shallow marine carbon and oxygen isotope and elemental records indicate icehouse greenhouse cycles during the early jurassic 1 for much of the mesozoic record there has been an inconclusive debate on the possible global significance of isotopic proxies for environmental change and of sequence stratigraphic depositional sequences we present a carbon and oxygen isotope and elemental record for part of the early jurassic based on marine benthic and nektobenthic molluscs and brachiopods from the shallow marine succession of the cleveland basin uk the invertebrate isotope record is supplemented with carbon isotope data from fossil wood which samples atmospheric carbon new data elucidate two major global carbon isotope events a negative excursion of 2 at the sinemurian pliensbachian boundary and a positive excursion of 2 in the late pliensbachian the sinemurian pliensbachian boundary event is similar to the slightly younger toarcian oceanic anoxic event and is characterized by deposition of relatively deepwater organic rich shale the late pliensbachian strata by contrast are characterized by shallow marine deposition oxygen isotope data imply cooling locally for both events however because deeper water conditions characterize the sinemurian pliensbachian boundary in the cleveland basin the temperature drop is likely of local significance in contrast a cool late pliensbachian shallow seafloor agrees with previous inference of partial icehouse conditions both the large scale long term and small scale short duration isotopic cycles occurred in concert with relative sea level changes documented previously from sequence stratigraphy isotope events and the sea level cycles are concluded to reflect processes of global significance supporting the idea of an early jurassic in which cyclic swings from icehouse to greenhouse and super greenhouse conditions occurred at timescales from 1 to 10 ma

heterogeneity cyclicity and diagenesis in a mississippian brachiopod shell of palaeoequatorial britain a shell of gigantoproductus okensis shows twenty growth lines with marked changes of fabric indicating periodical reduction of growth rates caused by environmental perturbations the number of growth lines suggests a lifespan of 20 years in agreement with the survival rates of extant brachiopods and with spiral deviation analysis geochemical analyses across the growth profile show a heterogeneous distribution of stable isotopes and trace elements it is possible to distinguish primary from altered carbonate and to interpret the isotopic data the oxygen isotope signal in the unaltered parts is periodical and annual with oscillation of 1 1 the higher values are at the growth lines winter and therefore most likely related to monsoon circulation during the visean the annual periodicity seems also present in the altered part of the shell suggesting that diagenesis could have reset the primary values but preserved their cyclicity

ionospheric convection at casey a southern polar cap station a digital ionosonde digisonde portable sounder 4 located at casey antarctica 66 3 s 110 5 e 80 8 corrected geomagnetic latitude has been operational since early 1993 and has accumulated 3 years of plasma drift measurements providing an excellent data set for studying the characteristics of ionospheric convection flow at a southern polar cap station the purpose of this study is to investigate the influence of the imf on the f region ionospheric convection over casey and to compare it to the heppner maynard satellite derived electric field models we find clear dependencies in the drift on the sign and strength of the imf by and bz components and with kp antisunward flow dominates during bz south conditions turning to have a sunward component around noon when bz is northward the by component causes the entire convection system to rotate and distorts the dayside flow in the proximity of the throat with a dawnward duskward component for by negative positive comparison with the bz south heppner maynard bc de and a patterns is favorable at most times although we predict a rounder more dominant dusk dawn cell and a smaller crescent shaped dawn dusk cell for by 0 there is a dependence on kp when bz is south in both the model and the drifts flow directions becoming more antisunward and velocities becoming higher on the dayside as kp increases this implies the polar cap is expanding under conditions of enhanced reconnection when bz is north the f region drift agreement with the bcp p and dep p models is excellent on the dawn dusk side for by 0 but diverges on the opposite side as the pattern flow lines twist sunward separation of the drifts into bz weakly 3 nt northward cases did not reveal any appreciable difference in the observed drift velocities

geochemistry of modern brachiopods applications and implications for oceanography and paleoceanography abstract modern rhynchonellida terebratulida thecideida and a few inarticulated brachiopods were collected live or soon after death at 30 locations from depths of 2 3940 m from the equator to high latitudes covering the atlantic pacific indian caribbean and mediterranean oceans seas well preserved fibers of the secondary layer combined with sr 450 1928 mg kg mn 1 199 mg kg and fe 1 610 mg kg contents confirm with few exceptions the excellent preservation of the modern brachiopod material evaluation of the stable isotope compositions was conducted on 276 specimens and on 374 published samples for a total database of 650 samples from 80 locations it revealed that oxygen isotope incorporation into shell calcite secondary layer of modern articulated brachiopods in most cases occurs in equilibrium with the ambient water and as such reflects surrounding environmental conditions thecideida as a group shallow water and low latitude incorporate oxygen isotopes 3 04 to 0 35 into shell calcite in equilibrium with their ambient water a large majority of genera and species of the terebratulida shallow water and low mid latitude also incorporate oxygen 2 30 to 3 48 into shell calcite in isotopic equilibrium with their surrounding water in contrast equilibrium incorporation of oxygen isotopes into shell calcite by the rhynchonellida is uncertain but a vital effect probably applies to the craniida the question of equilibrium disequilibrium incorporation of carbon isotopes into shell calcite of modern brachiopods remains unresolved at this time isotope compositions range up to 4 5 5 6 and 1 5 for 13 c and up to 5 5 5 9 and 0 3 for 18 o for shallow water low mid and high latitude brachiopods respectively strontium isotopes of modern brachiopods as expected show no significant variation with location latitude water depth temperature salinity and general water conditions once local habitat and ecological influences of fossil brachiopods from similar latitudes are considered their isotopic elemental values should be valuable diagenetic indicators and important oceanographic chemostratigraphic proxies

