and terrestrial sediments across the permian triassic boundary

carbon addition and removal during the late palaeocene thermal maximum basic theory with a preliminary treatment of the isotope record at odp site 1051 blake nose the late palaeocene thermal maximum lptm was a brief interval at c 55 ma characterized by a 2 5 to 3 per thousand shift in the 13c of global carbon reservoirs the geochemical perturbation probably represents a massive input of 12c rich carbon to the exogenic carbon cycle largely unresolved issues concerning this carbon injection during the lptm are the rates of carbon input and removal simple expressions are developed here to describe a 13c excursion in the exogenic carbon cycle after carbon input a change in global 13c d ex dt can be explained to a first approximation by a set of parameters the initial mass and isotopic composition of the global carbon cycle mex o ex o and the fluxes and isotopic compositions of external carbon inputs outputs and injected carbon fin in fout out fadd add in general for a given exogenic carbon cycle a large fadd or low add results in a larger 13c excursion likewise for a given negative 13c excursion a large mex or low ex requires a greater input of 12c differences in fin in fout and out cause changes in the response of ex over time for a negative 13c excursion of given magnitude a greater fin requires a greater input of 12c and lessens the time for ex to return to initial conditions a decrease in out caused by an increase in the relative output of organic matter and carbonate has a similar effect variable dmadd dt produces transients in ex that are related to the source function but modified by carbon removal in theory a well dated and representative global 13c excursion could be used to derive the carbon inputs and ouputs ocean drilling program odp site 1051 has an expanded early palaeogene section and recent work at this location has provided a well dated 13c record across the lptm this 13c record contains transient variations of apparently global nature these observed transients are best explained by a pulsed injection of ch4 into an exogenic carbon cycle with a greater carbon throughput or enhanced burial of organic matter after carbon addition

crustal velocity structure on the flanks of the mid atlantic ridge at 24 n summary a detailed seismic refraction survey was made over a 70 35 km area of young 9 ma crust at 24 n 45 w to investigate the detailed velocity structure of a portion of typical oceanic crust an array of eight ocean bottom hydrophones was used to monitor 358 explosive charges and over 750 airgun shots the crust exhibits normal oceanic structure with a steep vertical velocity gradient through layer 2 and almost 3 km of layer 3 material with a mean velocity of 6 65 km s 1 overlying probably anisotropic mantle ray tracing shows that the major amplitude variations observed in the upper crustal returns are governed more strongly by the seafloor topography than by the seismic velocity structure within the crust a delay time function method was used to invert the 1613 well determined first arrival travel times from the explosives we find that the delay time vanes by only 0 05 0 10s over most of the survey area but that there are rather larger delay times in the north west corner which may be associated with crust formed in the vicinity of a ridge jump we also find a systematic correlation between variations in delay time and in the seafloor depth at the ray entry point with smallest delay times where the seafloor is deepest this is consistent with a model of the crust in which the topography of the base of layer 2 and the moho is a subdued copy of the upper basement surface

early aptian algal bloom in a neritic proto north atlantic setting harbinger of global change related to oae 1a pervasive growth of microencrusters lithocodium bacinella characterizes short lived stratigraphic intervals in lower aptian shallow water carbonate platform settings of the central and southern tethys although a causal relationship between this peculiar shallow water facies and the early aptian oceanic anoxic event oae 1a seems likely the temporal and spatial distributions of lithocodium and bacinella mass occurrences in the rock record point to a rather complex pattern possible controls include various global and regional drivers expressed in environmental perturbation of neritic ecosystems microencruster blooms have hitherto been reported to occur at the onset and in the aftermath of oceanic anoxic event oae 1a slightly pre or postdating the deposition of dark organic rich black shales in pelagic basins this study presents evidence for a punctuated and well expressed early aptian bloom of the ulvophycean green algae lithocodium aggregatum observed at the proto north atlantic margin lusitanian basin portugal similar to southern and central tethyan sections a lithocodium dominated interval temporally replaces the rudist dominated platform ecosystem this lithocodium rich horizon is composed of oncoidal floatstones and boundstones including a well exposed biohermal level and reaches a thickness of up to 6 m high resolution carbon isotope stratigraphy shown here clearly argues for lithocodium to represent a biotic harbinger of environmental change in the run up of oae 1a chemostratigraphy provides evidence that the subsequent carbonate platform breakdown in the proto north atlantic was coeval with an early aptian carbonate platform drowning episode observed in the northern tethys prior to oae 1a a regionally enhanced nutrient level and a relative sea level rise are suggested to have triggered the transient mass occurrence of lithocodium microencrusters which also may have been influenced by elevated atmospheric co2 concentrations related to the emplacement of the ontong java large igneous province upwelling water masses and enhanced terrigenous runoff due to increased weathering may have served as possible sources of the required nutrients which tentatively diminished and finally interrupted the metazoan carbonate platform factory

