heterogeneity in geochemical expression of subaerial exposure in limestones and its implications for sampling to detect exposure surfaces abstract replicate sampling along stratigraphic horizons in one outcrop of ordovician limestones of the nashville dome reveals considerable variation in 13c 18o and sr concentration among seemingly identical samples ten vertical transects were sampled each consisting of samples taken at 10 cm intervals from 2 m below to 1 m above the m4 m5 sequence boundary of holland and patzkowsky 1997 1998 the 10 transects were each 2 m apart the resulting 10 replicate and thus seemingly interchangeable samples from each horizon have ranges of 13c as great as 2 4 and ranges of 18o as great as 2 8 and sr concentrations that vary by as much as a factor of four variance is great both just below and just above surfaces of subaerial exposure as a result maxima in variance in 13c as well as or in some cases instead of minima in mean 13c can be used to test such surfaces for exposure maxima in variance and minima in mean 13c may be found either below a surface of subaerial exposure in the meteorically altered sediment or above that surface in the transgressive lag where reworked pre exposure sediment is mixed with unaltered post exposure sediment the large variances and ranges found in this study and the dissimilarity of geochemical profiles from the 10 vertical transects indicate that single samples of stratigraphic horizons and thus single vertical transects can yield misleading characterizations of stratigraphic variation of geochemical parameters instead replicate sampling along horizons better characterizes isotopic and elemental variation through stratigraphic sections than does existing single sample strategies and it will be more useful in future testing of surfaces of subaerial exposure for example the replicate geochemical data used in this project confirm the presence of three surfaces of subaerial exposure whereas only two had been recognized as exposure surfaces before data from replicate samples also make unnecessary the use of arbitrary criteria for inferring exposure such as the predetermined numerical cutoffs used by railsback et al 2003

hypersaline burial diagenesis delineated by component isotopic analysis late paleozoic limestones west texas abstract pennsylvanian to lower permian cyclic platform carbonates from the southwest andrews area of west texas are now 3 km below the land surface isolated samples of brachiopod shell zones of calcite cement and saddle dolomite from limestone cores of the southwest andrews oilfields have been analyzed isotopically to determine if 1 diagenesis is dominated by early meteoric processes as suggested by bulk rock isotopes 2 whether significant burial diagenesis occurred in hypersaline waters and 3 whether most burial cements are allochthonous or internally derived bulk isotopic analyses of limestones show systematic variation in 13c and 18o according to length of subaerial exposure and stratigraphic position chemically zoned calcite cements show four growth stages stage 1 and 2 cements have average 13c values of 4 6o oo and average 18o of 5 0o oo and most are interpreted to have precipitated during subaerial exposure in meteoric water a few stage 1 cements with average 13c values of 4 7o oo and 18o of 1 4o oo apparently precipitated below the sediment surface from modified seawater in a sulfate reduction zone stage 3 and 4 calcite cements precipitated after compaction and compose up to 25 of the total rock volume they have 13c values 0 2o oo to 4 4o oo that vary according to stratigraphic position and are similar to bulk rock samples of the host limestone the 13c values indicate local derivation by dissolution of adjacent rock the 18o values of stage 3 and 4 cement 5 5o oo to 3 5o oo are indistinguishable from stages 1 and 2 cements precipitated from meteoric water the decrease in 18o expected from elevated temperatures during burial present formation temperature of 55 c is negated by precipitation from a late permian brine with 18o values of 5o oo stages 3 and 4 calcite and saddle dolomite cements have 87sr 86sr values that are lower than 87sr 86sr values for contemporaneous marine carbonate 0 70830 this is probably due to incorporation of sr with low 87sr 86sr values 0 70672 to 0 70682 also from the late permian hypersaline brine the southwest andrews area has several diagenetic features that are probably more common in ancient limestone than previously recognized 1 burial cements have 18o values similar to meteoric cements when precipitated from hypersaline waters with 18o values 5o oo greater than contemporary seawater 2 deep burial cements have 13c values similar to the bulk rock because carbon is derived from pressure dissolution of nearby carbonate 3 bulk rock isotope values are characteristic of marine limestones subjected to meteoric diagenesis even though they have suffered substantial deep burial diagenesis this study clearly illustrates the value of analyzing diagenetic components to accurately understand porosity evolution in limestones