large scale convection patterns observed by dmsp the authors present a comprehensive compilation of the average distribution of the electrostatic potential across the high latitude ionosphere the averages are compiled from potentials along the satellite path calculated from thermal ion drift data from instrumentation on the defense meterological satellite program dmsp flights 8 and 9 satellites data were collected from the dmsp f8 satellite during the period september 1987 to december 1990 and from the dmsp f9 satellite during the period march 1988 to december 1990 the potential distributions are separated by geomagnetic position season and orientation of the interplanetary magnetic field imf and then averages of the distributions are calculated the average potential distributions clearly show the displacement of polar cap convection contours to the dusk or dawk flanks under the influence of the imf b sub y component the cross cap potential decreases as imf b sub z changes from southward to northward the average distributions indicate that the development of more than two convection cells for northward imf is either uncommon or nonexistent for imf b sub z 0 and b sub z vert bar b sub y vert bar a distorted pattern is observed in the average potential distribution not a four cell pattern as some previous studies suggest it should be for allmore orientations of the imf the convection reversal boundary at the poleward edge of the auroral zone is observed in the average distributions to be a rotational boundary it is not a shear boundary as suggested by some previous investigations on average the harang discontinuity convection reversal in the auroral zone near midnight is observed to exist weakly or not at all when examining individual passes a strong eastward flow is present sometimes in the region of the harang discontinuity especially on the poleward boundary but not at all times as implied by the heppner maynard model 39 refs 9 figs 2 tabs less

seasonal effect for polar cap sunward plasma flows at strongly northward imf bz we use superdarn data to study polar cap ionospheric flow under strongly dominant positive interplanetary magnetic field imf bz component we show that the near noon flow along the magnetic meridian is predominantly sunward in summer the sunward velocity increase with intensification of the external driver the reverse convection electric field is also faster in summer and the rate of the increase is slightly larger for the southern hemisphere the sunward flows simultaneously detected in both hemispheres are faster in the summer hemisphere in addition while sunward flows are aligned with the midnight noon line in a winter hemisphere they are oriented toward earlier magnetic local hours in a summer hemisphere

δ18o and δ13c values of modern brachiopod shells abstract researchers have not rigorously tested the hypothesis that calcite from modern brachiopod shells is precipitated in oxygen isotope equilibrium with ambient seawater isotopic variability at the intraspecimen and intertaxon levels has not been examined without such data for modern brachiopods similar data from ancient brachiopods cannot be accurately interpreted in this study a survey is made of 18 o and 13 c values of terebratulid rhynchonellid thecideidine and craniacean brachiopods from antarctica the bay of fundy curacao japan new zealand norway puget sound palau sicily and south africa this suite of samples provides a wide range of taxonomic levels temperatures salinities and depositional environments for evaluating the degree of isotopic equilibrium attained during precipitation of brachiopod calcite new data indicate that modem brachiopod calcite is not always precipitated in oxygen and carbon isotope equilibrium with ambient seawater calcite from the primary layer and specialized shell structures hinge brachidium foramen interarea muscle scars are depleted in both 18 o and 13 c a characteristic of biological fractionation or vital effects often found in other calcerous marine organisms our findings suggest that these portions of the brachiopod shell should be avoided during sampling of ancient brachiopods secondary layer calcite the material most often analyzed in ancient brachiopods has higher 18 o and 13 c values which approach and sometimes correspond with predicted equilibrium values therefore secondary layer calcite is the most suitable portion of the brachiopod shell for use as an ancient seawater proxy although near equilibrium precipitation in secondary layer calcite is encouraging to those studying the isotopic composition of ancient oceans these data come with caveats large intraspecimen variability in the 18 o values of secondary layer calcite 1 in some samples limits the use of brachiopods as precise indicators of the 18 o value or temperature of ancient seawater the 18 o and 13 c values of secondary layer calcite deviate from predicted equilibrium values toward the lower values of the primary layer and specialized shell structures indicating a measurable vital effect in most cases measured 18 o values are lower than the calculated equilibrium values dictated by seasonal variations in the 18 o value and temperature of seawater ontogenetic variations in the 13 c values of secondary layer calcite have also been measured and must be assessed when interpreting ancient data modern brachiopods occur in a wide range of seawater salinities and temperatures therefore the relation between salinity and the 18 o value of seawater must be known to correctly calculate the temperature of calcite precipitation from secondary layer 18 o values errors of up to 15 c are found if ambient seawater is assumed to have a constant 18 o value e g 0 smow and the 18 o values of secondary layer calcite are used to calculate temperature coupled with vital effects and intraspecimen variability these errors can obscure the resolution of secular variations in the rock record

