missing molluscs as evidence of large scale early skeletal aragonite dissolution in a silurian sea in the silurian carbonate platform succession of gotland sweden interreef limestone and marl sequences have shallow marine shelly assemblages dominated by brachiopods typical of many paleozoic benthic faunas locally from similar facies where early skeletal silicification occurred associated with bentonites there are rich and diverse assemblages of originally aragonitic molluscs quantitative faunal analysis suggests that massive depletion of skeletal aragonite chiefly molluscs took place and the entire molluscan fauna may generally be underrepresented by at least 100 fold previous workers on the gotland succession have already shown that abundant diagenetic limestones were sourced by early aragonitic remobilization our findings indicate molluscs as the probable source of this diagenetic calcite not aragonite mud as was previously thought the taphonomic window represented by the silicified faunas illustrates the importance of aragonitic shells among the original silurian shelf benthos and raises the possibility that the paucity of such shells among many fossil faunas may be largely an early taphonomic effect

coping between crises early triassic early jurassic bivalve diversity dynamics abstract the triassic is bounded by two of the most severe biotic crises but nevertheless this time was for bivalves both a recovery and a diversification period and a moment to fully exploit some of their evolutionary novelties just how and when this was achieved is analyzed in this paper which covers induan to sinemurian bivalve diversity based on a newly compiled database taxonomic diversity and ecospace dynamics are examined separately diversity and evolutionary rates were assessed extinction selectivity was tested using a resampling algorithm and cohort analysis was used to study extinction patterns during the early triassic most bivalve genera were survivors from the permian and they were mainly cosmopolitan epifaunal and semi infaunal endobyssate taxa reclined epifaunal and semi infaunal bivalves increased in diversity during the triassic but from norian on their diversity declined and they were strongly affected by the t j crisis although the triassic jurassic extinction strongly impacted bivalve taxonomic diversity it had little impact on bivalve ecologic diversity not a single bivalve life strategy was eliminated at the end of the triassic the present study does not support previous conclusions that infaunal bivalves suffered greater extinction than epifaunal ones during the t j extinction not all life strategies were equally affected by the extinction event being the deep infaunal burrowers positively selected bivalves with major energetic requirements seem to have been more affected with negative selectivity on fast shallow burrowers the ecologic diversity changes of bivalves during the late triassic and several adaptations of durophagous predators reflect the effects of increasing predation pressure and support a late triassic origin of the mesozoic marine revolution

sustained mesozoic cenozoic diversification of marine metazoa a consistent signal from the fossil record paleobiological data provide a key historical record of global biodiversity dynamics but their interpretation is controversial due to geological and sampling biases raw data suggest that marine metazoans diversified dramatically during the late mesozoic and cenozoic whereas bias corrected analyses based on occurrence level data in the paleobiology database pbdb have indicated much less cenozoic diversification these standardized analyses are cited as evidence that biases strongly conceal underlying patterns in the global fossil record however we show that marine diversity did increase substantially and continuously from the jurassic to the neogene even after correcting for biases in pbdb data previous standardized analyses did not capture this diversification in full because they were based on incomplete data in the cenozoic observed richness rose to twice the paleozoic average which is within the range of values seen in analyses of raw data suggesting that even the raw global marine fossil record preserves first order signals of diversity history