giant ooids in a neoproterozoic carbonate shelf simla group lesser himalaya india an analogue related to neoproterozoic glacial deposits abstract this study is the first comprehensive documentation of giant ooids and related facies associations in the kunihar formation proterozoic simla group lesser himalaya himachal pradesh analysing the understanding of processes and controls on development of giant ooids based on field observations supplemented by outcrop based facies analysis petrography and delineation of environmental variations four facies associations have been delineated i peritidal siliciclastic carbonate fa1 ii shelf lagoon fa2 iii reef complex fa3 iv fore reef slope fa4 deposition of giant ooids and associated facies associations of kunihar formation occurred in a carbonate rimmed shelf with high tidal influence size of giant ooids from kunihar formation is the largest as compared to giant ooids from other geological formations kunihar giant ooids developed when normal ooids were washed from ooid shoals intertidal into slightly deeper regions shallow subtidal resulting in the increased dimension of ooids in suspension due to higher hydrodynamics scanning electron microscopy sem studies support microbial origin of ooids giant ooids and stromatolites of kunihar formation increased microbial activity in kunihar formation is attributed to increase in nutrients by virtue of weathering of underlying darla volcanics abundant carbonate and microbially induced sedimentary structures miss deposition in lower part of simla group points to increased microbial activity which likely increased the volume of oxygen in neoproterozoic atmosphere ushering in ice house conditions during the subsequent deposition of blaini group giant ooids associated with neoproterozoic glacial deposits throughout the world occupy stratigraphic positions below above or between glaciations simla group is another example where giant ooids lie stratigraphically below marinoan blaini tillites increased magmatic activity and weathering before and during the neoproterozoic glaciations increased nutrients in marine waters which increased algal growth thus giant ooids were deposited due to such phases of increased microbial activity before after glaciation and during interglacial periods

temperature limits for preservation of primary calcite clumped isotope paleotemperatures abstract solid state reordering of c o bonds in the calcite lattice can alter the clumped isotope composition of paleotemperature archives such as fossil brachiopod shells without inducing significant changes in shell microstructure and trace element concentrations metrics commonly used to gauge preservation quality to correctly interpret the paleoenvironmental significance of clumped isotope derived paleotemperatures it is necessary to understand the temperature time domain in which solid state c o bond reordering is important we address this question using a combination of laboratory and natural geological experiments on paleozoic brachiopod shells the laboratory experiments involve heating fossil brachiopod calcite at different temperatures and times to directly observe rates of 13 c 18 o bond reordering the resulting arrhenius parameters are indistinguishable from values previously determined for an optical calcite with similar trace element compositions we develop an alternative kinetic model for reordering that accounts for non first order reaction progress observed during the initial several hundred minutes of laboratory heating experiments and show that the simplified first order approximation model passey and henkes 2012 predicts reaction progress equally well for temperatures and timescales relevant to sedimentary basins we evaluate our laboratory based rate predictions by studying brachiopod calcite from several sedimentary basins with independently constrained burial temperature histories specifically we use the laboratory derived arrhenius parameters to predict the evolution of brachiopod calcite clumped isotope compositions during successive one million year time steps reflecting the burial and exhumation temperature paths of each basin while this exercise is limited by the relatively large uncertainties in the temperature histories of these basins we find general correspondence within error between predicted and observed clumped isotope values we present simplified temperature time diagrams for calcite showing domains where primary clumped isotope compositions will be preserved partially reordered and fully reordered in conclusion calcite samples dwelling at 100 c or lower for 10 6 10 8 year timescales should not be affected by solid state c o bond reordering

comparing oxygen isotope records of silurian calcite and phosphate δ18o compositions of brachiopods and conodonts oxygen isotope compositions of silurian calcitic brachiopod shells from gotland are compared with those of phosphatic inarticulate brachiopod shells and conodonts from identical stratigraphic horizons the 18ophosphate values of inarticulate brachiopod shells vary widely 13 0 to 17 5 v smow and are significantly lower than those of coeval conodont samples 17 5 to 19 5 v smow conodont samples from individual beds have homogenous 18ophosphate values with variations close to the analytical precision 0 2 together with a significant correlation of conodont 18ophosphate and articulate brachiopod shell 18ocarbonate values r 0 85 this observation suggests that primary marine oxygen isotope compositions are better preserved in conodont apatite than in inarticulate brachiopod shell apatite however with decreasing 18ocarbonate values the 18ophosphate values of conodonts are higher than expected for theoretical isotopic equilibrium with brachiopod calcite different habitats of conodonts and brachiopods cannot account for the observed offset in 18o because both organisms were thriving on a broad shallow marine carbonate platform whereas 18ophosphate values of silurian conodonts are close to 18ophosphate values of modern and mesozoic tropical biogenic apatites most silurian calcitic brachiopod shells from gotland are depleted in 18o when compared to modern or mesozoic brachiopods we therefore suggest that silurian conodonts from gotland record paleotemperature and 18o of silurian sea water more faithfully than coeval calcitic brachiopod shells

weathering by wetting and drying some experimental results a series of experiments on sandstone and dolerite was undertaken in an attempt to better understand the wetting and drying weathering process as rock samples are frequently subjected to wet dry cycles within the simulation of other weathering mechanisms e g freeze thaw three common methods of moisture application were used and the influences of these evaluated it was found that the method of moisture application could affect the nature of the weathering products resulting from wetting and drying it was also observed that there were changes in the internal properties of the rock e g porosity microporosity and that these could influence the synergistic operation of other weathering processes although not all of the observations could be explained it is apparent that wetting and drying has both a direct and an indirect effect on the weathering of rock that has not been taken into account in simulations greater cognizance needs to be given to the role of this process both in the field and in laboratory simulations