palaeoenvironmental significance of carbon and oxygen isotope stratigraphy of marine triassic jurassic boundary sections in sw britain abstract carbon isotope stratigraphy is a useful tool for stratigraphic correlation especially for strata deposited during major perturbations of the carbon cycle that affected the marine terrestrial and atmospheric reservoirs for the triassic jurassic boundary effectively defined by a first order mass extinction major fluctuations in carbon isotope values have been well documented but these datasets have generally been derived from bulk rock samples hence the extent to which features of the isotopic curve reflect diagenetic alteration or changing proportions of constituent materials is unconstrained here carbon and oxygen isotope data are presented from well preserved oyster shells liostrea comprising low magnesium calcite a mineral species relatively resistant to diagenetic alteration samples were obtained from lavernock point glamorgan wales a coastal section close to a candidate stratotype for the base of the jurassic at st audrie9s bay somerset england the carbon isotope signature from st audrie9s bay previously defined on the basis of analysis of bulk organic matter is confirmed by our new data major features are 1 the upper part of an initial9 negative isotope excursion in the lowest part of the section followed by 2 a pronounced positive excursion and 3 an extended main9 negative isotope excursion in the highest part of the section the data confirm that the carbon isotope stratigraphy previously documented from bulk organic matter in sw england records the chemical composition of the contemporaneous seawater bulk carbonates sampled over the same interval near lyme regis england show similar trends to those from oyster calcite in the lower part of the study section but there are more 13 c depleted values up section these lower values probably result from an admixture of primary and diagenetic carbonate palaeotemperatures calculated from oxygen isotope values from lavernock point oyster shells are relatively cool at the beginning of the positive carbon isotope excursion and increased by up to 10 c during the main negative carbon isotope excursion the new results are compatible with the view that positive carbon isotope excursions correspond to times of low atmospheric carbon dioxide content whereas negative carbon isotope excursions correspond to times of high atmospheric carbon dioxide content as is also found to be the case during the early jurassic toarcian oceanic anoxic event the mg ca and sr ca ratios and 18 o of investigated liostrea hisingeri show no correlation supporting data from modern bivalves that indicate that incorporation of mg and sr is controlled mainly by factors other than temperature

criteria for the retention of original seawater 87sr86sr in ancient shelf limestones abstract a suite of 40 pennsylvanian and permian shelf limestones from kansas and oklahoma has been analyzed for major and trace elements and for c o and sr isotopes the chemistry and c and o isotope ratios were compared to the difference between the measured 87 sr 86 sr and original seawater ratio concentrations of mn sr and fe varied most systematically with the change in the seawater sr ratio all samples containing 700 ppm mn yielded altered ratios fe and sr contents were also found to have a less well defined correlation with the retention of the original sr ratio all limestones with 900 ppm sr and 20 of 22 samples with a sr mn 2 0 yielded correct ratios the geochemical guidelines were applied to the chemical and isotopic results from 76 shelf limestones mostly of pennsylvanian age from texas and oklahoma more than 92 of the samples that contain sr mn 2 0 yielded correct seawater ratios every sample with sr mn 2 0 retained the original seawater ratio

signatures of the ionospheric cusp in digital ionosonde measurements of plasma drift above casey antarctica signatures of the ionospheric cusp in hf digital ionosonde measurements of plasma drift made at the polar cap station casey antarctica 80 8 geomagnetic latitude are investigated measurements recorded during the campaign interval february 13 17 1996 are considered in this case study because the summer dipole tilt effect and an interplanetary magnetic field imf northward condition on february 16 were favorable for the detection of the cusp at a higher than usual latitude on february 14 and 15 the magnitude of the imf was about 4 6 nt and the station probably passed just poleward of the cusp the most general signatures of the cusp were enhanced electric field and electric field turbulence shown by increased drift velocity and velocity scatter in the convection throat respectively broadband unstructured fluctuations in the geomagnetic field measured by magnetometers near to noon are well known to be a signature of cusp currents and were associated with the intervals of enhanced convection turbulence a major cusp event occurred above casey on february 16 when the magnitude of the imf increased from about 5 to 10 nt cusp signatures during this event included the drift velocity surging to large values just before and after an interval during which the f region echoes were lost because of an absence of f region ionization and the formation of electron density patches the loss of echoes was only partly explained by increased absorption and scatter of the transmitted radio waves although the spectral width of doppler peaks increased this by itself was not a unique signature of the cusp because the obliquity of echoes also controlled the spectral width in our near vertical interferometry however signatures of the cusp were easily recognized in digisonde data and the cusp s location and dynamics can be monitored using digisondes

comparison of an ionosonde drift model at a single station with polar convection patterns a study of convection at a single point in the polar cap is made by comparing vertical incidence ionosonde doppler drift data with the weimer 96 w96 convection model four years of data are used to construct a local convection model at casey antarctica geographic 66 3 s 110 5 w geomagnetic 80 8 s the casey 97 c97 model is capable of producing a drift velocity for any time and interplanetary magnetic field imf vector in the by bz plane for either summer or winter seasons the direct comparison of vectors predicted by c97 and w96 shows there is generally very good agreement between them at all imf angles because of this our findings consequently support the existence of multiple convection cells for imf angles within 30 of northward z for all other angles there is a distortion of the two cell pattern that becomes a minimum when the imf is strongly southward with slightly a positive by component drawing on an earlier study with the heppner maynard model smith et al 1998 but now using the c97 model for comparison we also find their patterns only begin to match the casey results when by bz a significant seasonal dependence also exists within the c97 and w96 models although the agreement with each other in winter is not as good the presence of additional summer lobe cells embedded within the merging cells is supported particularly for a rounder expanded dusk cell