natural mass dependent variations in the isotopic composition of molybdenum we present the first observations of natural mass dependent fractionation of the isotopic composition of molybdenum mo using multi collector inductively coupled plasma mass spectrometry variations in the isotopic composition of mo are reported as 97 95mo 97mo 95mo sample 97mo 95mo standard 1 1000 external analytical precision of 97 95mo is 1 between sediments deposited under anoxic conditions 97 95mo 1 02 to 1 52 relative to our in house standard and ferromanganese nodules 97 95mo 0 63 to 0 42 97 95mo of pacific ocean seawater 97 95mo 1 48 lies within the range of values for anoxic sediments closest to modern black sea anoxic sediments molybdenites from continental ore deposits have intermediate 97 95mo ranging from 0 26 to 0 09 variations in the abundances of 92mo 95mo 96mo 97mo and 98mo are consistent with mass dependent fractionation a sporadic unidentified interference occurs at mass 94 and 100mo is not measured we hypothesize that the 97 95mo offset between anoxic sediments and ferromanganese nodules results from mo isotope fractionation during inefficient scavenging of mo from seawater by mn oxides under oxic conditions the similarity in 97 95mo of anoxic sediments and seawater is consistent with the very efficient removal of mo from seawater under anoxic conditions in the presence of h2s the data can be interpreted in terms of a steady state mass balance between the mo flux into the oceans from the continents and the mo flux out of the oceans into oxic and anoxic sediments such an interpretation is quantitatively consistent with existing estimates of the removal fluxes of mo to anoxic and oxic sediments these findings suggest that 97 95mo in seawater may co vary with changes in the relative proportions of anoxic and oxic sedimentation in the oceans and that this variation may be recorded in 97 95mo of anoxic sediments hence the mo isotope system may be useful in paleoredox investigations

analysis of sample level properties along a paleoenvironmental gradient the behavior of evenness as a function of sample size abstract the assessment of fossil biodiversity is influenced by a range of sampling effects such as sample size evenness and lithological character many paleontologists have worked to understand and mitigate for such biasing effects generating standardized assessments of local regional and global biodiversity however when assessing biodiversity among individual communities with small sample sizes such as those from lithified paleozoic samples it is important to understand how these properties relate to one another qualitatively and quantitatively here the properties of richness and evenness were investigated at the sample level along an ecological gradient within a stratigraphically and geographically constrained interval the fulton submember of the kope formation upper ordovician of kentucky and ohio using a replicate subsampling procedure analogous to rarefaction evenness values were generated using four metrics j pie ess and essmin simulations using real and simulated data revealed that the pie metric generates the most stable assessments of evenness at very small sample sizes and relative evenness can be assessed at any sample size provided that replicate subsampling is performed comparisons of richness evenness and lithology revealed no directional trend along the ecological gradient but two distinct groups of samples were identified by differences in evenness as well as the number of sampled individuals such differences in abundance structure influence perceptions of biodiversity when samples from low evenness and high evenness assemblages are aggregated increasing the sampling intensity in low evenness communities may help to mitigate for these differences and more thoroughly capture the richness contained within small samples

burrow mediated carbonate dissolution in rudist biostromes aurisina italy implications for taphonomy in tropical shallow subtidal carbonate environments abstract in upper cretaceous rudist biostromes at aurisina italy marked taphonomic loss of radiolitids by burrow mediated dissolution and mechanical disintegration illustrates the relation between synecology and early diagenesis in the fossilization of tropical shallow water bioconstructions the studied platform succession accumulated in a prevalently open lagoon with bioturbated carbonate sand rudist thickets bioclastic dunes and areas with shelly lime ooze rudist biostromes consist of radiolitids and hippuritids or of radiolitids only and have an open parautochthonous rudist fabric with a matrix of bioclastic bioturbated wackestone swirly disorientation of bioclasts records early softground bioturbation later firmground burrows comprise an irregular network of tunnels and chambers filled with bioclastic packstone to grainstone the size geometry and sediment fill of the firmground burrows suggest that they were produced by crustaceans radiolitid preservation ranges from complete to relictic radiolitid relicts formed by a spalling and or dissolution of the cellular boxwork ostracum of the attached valve leaving calcite tubes built by a distinct thin ostracal shell layer or b dissolution of the entire shell leaving the sedimentary fills of the intertabular spaces of the attached valve as a diagnostic vestige or c shell dissolution within the firm sediment with subsequent filling of the biomould by bioclastic wackestone to packstone to grainstone loss of radiolitids produced a taphonomic bias towards rudists with non cellular ostracum locally taphonomic loss produced ghost biostromes composed nearly entirely of faint radiolitid relicts shell dissolution resulted from chemical gradients in the sediment within and near the burrows and from enhanced microboring and microbial infestation dissolution of radiolitids was favoured by the combined effects of chemical instability of hypostracal aragonite and by the structure of the calcitic boxwork ostracum of thin walled cells some biostromes are intercalated with or are capped or overlain within a short vertical distance by a hardly recognizable emersion surface as a consequence of the shallow depth of biostrome accumulation taphonomic loss by dissolution is widespread in open parautochthonous rudist fabrics and confirms actuogeological results of other authors that bioturbation mediates carbonate dissolution also under shallow tropical waters supersaturated for calcium carbonate the amount of carbonate dissolved upon burrowing of the biostrome matrix is hardly quantifiable but by analogy to recent carbonate environments may have been large within bioconstructions deep tiered burrowing occurred at least since the carboniferous taphonomic loss by dissolution thus may have been active in the fossilization of tropical shallow water mounds and biostromes over much of phanerozoic times