δ 18 o of carbonate quartz and phosphate from belemnite guards implications for the isotopic record of old fossils and the isotopic composition of ancient seawater belemnite guards of cretaceous and jurassic age were found to contain varying amounts of quartz deposited both on the external surface and inside the rostrum the oxygen isotopic composition of coexisting carbonate quartz and phosphate from the same rostrum was measured according to well established techniques none of these compounds showed isotopic values in equilibrium with one another assuming 18o values of the diagenetic water within the range of meteoric waters the 18o sio2 yield temperatures in agreement with the apparent secondary origin of this phase the 18o co32 range with a certain continuity between 10 8 and 0 97 pdb 1 with most of the intermediate values being within the range of the carbonate isotopic values of mesozoic fossils the most positive isotopic results obtained from phosphate are close to 23 24 v smow they can hardly be related to a secondary origin of the phosphate or to the presence of diagenetic effects since these results are among the most positive ever measured on phosphate as far as we know there is no widespread diagenetic process determining an 18o enrichment of phosphate the very low concentration of phosphate did not allow the determination of its mineralogical composition all the available 18o po43 values from belemnite and non belemnite fossils of marine origin of tertiary and mesozoic age are reported along with the newly measured belemnites the following conclusions may be drawn from the data reported 1 the pristine oxygen isotope composition of fossil marine organisms either carbonate or phosphate may easily undergo fairly large changes because of oxygen isotope exchange processes with diagenetic water this process is apparent even in the case of geologically recent fossils 2 the 18o po43 of belemnite rostra seems to be at least in the case of the most positive results in isotopic equilibrium with environmental water because of the similarity between the results from cretaceous belemnites and the results from cretaceous and lower tertiary pelecypods and fish teeth 3 if so the only feasible interpretation that can be suggested for the 18o enriched data is the possibility of a relatively large variation of the oxygen isotopic composition of ocean paleowater from jurassic to recent time

δ18o values 87sr86sr and sr mg ratios of late devonian abiotic marine calcite implications for the composition of ancient seawater abstract late devonian frasnian abiotic marine calcite has been microsampled and analyzed for 87 sr 86 sr ratios 18o and 13c values and minor element concentrations portions of marine cement crystals from the alberta and canning basins have escaped diagenetic alteration and preserve original marine 18o values 4 8 0 5 pdb 13c values 2 0 to 3 0 pdb 87 sr 86 sr ratios 0 70805 3 and sr mg weight ratios 0 04 to 0 05 marine 87 sr 86 sr ratios are globally consistent and can be correlated within the alberta basin and among the alberta canning and williston basins correlation of isotopic and chemical data strengthen the conclusion that marine cements from the leduc formation preserve original marine 18o values which are 3 to 4 lower than those of modern marine cements these low 18o values are best explained by precipitation from 18o depleted seawater and not by elevated seawater temperature or diagenetic alteration for comparison with devonian data analogous data were collected from holocene mg calcite and aragonite marine cements from enewetak atoll marshall islands mg calcite and aragonite marine cements are in isotopic equilibrium with ambient seawater and mg calcite cements are homogeneous with respect to sr and mg contents empirically derived homogeneous distribution coefficients for mg and sr in modern abiotic mg calcite from enewetak atoll are 0 034 and 0 15 respectively an equation describing the dependence of dsr on mg content was based on a compilation of sr and mg data from holocene abiotic marine calcite dsr 3 52 10 6 ppm mg 6 20 10 3 unlike that derived from experimental data this sr mg relation is consistent over a range of 4 to 20 mol mgco3 and may represent precipitation phenomena which are minimally controlled by kinetic effects comparison of sr and mg contents of analogous devonian and holocene marine cements suggests that the mg ca ratio of late devonian seawater was significantly lower and that the sr ca ratio was significantly higher than that of modern seawater

oxygen and carbon isotopic composition of ordovician brachiopods implications for coeval seawater one hundred and two articulate brachiopods and twenty one marine cements from ordovician strata were analyzed for oxygen and carbon isotopes these together with 200 previously published analyses of ordovician brachiopods mostly of upper ordovician age define a general trend of increasing 18o values with decreasing age from a minimum of 10 5 in the earliest ordovician to a maximum of 1 5 at the ordovician silurian boundary the 13c values show a general positive correlation with 18o and increase from 2 5 in the earliest ordovician to as high as 7 at the ordovician silurian boundary the magnitudes of ordovician excursions for both 18o and 13c are similar to those reported previously for the entire phanerozoic different genera of brachiopods from the same stratigraphie level did not yield any large differences in their isotopic compositions suggesting that vital effects did not influence decisively the isotope values of the studied skeletal parts petrographic examination and trace element analyses indicate that brachiopod shells from llandeilo to ashgill strata are well preserved and thus may have retained their primary isotopic signatures the same criteria for the tremadoc arenig and some llanvirn brachiopods suggest partial diagenetic alteration of 18o values in these samples but this does not preclude their utility as a recorder of a primary 13c signal the observed 18o trend for the well preserved portion of the record may reflect progressive cooling during ordovician perhaps complemented by a changing 18o values of seawater the latter could have been a consequence of a net increase in the rate of high temperature water rock interaction relative to its low temperature counterpart as indicated also by the large coeval decrease of marine 87sr86sr superimposed on the above long term 18o trend is a positive excursion of about 2 at the end of the ordovician probably reflecting a large expansion of polar ice caps the magnitude of this excursion indicates a degree of glaciation comparable to that at the height of the quaternary glacial episodes the secular 13c trend may reflect a progressive increase in marine organic productivity and or enhanced organic deposition in the ordovician oceans particularly noticeable at the time of the terminal ordovician glaciation