super dual auroral radar network radar imaging of dayside high latitude convection under northward interplanetary magnetic field toward resolving the distorted two cell versus multicell controversy data from the kapuskasing and saskatoon radars of the evolving super dual auroral radar network superdarn hf radar network have been analyzed to study the two dimensional structure and dynamics of dayside high latitude ionospheric convection under northward interplanetary magnetic field imf conditions a period extending from 1600 to 2030 ut 0900 1330 mlt on january 10 1994 was examined during this interval magnetic field data were available from the imp 8 satellite and indicated moderately stable northward imf conditions for the first few hours of observation the by component of the imf was positive reasonably steady and approximately twice the magnitude of bz during this interval the high latitude convection images obtained with the superdarn radars were very similar to the distorted two cell convection maps for positive by as presented by heppner and maynard 1987 at 1840 ut a decrease in by in association with an increase in bz led to an extended period with by bz during this second interval the convection patterns were highly variable and even chaotic finally a sharp decrease in the by component at 1914 ut probably in association with a rotational discontinuity in the solar wind led to an extended period with by bz during this third interval the high latitude convection pattern was again stable and exhibited a single counterclockwise rotating vortex consistent with one of the polar cap merging cells proposed by dungey 1963 burke et al 1979 and reiff and heelis 1994 the transition to the counterclockwise rotating vortex occurred over a large spatial area within two radar scans 200 s and appeared to proceed via an equatorward incursion of the vortex into the previous convection configuration once this polar cap merging cell was formed it remained stable for the remainder of the period 40 min that by remained small sunward convection was also observed in the polar cap of the conjugate hemisphere at 1932 ut by the dmsp f11 satellite confirming that both magnetopause and internal reconnection sites were active at that time

geochemical and climatic effects of increased marine organic carbon burial at the cenomanian turonian boundary perhaps the most significant event in the cretaceous record of the carbon isotope composition of carbonate1 2 other than the 1 2 5 negative shift in the carbon isotope composition of calcareous plankton at the cretaceous tertiary boundary3 is the rapid global positive excursion of 2 13c enrichment which took place between 91 5 myr and 90 3 myr late cenomanian to earliest turonian c t boundary event 1 4 5 this excursion has been attributed to a change in the isotope composition of the marine total dissolved carbon tdc reservoir resulting from an increase in rate of burial of 13c depleted organic carbon which coincided with a major global rise in sea level5 during the so called c t oceanic anoxic event oae 6 here we present new data from nine localities which demonstrate that a positive excursion in the carbon isotope composition of organic carbon at or near the c t boundary7 8 is nearly synchronous with that for carbonate and is widespread throughout the tethys and atlantic basins fig 1 as well as in more high latitude epicontinental seas the postulated increase in the rate of burial of organic carbon may have had a significant effect on co2 and o2 concentrations in the oceans and atmosphere and consequent effects on global climate and sedimentary facies

an isotopic study of biogeochemical relationships between carbonates and organic carbon in the greenhorn formation carbon isotopic compositions of total carbonate inoceramid carbonate micritic carbonate secondary cements total organic carbon and geoporphyrins have been measured in 76 different beds within a 17 m interval of a core through the greenhorn formation an interbedded limestone and calcareous shale unit of cretaceous age cenomanian turonian from the western interior seaway of north america results are considered in terms of variations in the processes of primary production which led to the biosynthesis of the molecular precursors of the geoporphyrins and in secondary processes those mediating the transformation of primary organic material into sedimentary total organic carbon it is shown that the porphyrin isotopic record reflects primary isotopic variations more closely than the toc isotopic record and that in these sediments toc is enriched in 13c relative to its primary precursor by 0 6 to 2 8 this enrichment is attributed to isotope effects within the consumer foodweb and is associated with respiratory heterotrophy variations in this secondary enrichment are correlated with variations in the isotopic composition of marine carbonate this correlation is attributed to effects of environmental changes on the marine foodweb these may have included increased atmospheric oxygen associated with the cenomanian turonian oceanic anoxic event the isotopic fractionation associated with fixation of carbon by primary producers is observed to have varied by 1 5 during the interval of deposition it is suggested that this change is due to a variation in the makeup of the community of primary producers and or to a decrease in the atmospheric abundance of co2 during the oceanic anoxic event

δ13c and δ18o values of triassic brachiopods and carbonate rocks as proxies for coeval seawater and palaeotemperature abstract a dataset of 160 isotope 13c and 18o values from anisian ladinian carnian and rhaetian articulate brachiopod shells complemented by 158 carbon and oxygen isotope values from whole rock carbonates define the first continuous stable isotope baseline trends for the triassic seawater the carbon isotope data suggest the existence of several short term high amplitude excursions in the early triassic followed by a predominance of values around 0 5 1 during the middle triassic a rise to 3 5 during the carnian plateau at this level during the late carnian to early norian and a 1 5 decline in the middle norian to values around 2 during the late norian rhaetian interval the causation scenarios for these rapid oscillations are at present equivocal but may in part reflect a biological instability of the carbon cycle following the recovery from the end permian extinction event and or an input of mantle derived co2 from enhanced volcanic activity the 18o values from well preserved brachiopods from the tethyan realm range from 3 9 to 0 6 v pdb these values require open marine triassic seawater 18o values close to 0 v smow for the calculated temperatures to be within the range of tolerance of the coexisting reef building corals a 2 18o increase within the uppermost cordevolian and early julian suggests either a distinct temperature decline in the southern alps during that time interval an increase in seawater salinity or their combination oxygen isotope values for the muschelkalk brachiopods range between 6 2 and 2 0 and likely reflect a strong influx of meteoric waters into the germanic basin that shifted the oxygen isotopes to more negative values