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

digital terrain modelling a review of hydrological geomorphological and biological applications the topography of a catchment has a major impact on the hydrological geomorphological and biological processes active in the landscape the spatial distribution of topographic attributes can often be used as an indirect measure of the spatial variability of these processes and allows them to be mapped using relatively simple techniques many geographic information systems are being developed that store topographic information as the primary data for analysing water resource and biological problems furthermore topography can be used to develop more physically realistic structures for hydrologic and water quality models that directly account for the impact of topography on the hydrology digital elevation models are the primary data used in the analysis of catchment topography we describe elevation data sources digital elevation model structures and the analysis of digital elevation data for hydrological geomorphological and biological applications some hydrologic models that make use of digital representations of topography are also considered

comment on non uniqueness and interpretation of the seawater 87sr 86sr curve by dave waltham and darren r grocke gca 70 2006 384 394

chemostratigraphy of the posidonia black shale sw germany i influence of sea level variation on organic facies evolution abstract during the toarcian widespread deposition of organic rich mudrocks black shales occurred across europe linked to a 3rd order sea level rise we provide a high resolution multi proxy chemostratigraphic study of the dotternhausen sections in sw germany embedded in a stratigraphic and palecological framework chemostratigraphy and paleoenvironmental reconstruction is based on rock eval and elemental analysis of bulk organic matter and biomarker analysis of aliphatic hydrocarbon fractions aliphatic biomarker ratios applied to reconstruct the facies development include pristane phytane and pristane phytane n c 17 n c 18 ratios carbon preference index c 27 c 29 sterane and hopane sterane distribution results show that sea level change exerted the major control on the composition of primary produced organic matter and on its redox controlled preservation changes in primary production and in the ratio of terrestrially versus marine derived organic matter take place drastically at the onset of black shale deposition the relative proportion of terrestrial versus marine biomass accumulation is independently assessed by highly correlative bulk organic matter properties hi values and biomarker ratios cpi the terrestrially derived end member yields hi values of 50 100 mg hc toc and cpi values of 1 4 2 0 whereas the marine end member has hi values of 600 700 mg hc toc and cpi values of 0 95 1 05 correspondingly the shift in plant versus bacteria derived biomass is discriminated by hi values and hopane sterane ratios data were grouped according to their position in the lowstand lst lower and upper transgressive tst highstand systems tract hst and maximum flooding surface mfz of the sequence stratigraphic framework occurrence of organic matter with a significant bacterial contribution is restricted to the lst and lowermost tst and shows hopane sterane ratios of 0 25 0 5 marine phytoplanktonic matter dominates the upper tst hst and mfz with organic matter giving hopane sterane ratios between 0 5 and 1 7 samples from the hst yield hi values between 450 and 500 mg hc toc which is consistently lower by 150 mg hc toc when compared to the tst the lower quality of the organic matter deposited in the hst is due to its longer settling times through a thicker oxygenated water column a strong correlation with relative sea level stand and the c 27 c 29 sterane ratio is observed lst exhibits c 27 c 29 sterane ratios of 0 7 during transgression values increase to 1 0 reaching a maximum at 1 4 during sea level highstand thus reflecting decreasing terrigenous input and shifts in near shore versus open marine algal communities episodic fluctuation of anoxia is discussed on the basis of a monsoonal climate model which infers significant surface run off during summer monsoon providing high nutrient supply and associated high primary productivity in combination with a density stratified water column excellent preservation conditions for organic matter were established dry winter conditions with inversion of the surface water flow regime led to seasonal export of bottom waters and episodic establishment of dysoxic conditions variations in the extent of annual flushing of sub basins on the toarcian shelf controlled long term organic matter accumulation rate paleoclimate variability and its connection with sea level stand is thus of paramount importance for different modes of organic matter accumulation and redox system stability