paleoceanographic significance of high resolution carbon isotope records across the cenomanian turonian boundary in the western interior and new jersey coastal plain usa carbon isotope measurements have been made on bulk carbonate and organic matter in three sections across the cenomanian turonian boundary the stratigraphy records the positive carbon isotopic excursion that is associated with the major oceanic anoxic event oae2 comparison of toc and isotopic data in sections from a range of different environments suggests that the positive excursion is a response to increased burial of organic matter in the deep ocean the lack of coupled negative shifts in 13corg and 13ccarb records at the onset of oae2 does not support a causal role for volcanism or methane hydrate dissociation

marine tectonics processes at mid ocean ridges the use of multibeam echo sounders and various side looking sonar systems has greatly increased our knowledge and understanding of mid ocean ridge mor processes since the last iugg review on mid ocean ridge research macdonald 1983 the extensive use of swath mapping tools such as sea beam deep tow seamarc i and ii the application of multichannel seismic reflection techniques to the detection of axial magma chambers and the use of satellite data in the study of oceanic lithosphere have rejuvenated marine tectonics and have generated and stimulated a large number of important contributions this review will focus on what we feel were the major developments in marine tectonics with an emphasis on mid ocean ridges in the last four years certain topics such as marine seismicity and satellite studies are only briefly mentionned for reference and will be covered in greater detail in other reviews because of the exponential growth in the number of publications related to mors during 1982 1986 only a few references pertinent to each of the major research topics will be quoted in the text as examples but the reader will find an extensive bibliography at the end of the text references published prior to 1982 were added only where appropriate

onset of the mid cretaceous greenhouse in the barremian aptian igneous events and the biological sedimentary and geochemical responses basalts and biostratigraphy dated at 125 120 ma from the ontong java and manihiki plateaus in the western pacific evidence the largest volcanic event in earth history in at least the past 160 m y the intervening nova canton trough rifted at about 121 118 ma and a number of guyots and seamounts formed concurrently or slightly later geological events that probably were responses to these volcanic tectonic events occurred in the following chronostratigraphic order biotic fluctuations began at about 122 5 ma at about 122 0 ma 87sr 86sr began to decline slowly metal concentrations of co mn pb yb and cu in sediments peaked at about 121 5 121 2 ma changes in planktonic communities and sedimentation culminated in a nannoconid crisis just prior to 120 5 ma and in the selli black shale oae la at about 120 5 119 5 ma a sharp drop in 13c occurred at the beginning of the selli event and rebounded into a longer positive excursion that reached a peak after the selli event at about 119 5 118 5 ma at 120 5 ma 87sr 86sr declined rapidly and reached a minimum at about 116 113 ma we speculate that the intensity of these latter responses suggests a corresponding peak in volcanic tectonic activity at about 121 119 ma

terrestrial record of methane hydrate dissociation in the early cretaceous reconstruction of changing c isotopic composition of early cretaceous atmospheric co2 from fossilized c3 vascular land plant tissue revealed a brief and striking negative excursion d 25 in atmospheric d 13 c followed by a rapid positive compensation d 15 during the aptian ca 117 ma mass balance calculations show that dissociation of a small amount of methane gas hydrate is the most tenable cause of the negative excursion this would also result in an increased co2 o2 mixing ratio as o2 is consumed during ch4 oxidation to co2 spurring the exponential phase of angiosperm biogeographic expansion