δ18o and δ13c of permian brachiopods a record of seawater evolution and continental glaciation abstract two hundred sixty four 18 o and 13 c values of permian articulate brachiopod shells were analyzed and 81 of these values were characterized as well preserved and biostratigraphically well defined these were then utilized for construction of baseline oxygen and carbon isotope curves for the permian interval in addition 21 13 c whole rock values are reported for the wordian and capitanian the early permian asselian to artinskian times are characterized by 2 5 decrease in oxygen isotope values from 0 7 to 3 3 v pdb this is attributed to a 4 7 c increase in temperature in the southern urals concomitant with the retreat of the permo carboniferous ice sheets and return of the 18 o depleted melt water into the oceans the late permian samples from iran jolfa at kuh e ali bashi and china meishan yield 18 o values and presumably temperatures similar to those that followed the termination of the large scale glaciation in the lower permian in between the upper kungurian to capitanian samples from the delaware basin guadalupe mountains are enriched in 18 o at 1 5 to 3 we have no definitive explanation for these data but tentatively suggest that the anomaly can potentially be a result of evaporative enrichment of seawater in 18 o due to intracratonic arid setting of the basin the 18 o enriched nature of the zechstein samples 1 2 to 2 5 on the other hand is in all probability a reflection of the high evaporation rates in the zechstein sea the permian interval is characterized by a relatively constant 13 c at about 4 the exceptions are again the brachiopods from the delaware basin guadalupe mountains which show 1 6 increase in the guadalupian to values of up to 5 9 in the wordian a tentative explanation as in the case of oxygen is based on the proposition that the semi enclosed delaware basin was likely stratified with sequestration of the 13 c depleted carbon to the deeper water layers and a complementary 13 c enrichment in the upper oxygenated layer the coeval open ocean water dic may have been similar to that of the remainder of the permian interval at 4 as indicated by whole rock carbonate samples from oman sicily and iran in the latest permian the trend mimics the well known 13 c drop at the permian triassic boundary

stability of atmospheric co2 levels across the triassic jurassic boundary the triassic jurassic boundary 208 million years ago is associated with widespread extinctions in both the marine and terrestrial biota the cause of these extinctions has been widely attributed to the eruption of flood basalts of the central atlantic magmatic province this volcanic event is thought to have released significant amounts of co2 into the atmosphere which could have led to catastrophic greenhouse warming but the evidence for co2 induced extinction remains equivocal here we present the carbon isotope compositions of pedogenic calcite from palaeosol formations spanning a 20 myr period across the triassic jurassic boundary using a standard diffusion model we interpret these isotopic data to represent a rise in atmospheric co2 concentrations of about 250 p p m across the boundary as compared with previous estimates of a 2 000 4 000 p p m increase the relative stability of atmospheric co2 across this boundary suggests that environmental degradation and extinctions during the early jurassic were not caused by volcanic outgassing of co2 other volcanic effects such as the release of atmospheric aerosols or tectonically driven sea level change may have been responsible for this event

deciphering kinetic metabolic and environmental controls on stable isotope fractionations between seawater and the shell of terebratalia transversa brachiopoda this study presents carbon and oxygen stable isotope data obtained from high resolution sampling 500 m of the brachiopod terebratalia transversa from the san juan islands washington state usa the aim of this study is to unravel the respective effects of physiological and environmental controls on the stable isotopic composition of brachiopod shell calcite based on an sem study of the shell a three dimensional sampling was performed in order to investigate carbon and oxygen isotope variations along isochrons and ontogenetic transects the primary as well as the outer part of the secondary shell layer display large and variable isotopic offsets as high as 7 for 13c and 6 for 18o relative to expected equilibrium values the significant positive correlations between 18o and 13c values indicate that the isotopic compositions of these shell domains are mainly controlled by kinetic isotope fractionation effects the ontogenetic 13c and 18o variations can be used to establish a growth curve of the studied specimen whose age is estimated close to 8 years the extent of apparent o 18 disequilibrium observed in this study is about as large as any yet measured in any organism which makes this brachiopod an interesting and important case the carbon and oxygen isotope compositions increase from the outer towards the inner part of the secondary layer where they reach values of 0 0 8 and 0 8 to 0 2 respectively corresponding calculated temperatures are between 12 and 13 8 c and fall in the range of seawater temperatures recorded off the san juan islands 7 13 c these results suggest that the calcite fibres were secreted more and more slowly during the thickening of the shell throughout the animal s life finally approximating isotopic equilibrium this study illustrates that the stable isotopic composition of the modern brachiopod t transversa is predominantly influenced by kinetic fractionation and to a minor degree by metabolic effects except for the innermost part of the shell the measured isotope ratios do not reflect environmental conditions during shell precipitation it remains to be tested by comparable studies whether other modern brachiopod species or even fossil brachiopods reflect comparable fractionation effects these results point out the need for re examination of paleozoic oceanographic conditions