evolutionary dynamics of pragian dacryoconarida lower devonian tentaculitoidea evidence from palaeontological data and 13c of marine carbonates from czech republic abstract a detailed palaeontological study was performed on 38 successive assemblages of dacryoconarids tentaculitoidea sampled along the cerna rokle section praha basin czech republic this section represents the whole pragian stage lower devonian and roughly corresponds to a 6 ma time span dacryoconarid assemblages change in abundance and diversity oxygen and carbon isotope compositions of the dacryoconarid bearing carbonates were measured to evaluate the possible influence and control of the environmental conditions on the evolutionary dynamics of the dacryoconarid faunas large and rapid 13c excursions occur in the marine carbonate sequence of the cerna rokle section some of which apparently correspond with faunal variations however no clear and simple relationship appears between carbon isotope ratio fluctuations and dacryoconarid evolutionary dynamics occurrence and extinction of the dacryoconarid species appear to be mainly related to biotic factors environmental variations cannot be regarded as having preponderantly influenced the dacryoconarid evolution which more probably resulted from intrinsic biotic factors

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

geochemistry of cambro ordovician arbuckle limestone oklahoma implications for diagenetic δ18o alteration and secular δ13c and variation abstract isotopic analyses of 227 limestone samples from the cambro ordovician arbuckle group oklahoma document slow secular changes in the chemistry of the limestones from late cambrian to early ordovician the 18o values of the limestones increase from 10 to 7 pdb 13c values decrease from 0 to 2 pdb and 87 sr 86 sr ratios decrease from 0 7091 to 0 7088 the light 18o values suggest that all arbuckle limestones underwent diagenetic alteration probably caused by meteoric water recharged during the development of the overlying pre middle ordovician unconformity the gradual 18o increase from late cambrian to early ordovician reflects reduced 18o depletion with decreasing burial temperature during alteration although the presence of additional primary secular 18o variation cannot be ruled out the 13c and 87 sr 86 sr variations in accord with 13c and 87 sr 86 sr variations in the literature represent primary secular variations the variations indicate that the 13c value and 87 sr 86 sr ratio of early paleozoic surface seawater decreased from late cambrian to early ordovician the 13c variation during this time period seems to correlate with sea level variation specifically during sea level fall an increase in the rate of oxidation of organic matter caused 13c depletion of inorganic bicarbonate in seawater as a result early ordovician carbonates probably deposited during the regression stage of the latest precambrian to latest early ordovician cycle became 13c depleted relative to late cambrian carbonates the decrease of seawater 87 sr 86 sr ratio from late cambrian to early ordovician may have resulted from decreased riverine sr input caused by decreased rate of continental weathering

delta 18 and delta 13 c variations in late devonian marine cements from the golden spike and nevis reefs alberta canada abstract for geologists and geochemists concerned about the chemical and thermal evolution of oceans deciphering primary marine 13c and 18o values from ancient marine carbonates can provide useful data for the physical and chemical modelling of ancient oceans although abiotic aragonite and magnesian calcite marine cements have commonly been overlooked because of their susceptibility to diagenetic alteration petrographic and chemical analysis of radiaxial fibrous calcite from two upper devonian frasnian pinnacle reefs golden spike and nevis of central alberta indicates that these magnesium rich cements preserve original marine 18o and 13c values in the pursuit of primary marine isotopic compositions it is necessary to characterize the isotopic compositions of diagenetic cement phases subaerial exposure and meteoric diagenesis of the interior of the golden spike reef is recorded by vadose cements caliche and clear non ferroan meteoric calcite spar with distinctive black and yellow cathodoluminescence cl zones 18o 13c crossplots of caliche and meteoric phreatic cement data form an inverted j curve typical of rock water reactions in meteoric systems a late phase of calcite spar with variable inclusion density and fe2 content and diffusely zoned red orange cl is associated with late fractures and stylolites and it precipitated during subsequent burial of the reef unaltered centers of radiaxial fibrous cement crystals are nonluminescent inclusion free and enriched in magnesium while diagenetically altered crystal terminations inter crystalline boundaries and crystal substrates are brightly luminescent inclusion rich microdolomite and fluid inclusions low mg calcite altered cloudy marine cements have variable 18o and invariant 13c values which define a trend that diverges from isotopically heavier unaltered marine cements the isotopic compositions of inclusionrich marine cement are coincident with those of meteoric phreatic spars although burial spars are present in all samples petrographic and chemical data suggest that portions of the marine cement crystals were altered by meteoric fluids the 18o values of unaltered radiaxial fibrous calcite are similar invariant throughout both reefs although marine cements from older deep parts of both reefs are enriched in 13c relative to younger shallow parts this records temporal changes in the 13c value of marine bicarbonate within the alberta basin variability of 18o and 13c values of unaltered marine cements is very small 0 5 for a given suite of samples making these cements ideal for h gh time resolution studies of the isotopic evolution of ancient oceans the presence of well preserved magnesium rich calcite from late devonian reefal sequences suggests that marine cements throughout the phanerozoic should be examined more carefully as they may preserve primary isotopic signatures independent confirmation of primary isotopic compositions is found in similar values for marine cements from middle frasnian reefs of the canning basin australia and the dinant synelinorium belgium the 18o values for the well preserved marine cements of this study are considerably lower than modern marine carbonate values by 3 to 4 detailed chemical and petrographic analysis indicates that diagenetic alteration is not the cause of these low 18o values in addition such a difference cannot be fully explained by warmer late devonian oceans these data suggest that late devonian seawater had a 18o value lower than modern oceans this in turn suggests that the balance of low temperature silicate weathering and high temperature seawater basalt exchange reactions must have been different from today s