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

sensitivity of watershed attributes to spatial resolution and interpolation method of lidar dems in three distinct landscapes this study investigates scaling relationships of watershed area and stream networks delineated from lidar dems the delineations are tested against spatial resolution including 1 5 10 25 and 50 m and interpolation method including inverse distance weighting idw moving average ma universal kriging uk natural neighbor nn and triangular irregular networks tin study sites include mosquito creek scotty creek and thomas brook representing landscapes with high low and moderate change in elevation respectively results show scale dependent irregularities in watershed area due to spatial resolution at thomas brook and mosquito creek the highest sensitivity of watershed area to spatial resolution occurred at scotty creek due to high incidence of lidar sensor measurement error and subtle changes in elevation length of drainage networks did not show a scaling relationship with spatial resolution due to algorithmic complications of the stream initiation threshold stream lengths of main channels at thomas brook and mosquito creek displayed systematic increases in length with increasing spatial resolution described through an average fractal dimension of 1 059 the scaling relationship between stream length and dem resolution allows estimation of stream lengths from low resolution dems in the absence of high resolution dems single stream validation at thomas brook showed the 1 m dem produced the lowest length error and highest spatial accuracy at 3 7 and 71 3 respectively single stream validation at mosquito creek showed the 25 m dem produced the lowest length error and the 1 m dem the highest spatial accuracy at 0 6 and 61 0 respectively

the impact of lithification on the diversity size distribution and recovery dynamics of marine invertebrate assemblages lithified marine sediments are not equitably distributed through time raising the possibility that lithification masks biological signals when data from unlithified and lithified sediments are compared or combined using mollusk dominated assemblages from the early cenozoic of the gulf coastal plain we find that lithification conceals small taxa decreases taxonomic resolution and exacerbates the undersampling of rare taxa lithified assemblages appear less diverse and have less even abundance distributions than coeval unlithified samples these limitations cannot be overcome by standardization procedures nor are they likely to be circumvented by collecting larger samples the effects of this bias however can be mitigated by restricting analyses to a single lithification state or to specific size classes since lithification selectively obscures small taxa the magnitude of this bias will be most severe when organisms are particularly small such as in the aftermath of mass extinctions in the study area lithification artificially protracts the recovery period following the cretaceous paleogene mass extinction by 7 m y

paleolatitudinal sampling bias phanerozoic species diversity and the end permian extinction species diversity varies with latitude almost all marine sedimentary rock in the paleozoic of europe and north america was deposited in the tropics whereas only 24 was deposited in these latitudes during mesozoic and tertiary time a new plot of species diversity through the phanerozoic takes into account the paleolatitudinal distribution of marine sedimentary rock this plot indicates that the permian witnessed the acme of species diversity prior to the cretaceous and that the triassic recovery following the end permian extinction was much less pronounced than suggested by previous studies