glacial interglacial surface water variations in the caribbean sea during the last 300 ky based on calcareous nannofossil analysis abstract quantitative and qualitative analyses of nannofossil assemblages from odp site 999 in the caribbean sea were performed to reveal changes in upper seawater structures and implications for climatic changes during the last 300 000 years overall there is an inverse relationship in stratigraphic fluctuations between the absolute abundance aa of a lower photic taxon florisphaera profunda and 18 o this fluctuation pattern is considered to represent climatic changes in the subtropical under water suw in the caribbean cross spectral analysis demonstrates that the suw was characterized by nutrient rich water during interglacials changes in orbitally driven gyre circulation system strongly affected fluctuations in the suw in the caribbean conversely the surface water was enriched in nutrients during glacials as the result of increased terrigenous materials and shallowing of the thermocline based on changes of the n ratio abundance of reticulofenestrids relative to f profunda and results of cross spectral analysis between them and 18 o the relationship between the thermocline variation and the ice volume fluctuation suggests that changes in the subtropical gyre circulation system in the north atlantic have had stronger influences on the caribbean thermocline variations and there may be a little contribution of climatic fluctuations that originated in the southern ocean to the caribbean sea a large floral turnover in nannofossil assemblages between oxygen isotope stages 6 and 5 was observed in hole 999a which corresponds to termination ii on the basis of floral compositions photosynthetic primary productions had been more active than today before termination ii

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

triassic jurassic organic carbon isotope stratigraphy of key sections in the western tethys realm austria the late triassic period is recognized as one of the five major mass extinctions in the fossil record all these important intervals in earth history are associated with excursions in c isotope records thought to have been caused by perturbations in the global carbon cycle the nature and causes of c isotopic events across the triassic jurassic t j transition however are poorly understood we present several new high resolution organic c isotope records from the eiberg basin austria including the proposed global boundary stratotype section and point gssp for the base of the jurassic the triassic jurassic boundary interval in these records is characterized by the initial and main negative organic carbon isotope excursions cie of up to 8 the initial and main cies are biostratigraphically constrained by first and last occurrences of boundary defining macro and microfossils e g ammonites high resolution c isotope records appear to be an excellent correlation proxy for this period in the eiberg basin pyrolysis analysis demonstrates increased hydrogen index hi values for organic matter coinciding with the initial cie terrestrial organic matter influx and mass occurrences of green algae remains may have influenced the c isotope composition of the sedimentary organic matter this may have contributed to the extreme amplitude of the initial cie in the eiberg basin

oxygen isotope fluctuations in a modern north sea oyster crassostrea gigas compared with annual variations in seawater temperature implications for palaeoclimate studies abstract a total of 181 oxygen isotope values from sequential samples of the left shell of a modern pacific oyster crassostrea gigas that lived on a sub tidal oyster bank in the list basin north sea germany shows periodically varying values between 1 3 and 2 5 in order to test whether these 18 o fluctuations reflect seawater temperature changes the isotope values of the shell were compared to actual seawater temperature variations from the region c gigas serves as an excellent proxy for temperature of palaeoseawater and the results show that the examined oyster precipitated its shell in 18 o equilibrium with the ambient seawater a cessation of the oyster shell calcification starts at water temperatures below 6 c at lower temperatures than previously thought for crassostrea for palaeoclimate investigations the termination of shell production is important because the lowest temperatures might not be represented in the shell higher water temperatures are therefore potentially over represented in time spans when the shell grows faster and mean oxygen isotope values may potentially bias the mean seawater temperatures of annual variations towards higher temperatures

geochemistry of brachiopods oxygen and carbon isotopic records of paleozoic oceans abstract combined trace element and isotope studies of 319 brachiopods covering the ordovician to permian time span show that 13c and 18o in well preserved specimens varied during the paleozoic the overall 13c secular trend is in accord with the previously published observations but its details are obscured by vital isotopic fractionation effects at generic level nonetheless the results suggest that the negative correlation between marine 13ccarbonate and 34ssulphate deteriorates at time scales of 106 years due to the long residence time and thus slow response of so42 in the ocean for oxygen isotopes all devonian and older specimens have 18o of 4 while the well preserved permian samples have near present day 18o of about 1 pdb this isotopic dichotomy is probably not due to post depositional phenomena salinity or biogenic fractionation effects this leaves open the perennial arguments for a change in 18 o 16 o of sea water versus warmer ancient oceans the present data are difficult to explain solely by the temperature alternative the coincidence of the proposed shift in 18o with the large late paleozoic changes in marine 87 sr 86 sr 13 c 12 c 34 s 32 s and sea level stands argues for a tectonic cause and for a change in 18 o 16 o of sea water although such explanation is difficult to reconcile with global balance considerations and with isotopic patterns observed in alteration products of ancient basalts and ophiolites whatever the precise cause or combination of causes the implications for tectonism and or paleoclimatology are of first order significance

oxygen isotope values from high latitudes clues for permian sea surface temperature gradients and late palaeozoic deglaciation abstract the permian was a period of waning large scale continental glaciations in the southern hemisphere the waning of these ice sheets during the early permian led to discharge of 18o depleted ice melt water into the oceans this coupled with rising seawater temperatures resulted in a concomitant decline of about 2 5 in the 18o of seawater as recorded by brachiopod shells from low latitude