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

carbon isotope stratigraphy a tool for basin to carbonate platform correlation the early cretaceous carbonate carbon isotope stratigraphy established in pelagic limestones is marked by several pronounced excursions towards positive 13c values we investigated a biostratigraphically and palaeomagnetically calibrated aptian section to see whether c isotope stratigraphy could be recognized in shallow water carbonates the chosen carbonate platform sections are located in southern italy and have been dated by biostratigraphy bulk samples chosen from the barremian albian part of the sequence were analysed for their o and c isotope compositions the c isotope curve established shows two major positive excursions which can be correlated with the synchronous and globally recognized aptian c isotope events the data provide evidence that c isotope stratigraphy can be used as a powerful correlation tool between pelagic and shallow water limestone sequences

geochemical and sedimentological evidence of productivity cycles recorded in toarcian black shales from the belluno basin southern alps northern italy abstract organic carbon rich black shales cyclically alternating with bioturbated limestones and gray marlstones which crop out in the belluno trough southern alps italy are expression of the early toarcian anoxic event in the tethys basin sedimentological and geochemical features such as well developed lamination common very small a negative 13c excursion at the middle part of the studied interval is correlative to maxima in toc v rb and ba rb values identifying within the toarcian anoxic event a stage of more intense bottom water anoxia coupled with high surface fertility

carbon isotope composition of lower cretaceous fossil wood ocean atmosphere chemistry and relation to sea level change the carbon isotope composition of fossil wood fragments collected through a biostratigraphically well constructed aptian lower cretaceous shallow marine siliciclastic succession on the isle of wight southern britain shows distinct variations with time the results indicate that the stratigraphic signature of delta sup 13 c sub wood through the aptian was influenced primarily by fluctuations in the isotopic composition of co sub 2 in the global ocean atmosphere system as registered in marine carbonates elsewhere and was not governed by local paleoenvironmental and or paleoecological factors negative and positive excursions in delta sup 13 c sub wood through the lower aptian occur in phase with inferred transgressions and regressions respectively a pattern that contrasts with that observed in many previous studies for different time intervals the relationship between delta sup 13 c variations and relative sea level change is tentatively interpreted as a response to various climatic and eustatic factors relating to rapid sea floor spreading thermal uplift of ocean floor emplacement of plateaus volcanic co sub 2 emissions weathering and sedimentary rate

the early toarcian jurassic anoxic event stratigraphic sedimentary and geochemical evidence detailed paleogeographic studies in the southern alps suggest that an oxygen minimum model is appropriate for interpreting conditions on tethyan continental margins advection of manganese in this low oxygen layer may explain the local occurrence of carbonate ores of this element carbon sulfur and iron sulfur ratios although showing considerable scatter suggest that bottom waters at this time were locally euxinic containing free hydrogen sulfide this oceanic anoxic event was preceded by significant faunal turnover of ammonites in tethys and accompanied by widespread extinction of benthos in northern europe in response to the lateral spread of anoxic bottom waters during transgression similar changes may be recognized in other parts of the world other anoxic events may have taken place during the jurassic but documentation is as yet meagre furthermore models for such phenomena remain largely speculative although upwelling and increased planktonic productivity commencing in pre toarcian time are favored for the falciferum zone event documented here from author

a upb age from the toarcian lower jurassic and its use for time scale calibration through error analysis of biochronologic dating abstract the early jurassic is the most poorly constrained part of the geochronologic scale within the mesozoic owing to the scarcity of calibration points we report a new u pb zircon age of 181 4 1 2 ma 2 from an ash layer within the type section of crassicosta zone a north american regional standard ammonite zone in the middle toarcian for time scale calibration the isotopically dated level needs to be tied to the jurassic chronostratigraphic framework established in northwest european successions three major contributing factors difference between local observed and true ranges uncertainties of correlation and taxonomic noise are assessed to determine the maximum biochronologic dating error the local ammonite distribution data are robust in that 95 confidence level extensions of observed ranges of common ammonite taxa would not significantly modify the zonal assignment the computer assisted unitary association ua method is used for world wide biochronologic correlation representative north and south american western tethyan and northwest european sections were selected to establish the global maximum ranges for 103 middle toarcian ammonite taxa and a sequence of 40 ua maximum permissible correlatives of the crassicosta zone lie between the northwest european standard semipolitum subzone late bifrons zone and bingmanni subzone early thouarsense zone in particular the dated tuff layer cannot be older than the semipolitum subzone or younger than the illustris subzone variabilis zone taxonomic inconsistencies are present in the source data but their effect in corrupting ua based correlation is shown to be negligible the precise u pb age 2 error