skeletal mineralogy and biodiversity of marine invertebrates size matters more than seawater chemistry it is now well established that seawater chemistry as well as influencing non skeletal marine precipitation calcite and aragonite seas has affected skeletal mineral secretion in some algal and marine invertebrate groups skeletal mineralogy has had a yet more profound consequence on fossil preservation the realization that the fossil record of marine organisms with an aragonite shell is widely depleted in some shelf settings through early effectively syn depositional dissolution missing molluscs effect has led to a re evaluation of the composition diversity ecological and trophic structure of marine benthic communities comparisons of molluscan lagerstatten from calcite and aragonite seas show a similar pattern of skeletal mineralogical loss that is no differences are discernibly linked to changed seawater geochemistry it is notable that the rare mollusc rich skeletal lagerstatten faunas in the fossil record include many small individuals micromolluscs are quantitatively important among modern shell assemblages yet small size is a major source of taphonomic and biodiversity loss in the fossil record in skeletal lagerstatten faunas micromolluscs contribute variably to mollusc biodiversity but appear particularly significant through at least to triassic times they highlight a further missing molluscs effect of taphonomic loss through early dissolution

improving with age the fossil record of lingulide brachiopods and the nature of taphonomic megabiases variation in the actual preservation of fossils can be used to detect taphonomic megabiases large scale distortions caused by changes in the quality of the fossil record for example temporal changes in the taphonomy of lingulide brachiopods suggest that paleozoic lingulides had a higher fossilization potential than the post paleozoic ones the frequency of stratigraphic occurrences of lingulides covaries with their fossilization potential thus the apparent decline in the importance of lingulides in phanerozoic communities may be due to changes in their fossilization potential taphonomic megabiases may be very common we distinguish four major types of megabiases within taxon static among taxon dynamic among taxon and global

mo total organic carbon covariation in modern anoxic marine environments implications for analysis of paleoredox and paleohydrographic conditions 1 sedimentary molybdenum mo s has been widely used as a proxy for benthic redox potential owing to its generally strong enrichment in organic rich marine facies deposited under oxygen depleted conditions a detailed analysis of mo s total organic carbon toc covariation in modern anoxic marine environments and its relationship to ambient water chemistry suggests that 1 mo s while useful in distinguishing oxic from anoxic facies is not related in a simple manner to dissolved sulfide concentrations within euxinic environments and 2 patterns of mo s toc covariation can provide information about paleohydrographic conditions especially the degree of restriction of the subchemoclinal water mass and temporal changes thereof related to deepwater renewal these inferences are based on data from four anoxic silled basins the black sea framvaren fjord cariaco basin and saanich inlet and one upwelling zone the namibian shelf representing a spectrum of aqueous chemical conditions related to water mass restriction in the silled basin environments increasing restriction is correlated with a systematic decrease in mo s toc ratios from 45 5 for saanich inlet to 4 5 1 for the black sea this variation reflects control of mo s by mo aq which becomes depleted in stagnant basins through removal to the sediment without adequate resupply by deepwater renewal the basin reservoir effect the temporal dynamics of this process are revealed by high resolution chemostratigraphic data from framvaren fjord and cariaco basin sediment cores which exhibit long term trends toward lower mo s toc ratios following development of water column stratification and deepwater anoxia mo burial fluxes peak in weakly sulfidic environments such as saanich inlet owing to a combination of greater mo aq availability and enhanced mo transport to the sediment water interface via fe mn redox cycling and are lower in strongly sulfidic environments such as the black sea and framvaren fjord these observations demonstrate that at timescales associated with deepwater renewal in anoxic silled basins decreased sedimentary mo concentrations and burial fluxes are associated with lower benthic redox potentials i e more sulfidic conditions these conclusions apply only to anoxic marine environments exhibiting some degree of water mass restriction e g silled basins and are not valid for low oxygen facies in open marine settings such as continent margin upwelling systems