intraspecific variations in carbon isotope and oxygen isotope compositions of a brachiopod basiliola lucida collected off okinawa jima southwestern japan abstract this study presents intraspecific variations in carbon isotope 13 c and oxygen isotope 18 o compositions of nine specimens of a subtropical brachiopod basiliola lucida collected west of okinawa jima ryukyu islands southwestern japan the 13 c values of samples collected along the maximum growth axis ontogenetic samples from two modern and seven older pre 1945 cal ad shells show no seasonal changes the modern shells which were collected from comparable depths have similar 13 c values that fall within the range of calcite precipitated in isotopic equilibrium with ambient seawater equilibrium calcite 13 c ec values and their mean 13 c values are 1 1 1 6 less than those from the older shells this decrease in 13 c values is similar in magnitude to the decreases in atmospheric co 2 and the oceanic dissolved inorganic carbon at the sea surface in recent years 13 c suess effect suggesting that the effect can even be detected at water depths of 200 300 m in the subtropical northwestern pacific ocean the 18 o values fluctuate within a narrow range 0 26 0 41 with no seasonal changes and they exhibit small 0 14 0 51 offsets from those of equilibrium calcite 18 o ec values a statistically significant negative linear relationship is established between seawater temperature and mean 18 o values of the nine shells but the slope 0 31 c is steeper than those of equilibrium calcite 0 23 c and other calcareous organisms 0 15 to 0 26 c the cross plots of the 13 c and 18 o values suggest that the degree of the vital effect varies among individuals in this species the 13 c and 18 o values of b lucida are potentially useful for reconstructing the 13 c and 18 o evolution of ancient oceans because both values show small intraspecific variations the former is identical to the 13 c ec values and the latter shows small within shell variations and small nearly constant offsets from the 18 o ec values

differential preservation of primary isotopic signatures in silurian brachiopods from northern europe abstract isotopic and geochemical compositions of silurian calcitic brachiopod shells carbonate host rocks and calcite cements from two northern european sites with different diagenetic histories are compared gotland sweden oslo graben norway the sequence exposed on gotland is characterized by repeated synsedimentary subaerial exposure and meteoric influence but only insignificant thermal alteration during shallow burial diagenesis in contrast silurian sediments exposed in the oslo area show only few signs of early exposure but were severely affected by thermal alteration during burial of the succession the preservation potential of brachiopod shells in each basin is different most nonluminescent brachiopods from gotland show well preserved microstructures and trace element contents very similar to those of modern brachiopod shells tightly clustered isotope values of individual brachiopod populations do not suggest significant meteoric alteration of primary marine isotope signatures in contrast there is strong evidence that all brachiopod shells from the oslo area contain diagenetically altered 18o values alteration of norwegian brachiopods is not always reflected in their trace element contents and cathodoluminescence characteristics but under sem the shells often display corrosion and recrystallization scattered oxygen isotope values in nonluminescent brachiopods 18o 18 to 5 5o oo together with low 18o values the case study from the oslo graben demonstrates that high fluid rock ratios and thermal overprinting during burial diagenesis may result in pervasive diagenetic alteration of 18o values in nonluminescent brachiopod shells this alteration can be detected by combined petrographic and geochemical investigations on brachiopod shells diagenetic cements and carbonate host rocks brachiopods from deeply buried and hydrothermally altered sites should not be used for deciphering marine 18o signals however the data presented here argue against a general rejection of lower paleozoic brachiopods as indicators of marine isotope signatures

low limit of mn2 activated cathodoluminescence of calcite state of the art in the literature the lower limit for mn2 activated cathodoluminescence cl of calcite is variously reputed to over a very wide range of values above 10 ppm mn our spectroscopic investigations of the cl response in natural calcite reveal that below 10 ppm manganese content mn2 activation is also present using the quantitative high resolution spectral analysis of cl qhrs cl an activation by mn2 in the range of 700 ppb is proved which cannot be determined visually so if not quenched the minimum mn2 content for mn2 activation is one atom in the irradiated calcite crystal lattice volume as the intrinsic background blue luminescence is used to determine non altered biogenic calcite the limit of mn2 activation plays an important role in the interpretation of diagenetic processes our results of spectroscopic analyses require a revision of current opinions about the diagenesis of calcite as revealed by cl investigation