on the nature of methane gas hydrate dissociation during the toarcian and aptian oceanic anoxic events the magnitude and timing of a major rapid negative carbon isotope excursion recorded in marine and terrestrial matter through the early toarcian early jurassic and early aptian early cretaceous oceanic anoxic events oaes have been proposed to be the result of large methane gas hydrate dissociation events here we develop and evaluate a global carbon isotope mass balance approach for determining the responses of each component of the exogenic carbon cycle terrestrial biosphere atmosphere and ocean the approach includes a dynamic response of the terrestrial carbon cycle to methane related co2 increases and climatic warming our analyses support the idea that both the early toarcian and early aptian isotopic curves were indicative of large episodic methane releases 5000 and 3000 gt respectively promoting warm greenhouse conditions in the mesozoic these events are calculated to have increased the atmospheric co2 concentration by 900 and 600 ppmv respectively and land surface temperatures by 2 5 to 3 0 c however we show that much of the methane released from oceanic sediments is rapidly sequestered by terrestrial and marine components in the global carbon cycle and this effect strongly attenuated the potential for ancient methane gas hydrate dissociation events to act as major amplifiers in global warming an increase in oceanic carbon sequestration is consistent with the deposition of globally distributed black shales during these oaes our analyses point to the urgent need for high resolution marine and terrestrial carbon isotope records to better characterize the nature of the toarcian and aptian events and improve our interpretation of their consequences for the global carbon cycle

the source and fate of massive carbon input during the latest paleocene thermal maximum lithologic faunal seismic and isotopic evidence from the blake nose subtropical western north atlantic links a massive release of biogenic methane 55 5 million years ago to a warming of deep ocean and high latitude surface waters a large perturbation in the combined ocean atmosphere carbon cycle the largest of the past 90 million years a mass extinction event in benthic faunas and a radiation of mammalian orders the deposition of a mud clast interval and seismic evidence for slope disturbance are associated with intermediate water warming massive carbon input to the global exogenic carbon cycle pelagic carbonate dissolution a decrease in dissolved oxygen and a benthic foraminiferal extinction event these events provide evidence to confirm the gas hydrate dissociation hypothesis and identify the blake nose as a site of methane release

quantitative palaeoclimate gcm validation late jurassic and mid cretaceous case studies using quantitative proxy models of peat and bauxite formation based upon modern analogues predictions of the distribution of peats coals and bauxites for both the late jurassic kimmeridgian and mid cretaceous cenomanian have been made using a general circulation model gcm the predictions have been compared with the known distribution of these deposits such an approach provides a means of identifying and isolating which components of the gcm accurately reflect palaeoclimate the prediction of peats and bauxites for the jurassic and cretaceous show good correspondence with known coals and bauxites and provides otherwise elusive quantitative estimates of high latitude climate the gcm is thus successfully replicating precipitation and mean annual temperatures for both the kimmeridgian and cenomanian investigations of mesozoic palaeoclimate using general circulation models gcms have often focused on qualitative evaluation of results e g barron washington 1982 valdes et al 1995 chandler et al 1992 a number of recent studies have however highlighted the need for gcm predictions to be quantitatively validated against the sedimentological and palaeobiological record e g poore chandler 1994 pollard schulz 1994 price et al 1995 quantitative estimates of terrestrial climate have been produced by physiognomic analysis of floras e g herman spicer 1996 wolfe upchurch 1987 and have also been derived from organisms such as crocodiles e g markwick 1994 comparable results have been used effectively for validating gcm predictions e g sloan barron 1992 but data tend to be restricted to north america and are often latest cretaceous or younger in age