within shell variations in carbon and oxygen isotope compositions of two modern brachiopods from a subtropical shelf environment off amami o shima southwestern japan 1 this study examines the fidelity of carbon 13c and oxygen 18o isotope compositions of two modern brachiopod species kikaithyris hanzawai and basiliola lucida to use as proxies of 13c values of total dissolved inorganic carbon and temperature and or 18o values of seawater respectively well preserved shells of k hanzawai and living individuals of b lucida were collected from a subtropical shelf environment off amami o shima southwestern japan some portions of the shells are in isotopic equilibrium with the ambient seawater while other portions are not the degree of disequilibrium differs between the two species and between different portions of each shell statistically significant positive correlations are recognized between the 13c and 18o values of these samples which can be ascribed to a kinetic fractionation effect far from the posterior shell edge and along the axis of shell growth the 18o values of the secondary shell layer of k hanzawai mostly fall within the expected range of equilibrium calcite the 13c values from the inner surface of the secondary shell layer in both species are relatively constant and are equivalent to or very close to equilibrium calcite therefore these portions of the shells are most suitable for collecting reliable environmental proxy data although the 13c and 18o values of modern brachiopod shells are predominantly controlled by a kinetic fractionation effect the appropriate selection of species and shell portions that reflect the isotopic composition of ambient seawater can facilitate the reconstruction of secular variations in oceanic 13c or 18o values

palaeoenvironmental changes in the late triassic rhaetian of the northern calcareous alps clues from stable isotopes and microfossils abstract the triassic kossen formation rhaetian in the northern calcareous alps has been studied with respect to oxygen and carbon isotope ratios of brachiopod shells and whole rocks together with the stratigraphic range and ecology of ostracod assemblages in order to detect palaeotemperature changes in the kossen formation and determine their significance for environmental and climatic conditions in the rhaetian sea of the eiberg basin northern calcareous alps for this purpose 60 18 o measurements on 43 articulate brachiopods samples from 8 different horizons were carried out the results indicate a progressive decrease of 2 5 c in seawater bottom temperature in the middle late rhaetian the carbonate microfacies as well as macro and microfossil assemblages indicate a gradual increase of water depth in the eiberg basin and thereby explain the observed bottom water cooling the oxygen isotope data further suggest that the palaeobathymetric temperature gradient in the eiberg basin was rather low indicating that almost no deep water exchange with tethys existed at that time carbon isotopes from brachiopod shells and from bulk carbonates show a trend to heavier values from the hochalm member up to the lowest unit 3 of the eiberg member that parallels oxygen isotopes this positive 13 c trend is interrupted by a sudden 1 5 negative excursion in the late rhaetian late rhaetian event a time span when the oxygen isotopes remain heavy

electric potential patterns in the northern and southern polar regions parameterized by the interplanetary magnetic field electric potential patterns have been obtained from the izmiran electrodynamic model izmem for the northern and southern polar regions during summer winter and equinox the model is derived from a large quantity of high latitude ground based geomagnetic data above 4 57 o corrected geomagnetic latitude at all magnetic local time hours a linear regression analysis technique has been used to obtain the quantitative response of each magnetic observatory to changes of interplanetary magnetic field imf components since no ionospheric conductivity model exists specifically for the southern polar region the statistical model of wallis and budzinski 1981 has been applied in both hemispheres a cross polar background potential of 35 kv derived by reiff et al 1981 is used to calibrate izmem s potential patterns the model s responses to changes in the imf by and bcomponents are analyzed to obtain a set of elementary convection patterns in both polar regions for each season of the year asymmetry in the potential pattern geometry in both hemispheres can be attributed either to the influence of the northern ionospheric conductivity model which was applied to the southern polar region or to some natural phenomena the modeled background cross polar potential for the condition when b by 0 is found to be 37 kv average values of the modeled potential drop caused by each nanotesla of the imf are the following 14 kv for southward b 4 kv for northward b and 4 4 5 kv for by components the latter is not applicable to the dawn dusk potential drop it may be applied across the cusp region only nevertheless a combination of the background and elementary potential patterns in the case studies gives a certain estimation of the cross polar potential drop which may be strongly distorted during time of large by it is concluded that izmem provides realistic convection patterns parameterized by the imf component directions and magnitudes and may be used to provide routine estimates of convection patterns and electric potentials if imf data are available