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

the silurian of gotland sweden facies interpretation based on stable isotopes in brachiopod shells the silurian of gotland sweden consists of 440 m of carbonate deposits repeatedly uniform sequences of micritic limestones and marls are interrupted by complex structured reefs and by adjacent platform sediments generally the alteration of facies is interpreted as the result of sea level fluctuations caused by a gradual regression with superimposed minor transgressive pulses the purpose of this study is a facies interpretation based on both field observations and stable isotope measurements of brachiopod shells approximately 700 samples from stratigraphically arranged localities in different facies areas have been investigated the carbon and oxygen isotopes show principally parallel curves and a close relationship to the stratigraphic sequence lower values occur in periods dominated by deposition of marly sequences higher values are observed in periods dominated by reefs and extended carbonate platforms the oxygen isotope ratios are interpreted to reflect paleosalinity changes due to varying freshwater input rather than to paleotemperature carbon isotope ratios are believed to have been connected to global changes in the burial of organic carbon in black shales during periods of euxinic deep water conditions consequently the facies succession on gotland results from global paleoclimatic conditions changes in terrigenous input due to different rates of weathering and freshwater runoff rather than sea level fluctuations control the carbonate formation of the silurian on gotland

comment on oxygen and carbon isotopic composition of ordovician brachiopods implications for coeval seawater by h qing and j veizer

the isotopic record of ocean chemistry and diagenesis preserved in non luminescent brachiopods from mississippian carbonate rocks illinois and missouri fossil brachiopod shells are commonly used as a faithful record of the chemical and isotopic composition of the ancient ocean water in which they grew owing to their stable mineralogy and their presence in marine strata throughout phanerozoic time the utility of these fossils for preserving ocean water compositions is evaluated through analyses of multiple specimens that grew in contemporaneous sea water at individual stratigraphic horizons in the mississippian age burlington keokuk formation of mid continent north america significant variations in sr c and o isotopic compositions are observed in contemporaneous non luminescent shells indicating that apparently pristine brachiopod shell material has been altered by post depositional processes of diagenesis the least altered compositions at each horizon are used to estimate secular variations of 87 sr 86 sr in early mississippian ocean water the estimated rates of change of ocean water 87 sr 86 sr are comparable in magnitude to the rapid sr isotopic changes documented for cenozoic ocean water for which high frequency glacial cycles are inferred as a driving mechanism the rigorous assessment of diagenetic effects on the isotopic signatures of marine phases is requisite for the advancement of high resolution reconstructions of paleo ocean chemistry

stable isotopes in late pennsylvanian brachiopods from the united states implications for carboniferous paleoceanography stable isotopic analyses have been performed on nearly 500 nonluminescent brachiopod shells from kansas new mexico and texas to evaluate temporal and geographic variability in the carbon and oxygen isotopic record for late pennsylvanian time brachiopod specimens were collected from missourian and virgilian shales and examined in thin section for preservation of microstructure and absence of cathodoluminescence as a primary test for shell preservation regional variations are observed in the 18o and 13c values of nonluminescent brachiopod shells of the same genera average 18o values for the three genera analyzed crurithyris composita and neospirifer are highest in kansas 1 9 intermediate in texas 2 3 and lowest in new mexico 3 6 vital effect on the 18o of these genera appears minimal 18o data and other evidence suggest warmer temperatures for the shallow new mexico localities and slightly higher salinity for the kansas sea relative to the texas sea the 13c values of composita average about 1 higher than those of co occurring crurithyris and neospirifer suggesting microhabitat differences or vital effects preservation of this species effect argues for preservation of original 13c values average 13c values are highest in texas intermediate in kansas and lowest in new mexico although these values range from 2 6 to 4 9 for individual genera the regional variation in 13c averages less than 1 data for nonluminescent brachiopods and marine cements reveal a mid carboniferous 13c increase of 2 in paleotethyan sea water this increase is not seen in samples from the north american epicontinental seas which opened to the panthalassa ocean this regional difference in 13c appears to be due to changes in ocean circulation associated with the closing of the equatorial seaway and formation of pangea

fabric retentive non luminescent brachiopods as indicators of original delta 13 c and delta 18 o composition a test abstract fabric retentive non luminescent helderbergian brachiopods from open marine deposits analogous to brachiopods used in other studies to determine paleo oceanic stable isotopic composition exhibit depleted 13c values immediately below the erosional contact and more enriched values of 13c at greater stratigraphic distances below this surface in addition 13c values of these brachiopods display a marked shift across the erosional surface and values above it are essentially the same as those 30 m or more beneath the surface complementing these trends whole rock analyses metastable high agnesian calcite constituents echinoderms as well as early equant calcite cement also exhibit an increased depletion trend in 13c values as the helderberg oriskany contact is approached from below fabric retentive luminescent as well as obviously recrystallized brachiopods display stable isotopic values similar to those of fabric retentive non luminescent brachiopods thus best preserved fabric retentive non luminescent brachiopods exhibit values similar to obviously recrystallized brachiopods diagenetically altered metastable constituents echinoderms stable early meteoric cements and whole rock samples helderberg strata of new york state were affected by early meteoric diagenesis during a siegenian exposure event prior to deposition of the overlying marine oriskany sandstone meteoric diagenesis during this exposure gave rise to a characteristic carbon isotopic depletion trend approaching the helderberg oriskany contact all of the skeletal constituents underwent isotopic exchange during early diagenetic exposure to meteoric waters consequently the practice of using fabric retentive non luminescent brachiopods as indicators of their original chemistry and by extension paleo oceanic chemistry is questioned