a review of methane and gas hydrates in the dynamic stratified system of the blake ridge region offshore southeastern north america abstract the blake ridge region lies on the passive margin of southeastern north america and contains a large amount of methane gas hydrate the methane and methane gas hydrate are predominantly biogenic apparently produced by co2 reduction reflection seismics indicate that bottom simulating reflectors bsrs enclose 55 000 km2 with high amplitude bsrs covering 26 000 km2 ocean drilling program odp leg 164 drilled three deep holes on a 10 km long transect sites 994 995 and 997 water depth 2770 2798 m based on sampling and geochemical thermal seismic and borehole geophysical measurements gas hydrates are most likely present between 190 and 450 m in sediment column gas hydrate is most often disseminated throughout the sediment column although concentrations occur within specific sedimentary horizons within supposed fault zones and at the base of gas hydrate stability bghs where methane recycling produces more pervasive concentrations of gas hydrate estimates of gas hydrate inventory are based on a variety of methods including geochemical proxies vertical seismic profiling electric logging and measurements of in situ methane over the entire sediment column at least 2 4 of pore space volume 1 2 sediment volume is occupied by methane gas hydrates but average and maximum estimates are 5 4 and 12 respectively extrapolation of vertical gas hydrate and methane inventory over the area containing high amplitude bsrs yields estimates of 67 406 gt gigatons 1015 g of methane gas hydrate or 9 52 gt of methane and 2 6 27 gt of methane occurring as gas bubbles below the bghs average values are 185 gt of gas hydrate and 24 gt of methane as gas hydrate any gas hydrate occurring outside the area underlain by bsrs as suggested by geochemical evidence or that associated with low amplitude bsrs may increase these estimates by an unknown factor various data give conflicting pictures of mass transport with blake ridge sediments the data can be reconciled by viewing the upper sedimentary section accumulation of gas hydrate in the blake ridge sediments depends on the amount of methane leaving the system versus the amount of methane entering the gas hydrate stability zone ghsz over geologic time although there are some point sources of methane loss from the sediments e g seafloor seeps odp site 996 of unknown magnitude most of the documented methane loss occurs through diffusion and consumption at the sulfate methane interface smi by anaerobic methane oxidation amo 2 108 mol year 1 methane entering the ghsz at a rate of 1 3 109 mol year 1 indicates a methane trapping efficiency of 85 129i measurements suggest that the blake ridge system has accumulated gas hydrate over as much as 55 million years

black shales and carbon isotopes in pelagic sediments from the tethyan lower jurassic detailed sampling and analysis of jurassic pelagic limestones and marls from italy hungary and switzerland have enabled construction of an isotope stratigraphy across the pliensbachian toarcian boundary with resolution to the zonal level the oxygen isotope record is unremarkable the carbon isotopes however show two positive excursions one relatively minor during the pliensbachian margaritatus zone subnodosus subzone the other more major during the toarcian early falciferum zone where a maximum 13c value of 4 52 pdb is attained these intervals are known to be favoured periods of organic rich sedimentation in diverse parts of the globe and the isotopic excursions are interpreted as a response to abnormally high rates of storage of organic carbon in the sedimentary record a comparable phenomenon has been documented from the cenomanian turonian boundary in the cretaceous where it has been referred to the influence of an oceanic anoxic event some italian sections spanning this lower jurassic interval contain organic rich shales in the falciferum zone the isotopic signatures from their included locally manganiferous carbonate betray a considerable diagenetic overprint and they cannot therefore be incorporated in a composite isotopic curve carbon isotopes from the organic carbon itself are extremely negative falling to 33 pdb and in one section examined in detail correlate with the calcium carbonate content of the shales they may reflect a partial change to a non calcified planktonic biota during deposition of this lime poor interval possibly responding to upwelling and increased fertility of near surface waters the onset of upwelling may have been as early as spinatum tenuicostatum zone time that is at the pliensbachian toarcian boundary

the early toarcian early jurassic ostracod extinction events in the iberian range the effect of temperature changes and prolonged exposure to low dissolved oxygen concentrations abstract the early toarcian extinction event once regarded as spanning in the late pliensbachian early toarcian boundary period actually occurred in two distinct phases in the early toarcian of the iberian range the first episode occurred in the mirabile subzone tenuicostatum zone and is characterised by the disappearance of about 12 of ostracod species and one suborder the metacopina superfamily healdioidea this study shows that this suborder disappears during stratigraphical intervals that are considered to be representative of low oxygen conditions in the iberian range the collapse of marine ecosystems affecting up to 40 of ostracod species in the elegantulum subzone serpentinum zone was most likely triggered by a sudden and widespread lethal increase in seawater temperature these combined processes of oxygenation and temperature change are considered to be responsible for the profound changes in marine environments that resulted in the early toarcian ostracod extinction in the iberian range