shell structure patterns and trends of oxygen and carbon stable isotopes in modern brachiopod shells this study investigates 13c and 18o variations in the shells of modern brachiopods representing all extant groups of calcite precipitating brachiopods collected live from 8 locations sem examinations determined the ultrastructural characteristics of each species prior to isotope analyses 13c and 18o analyses of shell carbonate were carried out with samples representing disparate morphological features and ultrastructural shell layers of both ventral and dorsal valves generally 18o values from the fibrous secondary or prismatic tertiary shell layers of the articulated terebratulida and rhynchonellida species were in oxygen isotopic equilibrium with ambient seawater isotopic temperatures extrapolated from these values are close to measured annual mean seawater temperatures 18o values were relatively unaffected by shell specialisation the only exception was antarctic species liothyrella uva which did not have a complete tertiary shell layer typical of this genus and had 18o values of the innermost layer strongly correlated with 13c and mostly not in oxygen isotopic equilibrium with ambient seawater with the exception of the rhynchonellid notosaria nigricans the outer primary layer material was depleted in 13c and 18o and highly variable inclusion of this material even as part of a whole shell sample could lead to misinterpretation of seawater temperature therefore only fossil secondary layer material should be used the anomalous articulated thecideidine brachiopod thecidellina barretti is composed of mainly primary shell material and was not in oxygen isotope equilibrium 18o values from the laminar secondary layer material of the inarticulated craniida are highly variable therefore these species are not recommended for use as palaeoenvironmental proxies this study suggests caution when employing fossil brachiopod shells with similar ultrastructures to modern craniid and thecideidine brachiopods the carbon isotope composition is highly variable in all of the brachiopods studied analysis of samples from specialised regions of the secondary shell layer show a pattern of depletion in 13c relative to non specialised secondary material the carbon isotope variability is independent of 18o and is repeated in most of the articulated species regardless of geographical location this is possibly a vital effect produced by metabolic prioritisation

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

empirical high latitude electric field models electric field measurements from the dynamics explorer 2 satellite have been analyzed to extend the empirical models previously developed from dawn dusk ogo 6 measurements j p heppner 1977 the analysis embraces large quantities of data from polar crossings entering and exiting the high latitudes in all magnetic local time zones paralleling the previous analysis the modeling is based on the distinctly different polar cap and dayside convective patterns that occur as a function of the sign of the y component of the interplanetary magnetic field the objective which is to represent the typical distributions of convective electric fields with a minimum number of characteristic patterns is met by deriving one pattern model bc for the northern hemisphere with a y interplanetary magnetic field imf and southern hemisphere with a y imf and two patterns models a and de for the northern hemisphere with a y imf and southern hemisphere with a y imf the most significant large scale revisions of the ogo 6 models are 1 on the dayside where the latitudinal overlap of morning and evening convection cells reverses with the sign of the imf y component 2 on the nightside where a westward flow region poleward from the harang discontinuity appears under model bc conditions and 3 magnetic local time shifts in the positions of the convection cell foci the modeling above was followed by a detailed examination of cases where the imf z component was clearly positive northward neglecting the seasonally dependent cases where irregularities obscure pattern recognition the observations range from reasonable agreement with the new bc and de models to cases where different characteristics appeared primarily at dayside high latitudes the analysis of these differences for strong bz conditions using passes through different mlt zones led to finding that the deviations from both the bc and de models could be modeled by extending the evening cell foci toward and beyond noon with a deformation that roughly resembles a rotational twist of the extended evening cell the morning cell accommodation to the model bc deformation produces a narrow tongue of eastward convection near 75 invariant latitude on the dayside the model de deformation compresses the morning cell toward the night hours such that the sunward convection in the morning cell is diverted to polar latitudes in the 0400 to 0700 mlt sector in both cases the deformations of the two cell patterns lead to sunward convection in dayside polar regions while maintaining the integrity of the nightside convection pattern thus the nightside dilemmas that plague three and four cell models designed to explain sunward convection in polar regions under bz conditions do not appear

strontium isotope evolution of late permian and triassic seawater the 87sr 86sr values based on brachiopods and conodonts define a nearly continuous record for the late permian and triassic intervals minor gaps in measurements exist only for the uppermost brahmanian lower part of the upper olenekian and middle norian and only sparse data are available for the late permian these 219 measurements include 67 brachiopods and 114 conodont samples from the tethyan realm as well as 37 brachiopods and one conodont sample from the mid european middle triassic muschelkalk sea the late permian lower triassic interval is characterized by a steep 1 3 10 3 rise from 0 7070 at the base of the dzhulfian to 0 7082 in the late olenekian a rate of change comparable to that in the cenozoic in the mid triassic anisian and ladinian the isotope values fall to 0 7075 followed again by a rise to 0 7081 in the middle late norian the 87sr 86sr values decline again in the late norian sevatian and rhaetian to 0 7076 the sharp rise in the 87sr 86sr values during the late permian early triassic was coincident with widespread clastic sedimentation because of the paucity of tectonic uplifts the enhanced erosion may have been due to intermittent humid phases during mainly an arid interval coupled with the absence of a dense protective land plant cover following the mass extinction during the latest permian the apex of the 87sr 86sr curve at the olenekian anisian boundary coincides with cessation of the large scale clastic sedimentation and also marks the final recovery of land vegetation as indicated by the renewed onset of coal formation in the middle triassic the rising 87sr 86sr values from the middle carnian to the late norian coincide with the uplift and erosion of the cimmeride indosinian orogens marking the closure of the palaeotethys the subsequent rhaetian decline that continues into jurassic pliensbachian toarcian boundary on the other hand coincides with the opening of the vardar ocean and its eastern continuation in the izmir ankara ophiolitic belt samples from the upper muschelkalk are more radiogenic than the global trend this may reflect separation of the basin from the open ocean due to strong meteoric influx from a large land mass in the north the germanic basin became increasing brackish up section in the north and east but because of the high evaporation rates the salt content was not much reduced in the southern and central basin where a rich but increasingly endemic marine fauna survived