sr c and o isotope geochemistry of ordovician brachiopods a major isotopic event around the middle late ordovician transition here we present sr c and o isotope curves for ordovician marine calcite based on analyses of 206 calcitic brachiopods from 10 localities worldwide these are the first ordovician wide isotope curves that can be placed within the newly emerging global biostratigraphic framework a total of 182 brachiopods were selected for c and o isotope analysis and 122 were selected for sr isotope analysis seawater 87sr 86sr decreased from 0 7090 to 0 7078 during the ordovician with a major quite rapid fall around the middle late ordovician transition most probably caused by a combination of low continental erosion rates and increased submarine hydrothermal exchange rates mean 18o values increase from 10 to 3 through the ordovician with an additional short lived increase of 2 to 3 during the latest ordovician due to glaciation although diagenetic alteration may have lowered 18o in some samples particularly those from the lower ordovician maximum 18o values which are less likely to be altered increase by more than 3 through the ordovician in both our data and literature data we consider that this long term rise in calcite 18o records the effect of decreasing tropical seawater temperatures across the middle late ordovician transition superimposed on seawater 18o that was steadily increasing from 3 standard mean ocean water smow by contrast 13c variation seems to have been relatively modest during most of the ordovician with the exception of the globally documented but short lived latest ordovician 13c excursion up to 7 nevertheless an underlying trend in mean 13c can be discerned changing from moderately negative values in the early ordovician to moderately positive values by the latest ordovician these new isotopic data confirm a major reorganization of ocean chemistry and the surface environment around 465 to 455 ma the juxtaposition of the greatest recorded swings in phanerozoic seawater 87sr 86sr and 18o at the same time as one of the largest marine transgressions in phanerozoic earth history suggests a causal link between tectonic and climatic change and emphasizes an endogenic control on the o isotope budget during the early paleozoic better isotopic and biostratigraphic constraints are still required if we are to understand the true significance of these changes we recommend that future work on ordovician isotope stratigraphy focus on this outstanding middle late ordovician event

nodular chert from the arbuckle group slick hills sw oklahoma a combined field petrographic and isotopic study nodular chert from the middle and upper arbuckle group early ordovician in the slick hills sw oklahoma was formed by selective replacement of grainstones burrow fillings algal structures and evaporite nodules chert nodules are dominantly microquartz with minor fibrous quartz both quartzine and chalcedony megaquartz and microflamboyant quartz lepisphere textures of an opal ct precursor are preserved in many especially in finely crystalline chert nodules the 18o values of microquartz chert range from 23 4 to 28 80 00 smow significantly lower than those of cenozoic and mesozoic microquartz chert formed both in the deep sea and from near surface sea water the 18o values of chert decrease with increasing quartz crystal size silicification in the arbuckle group occurred during early diagenesis with the timing constrained by the relative temporal relationships among silicification burial compaction and early dolomite stabilization silica for initial chert nucleation may have been derived from both dissolution of sponge spicules and silica enriched sea water chert nucleation appears to have been controlled by the porosity permeability and organic matter content of precursor sediments this conclusion is based on the fact that chert selectively replaced both porous grainstones and burrows and algal structures enriched in organic matter growth of chert probably occurred by a maturation process from opal a to opal ct to quartz as indicated by the presence of opal ct precursor textures in many chert nodules although field and petrographic evidence argues for an early marine origin for chert in the arbuckle group the light 18o values are inconsistent with this origin meteoric resetting of the 18o values of the chert during exposure of the carbonate platform best explains the light 18o values because i the 18o values of chert nodules decrease with decreasing 18o values of host limestones and ii chert nodules from early dolomite which underwent more extensive meteoric modification than associated limestones have lighter 18o values than chert nodules from limestones increasing recrystallization of chert nodules by meteoric water resulted in progressive 18o depletion and quartz crystal enlargement