interpreting carbon isotope excursions carbonates and organic matter abstract variations in the carbon isotopic compositions of marine carbonate and organic carbon provide a record of changes in the fraction of organic carbon buried through time and may provide clues to changes in rates of weathering and sources of organic carbon paired carbonate and organic carbon isotope determinations provide a possibility of interpreting not only changes in the global carbon cycle through time but changes in atmospheric pco2 as well interpretations of these types of data are typically rather qualitative a quantitative basis is required to develop a better understanding of changes in the carbon cycle for this purpose we employ a simple model of the global carbon cycle which is subjected to a number of different perturbations each lasting 500 ky i e much longer than the residence times of carbon and phosphorus in the ocean atmosphere system in addition to standard considerations of carbon mass and isotopic fluxes to the ocean atmosphere system from weathering and volcanism and fluxes of organic carbon and carbonate carbon to sediments the model incorporates sensitivity of the photosynthetic carbon isotope effect to changes in pco2 the inclusion of this parameter leads to unexpected carbon isotope responses to forcing that causes increased rates of organic carbon burial a series of simple to more complex simulations illustrates the significant effects of varying differences between the carbon isotopic composition of sedimented carbonate and organic carbon b with constant b a 50 increase in organic carbon burial produces a parallel increase in carbonate and organic carbon isotopic compositions however the same simulation with b responsive to pco2 changes produces an initial parallel 13 c increase but this is followed by an even greater 13 c enrichment in organic carbon because pco2 falls in response to increased organic carbon burial the counterintuitive overall result of the enhanced organic carbon burial event is that the carbonate carbon isotopic composition actually decreases because of the more substantial increase in 13 c org in addition we illustrate the effects on carbon isotopic compositions of the oceanic inorganic carbon reservoir and buried organic matter of a 50 increase in volcanic co2 outgassing a 50 increase in weathering rate with coupled phosphate and riverine carbon flux responses a 50 decrease in shale associated organic carbon weathering a 50 decrease in silicate weathering rate and the possible effects of the rise in abundance of c4 plants in the late miocene to recent we compare the model simulated carbon isotopic responses for some of these experiments to paired carbonate and organic carbon records to illustrate how these records might be interpreted in light of the model response

paleomagnetic calibration of milankovitch cyclicity in lower cretaceous sediments abstract bedded pelagic limestones of early to mid cretaceous age may record cyclic climate variability driven by changes in the earth s orbit milankovitch cycles if properly calibrated the lithologic cycles could provide sedimentary chronometry at far shorter time scales than biostratigraphy and magnetic stratigraphy the lower cretaceous maiolica formation of northern and central italy displays rythmic bedding in sections whose magnetic reversal sequence is clearly correlated to the upper m series marine anomalies the magnetic reversals divide measured sections into 0 5 2 myr increments in which sedimentation rate assuming continous deposition can be measured to 15 25 accuracy paleomagnetically estimated sedimentation rates over a number of polarity zones establish with one exception a narrow range in periods of the carbonate cycles carbonate couplets have an estimated mean period of 23 5 kyr and prominent modulations in bedding thickness bundling have a mean period of 117 kyr kent and gradstein 1985 time scale the close match of sedimentary periodicities to orbital repeat times implies that the carbonate cycles reflect a combination of professional and eccentricity climatic forcing comparison of measured sections from different locations shows that bed to bed correlations are possible regionally cosistent with the milankovitch hypothesis episodes of barremian black shale deposition occur in the troughs of the short eccentricity cycle as roughly one half of the carbonate marl couplet or about 10 kyr duration it is possible to estimate both the timing relative to the top of reversed chron m 0 and duration of a period of unusual organic carbon rich deposition selli horizon in the early part of the cretaceous long normal magnetic polarity interval by tracing the sedimentary cycles up from the barremian strata

evidence for late jurassic release of methane from gas hydrate four late jurassic carbonate successions deposited in the tethys atlantic ocean record a negative carbon isotope excursion of at least 2 the excursion is present in both organic and carbonate carbon records and is comparable in magnitude and duration to isotopic changes during the late paleocene thermal maximum our results indicate that during the late jurassic long considered a warm greenhouse time additional greenhouse gas was input to the atmosphere by a sudden release of methane from buried gas hydrate a potential triggering mechanism may have been the opening of an oceanic gateway through the early atlantic between the ancient tethys and pacific oceans

increased terrestrial carbon storage across the palaeocene eocene boundary abstract a rapid short lived global warming event at the close of the palaeocene ca 55 5 million years ago ma was associated with the addition of massive amounts of carbon to the exogenic carbon cycle and major benthic extinctions evaluation of the response of the terrestrial biosphere to this transient ancient greenhouse world with a process based terrestrial carbon cycle model and global palaeoclimate simulations indicates the sequestration of ca 1112 gt c into the vegetation and soil carbon pools a largely independent global carbon isotope mass balance analysis also points to the strong potential for the terrestrial biosphere to sequester substantial amounts of carbon at this time with the increase being calculated at between 2107 and 3060 gt c this analysis suggests that some other feature of global environmental change across the palaeocene eocene boundary may have been forcing terrestrial carbon cycle dynamics at this time however even a minimum increase of 1112 gt c would have been sufficiently large to implicate the response of the terrestrial carbon cycle as a negative feedback mechanism on climate at 55 5 ma possibly reducing global temperatures by up to 2 5 c

calcareous nannofossils and mesozoic oceanic anoxic events abstract twenty five years ago mesozoic oceanic anoxic events oaes were documented and formalised as intervals of widespread to global deposition of organic matter the toarcian early aptian oae1a and latest cenomanian oae2 oaes are truly global in nature commonly carbonate poor and typically represented by organic carbon rich black shales in some areas these oaes are also characterised by abundant radiolarian sands and silts they are associated with negative and positive excursions in the 87sr 86sr record in addition to large global carbon isotope anomalies in carbonate and or organic matter caused by a major perturbation of the global carbon budget increased rates of volcanism during the formation of the ontong java and manihiki and caribbean plates and the karoo ferrar traps are believed to have caused the geological responses associated with oae1a oae2 and the toarcian oae respectively excess volcanogenic co2 in the atmosphere most probably turned the climate into a greenhouse mode accelerating continental weathering and increasing nutrient content in oceanic surface waters via river run off higher fertility in the global ocean was also probably triggered directly by submarine igneous events that introduced enormous quantities of biolimiting metals within hydrothermal plumes because mesozoic oaes are often represented by carbonate poor sediments quantitative studies of calcareous nannofossils have been applied to explore a the causes and effects of igneous tectonic events and climate changes relative to nannofloral increases and crises as well as b dissolution events and c diagenetic modifications characterization of calcareous nannofloras in oae intervals can improve our understanding of the marine ecosystem and biological processes such as photosynthesis biological pump and biomineralisation carbonate pump that affect the organic and inorganic carbon cycle as well as adsorption of atmospheric co2 in the oceans types and rates of nannoplankton production and evolution are interpreted to trace the impact of major palaeoceanographic and palaeoclimatic events in selected sections it has been documented that calcareous nannofloras rapidly reacted to new conditions of fertility and higher pco2 by drastically reducing calcification as in the modern oceans during oaes the increase of nutrients and atmospheric co2 induced higher abundances of nannoplankton producing small placoliths and inhibited the deep photic zone nannoconids and schizosphaerellids similarly to the nannoconid crisis preceding deposition of the early aptian oae1a black shales a schizosphaerellid crisis is detected prior to the toarcian oae both oaes are further characterised by a rapid nannofloral speciation beginning approximately 1 5 myr before the oae but without extinctions global changes during the latest cenomanian oae 2 exerted different influences on calcareous nannoplankton that experienced a turnover due to most extreme environmental conditions this event in fact was a time of extinctions followed by originations within calcareous nannofossils precise timing of the events before during and after oae1a oae2 and the toarcian oae indicates that they were intervals of enhanced oceanic productivity and that anoxia dysoxia post dated biotic changes

strontium isotope profile of the early toarcian jurassic oceanic anoxic event the duration of ammonite biozones and belemnite palaeotemperatures we profile 87sr 86sr 13c 18o sr ca mg ca and na ca in belemnites through pliensbachian and toarcian strata on the yorkshire coast uk which include the early jurassic oceanic anoxic event the 87sr 86sr profile shows that the relative duration of ammonite subzones differ by a factor of up to 30 the lower jurassic exaratum subzone is 30 times longer than the clevelandicum subzone because the exaratum subzone in yorkshire which contains the anoxic event is condensed by a factor of between 6 5 and 12 2 times relative to adjacent strata using our 87sr 86sr profile the resolution in correlation and dating attainable in the interval is between 1 5 m and 15 m of section and better than 0 25 myr in parts of the sequence this stratigraphic resolution equals that attainable with ammonites a new age model is provided for late pliensbachian and early toarcian time that is based on the 87sr 86sr profile through the sequence the sr ca mg ca na ca and 18o of belemnite carbonate covary suggesting that elemental ratios may be useful for palaeotemperature measurement our 13cbelemnite data splits into three the previously reported positive isotope excursion to 6 5 in the early toarcian we speculate that the excursion s resulted from addition to surface waters of isotopically heavy co2 via ebullition of methanogenic co2 from the sediment during early burial of organic rich 10 toc sediments