paleoceanographic conditions at approximately 20 and 70 ka recorded in kikaithyris hanzawai brachiopoda shells abstract the 13c and 18o values of fossil brachiopod shells have been widely used as paleoenvironmental proxies in this study we investigated intrashell and intraspecific variations in the isotopic and minor element concentrations of well preserved shells of the brachiopod kikaithyris hanzawai yabe from the last glacial period 20 ka last glacial maximum lgm and 70 ka marine isotope stage 4 mis4 collected in the central ryukyus and used these data to estimate the paleoceanographic conditions seawater temperature concentration of dissolved inorganic carbon dic and 13c value of dic 13cdic the 13c and 18o profiles along the maximum growth axis obtained from the inner shell surface show three distinct intervals corresponding to changes in shell morphology these results suggest that the bulk isotopic compositions of brachiopods with complex shell morphologies are unsuitable for paleoenvironmental reconstructions nevertheless there exists a specific shell portion with relatively small intrashell and intraspecific variations the past seawater temperatures derived from the 18o values of this portion are consistent with the alkenone and planktic foraminiferal mg ca based past seawater temperatures reported in previous studies the past 13cdic values estimated from the 13c values of the specific shell portion are within the range of the past 13cdic values calculated from known atmospheric and oceanographic parameters the past dic concentrations reconstructed from the brachiopod based 13cdic values are lower than the present concentrations in the east china sea which can be explained by low partial pressure of co2 during the last glacial period these results indicate that the 13c and 18o values obtained from k hanzawai shells are potential paleoenvironmental indicators the intrashell and intraspecific variations in the k hanzawai shells are different for each minor element some anomalously high mn and fe concentrations in the shells are probably caused by metabolic factor s not by meteoric diagenesis this suggests that the minor element concentrations are useful but not perfect for distinguishing diagenetically altered and unaltered portions of the shells of k hanzawai in the studied succession

a survey of multi stage diagenesis and dolomitization of jurassic limestones along a regional shelf to basin transect in the ziz valley central high atlas mountains morocco abstract petrographic and geochemical data from five localities in the ziz valley of morocco indicate that jurassic limestones have undergone early diagenesis that varied with location from shelf to basinal settings burial diagenesis that was most pronounced in basinal settings and late diagenesis caused by compression and uplift of the high atlas mountains marine cements occur at all five localities from shelf to basin center although cement types vary from peloidal microcrystalline cements updip on the shelf to equant calcite in basinal settings presence of moldic grains and or mg poor fe poor blocky cements suggest that meteoric waters influenced early diagenesis at all shelf localities and on an upturned fault block in the basinal region leaving only one locality unaffected by early meteoric processes 87 sr 86 sr ratios of 0 70810 0 70895 greater than 87 sr 86 sr of coeval limestones mg contents that decrease upward from 47 5 to 43 0 mol mgco 3 presence of dolomitized marine cements and dolomite cements that postdate marine cements but predate meteoric to burial cements suggest that dolomitization and dolomite cementation at two shelf localities took place in mixed meteoric and marine waters early in diagenesis however poorer preservation of depositional fabrics lower 18 o values and larger and more anhedral crystals suggest that dolostones downdip underwent later modification during burial whereas those updip did not compaction during diagenesis generated numerous concavo convex and sutured intergranular contacts at updip shelf downdip shelf and basinal localities where earlier meteoric cementation was not extensive compaction was insignificant in more extensively cemented mid shelf settings high sr 1200 3800 ppm and fe 1000 2300 ppm contents in brachiopod grains suggest that lmc components underwent some modification during burial in basinal settings in sr rich reducing waters fe contents of late intergranular cements increase from 2000 ppm at the basin s edge to as much as 6000 ppm in the basin s center bedding parallel stylolites occur at all localities the most negative 18 o values of sparry dolomites near the tizi n firest fault 6 2 vs pdb imply diagenetic temperatures of 65 85 c assuming water 18 o values of 0 0 2 0 vs smow those temperatures are much less than previous estimates of burial temperatures in the high atlas basin an isotopic gradient extrapolating to roughly 5 km in diagenetically modified dolostones likewise suggests a geothermal gradient less than gradients previously proposed for at least parts of the area comparison of morphologies of transverse stylolites which are found at all localities with morphologies of bedding parallel stylolites suggests that transverse stylolites formed due to compression during late diagenesis uplift accompanying that compression allowed influx of low mg waters that along with other factors caused calcitization of dolomites the fe concentration of calcite that fills late fractures increases from less than 2000 ppm at the basin center to values in excess of 3000 ppm at the basin edge opposite trends in earlier cements and reflecting uplift of the high atlas mountains and resultant changes in patterns of groundwater flow

on plate tectonics and ocean temperatures plate tectonics the principal vehicle for dissipation of planetary energy is believed to buffer the 18o of seawater at its near modern value of 0 smow standard mean ocean water because the hot and cold cells of hydrothermal circulation at oceanic ridges cancel each other the persistence of plate tectonics over eons apparently favors attribution of the well documented oxygen isotope secular trends for carbonates cherts phosphates to progressively warmer oceans from 40 70 c in the early paleozoic to 60 100 c in the archean we argue that these oceanic hydrothermal systems are dominated by low temperature 350 c cells that deplete the percolating water in 18o seawater 18o is therefore a proxy for rather than being buffered by the intensity of plate tectonics detrending the phanerozoic carbonate 18oc secular trend for its tectonic component yields a stationary time series that interpreted as a proxy for phanerozoic climate indicates low latitude shallow ocean temperatures oscillating between 10 and 30 c around a baseline of 17 c attributes comparable to modern temperature values