erratum to the posidonia shale lower toarcian of sw germany an oxygen depleted ecosystem controlled by sea level and palaeoclimate palaeogeogr palaeoclimatol palaeocol 165 2001 27 52 abstract the lower toarcian posidonia shale is famous for its excellently preserved fossils and its high amount of organic matter up to 16 both quality of preservation and accumulation of organic matter have been explained by permanent anoxic bottom water conditions high resolution geochemical sedimentological and palaeoecological investigations of various sections of the posidonia shale in sw germany however indicate that oxygen availability was variable and ranged from short oxygenated periods to longer term anoxia the benthic macrofauna consists of nine fossil communities and was used in combination with geochemical data to reconstruct a time averaged oxygen curve anoxic conditions prevailed during the deposition of the toarcian black shales they were however punctuated by various short periods weeks to years with oxygenated bottom water conditions sedimentological e g distinctiveness of microlamination siliciclastic content and geochemical parameters e g organic matter content isotopic signatures 18 o and 13 c molecular redox parameters pristane phytane ratio arylisoprenoids exhibit a remarkable covariation and seem to be controlled by sea level fluctuation maximum oxygen depletion and an extreme negative shift of 13 c org values 34 occurred during the early falciferum zone this is explained by the recycling of 12 c enriched carbon derived from remineralization of organic matter on and within the substrate during low sea level stand and a highly elevated redox boundary including photic zone anoxia the subsequent transgression permitted enhanced water exchange with the tethyan ocean and caused improvement of living conditions at the end of the falciferum zone other important factors controlling the depositional environment are the overall palaeogeographic situation and climate the early jurassic is the latest period before break up of pangaea and probably was ruled by a strong meridional atmospheric circulation system with pronounced seasonal changes of prevailing trade and monsoon wind systems an estuarine circulation with a positive water balance and surface water with slightly reduced salinity in the summer alternated with an anti estuarine circulation and a negative water balance in the winter during the summer months a stratified water column with anoxic conditions below the halocline developed 18 o data indicate low salinity in the surface water during the monsoon influenced summer high productivity was then located in the photic zone and the corresponding isotopically light 18 o signal was fixed in the calcareous nannoplankton during the winter months a saline circulation system brought oxygen to the benthic environment favouring temporary benthic colonization especially during times of relative sea level high stand

stream network modelling using lidar and photogrammetric digital elevation models a comparison and field verification a conventional photogrammetrically derived digital elevation model dem 10 m resolution and a light detection and ranging lidar derived dem 1 m resolution were used to model the stream network of a 193 ha watershed in the swan hills of alberta canada stream networks modelled using both hydrologically corrected and uncorrected versions of the dems and derived from aerial photographs were compared the actual network mapped in the field was used as verification the lidar dem derived network was the most accurate representation of the field mapped network being more accurate even than the photo derived network this was likely due to the greater initial point density accuracy and resolution of the lidar dem compared with the conventional dem lidar dems have great potential for application in land use planning and management and hydrologic modelling the network derived from the hydrologically corrected conventional dem was more accurate than that derived from the uncorrected one but this was not the case with the lidar dem copyright 2007 john wiley sons ltd

provenance of microcrystalline carbonate cement in limestone marl alternations lma aragonite mud or molluscs aragonite derived from marine molluscs is evaluated as the source for microcrystalline carbonate cements of limestone marl alternations lma calculations demonstrate that extremely low levels of mollusc derived aragonite well below the production rates of molluscs in modern marine settings could have provided sufficient carbonate to cement examples of ordovician silurian and jurassic lma in non tropical or tropical settings it is likely that even in the palaeozoic molluscs provided sufficient carbonate entirely to source microcrystalline cements of lma autochthonous molluscan aragonite is the only viable aragonite precursor for lma microcrystalline cements of cool water settings where temperatures precluded calcified algae and abiotic carbonate precipitation in calcite seas where mg ca ratios inhibited both abiogenic aragonite precipitation and aragonite generation by calcified algae molluscan aragonite was again the most likely main contributor in some epeiric seas where brackish wedges switched off the shallow water carbonate mud factories molluscan aragonite is the parsimonious source of carbonate for lma microcrystalline cements

applications of hydrologic information automatically extracted from digital elevation models digital elevation models dems can be used to derive a wealth of information about the morphology of a land surface traditional raster analysis methods can be used to derive slope aspect and shaded relief information recently developed computer programs can be used to delineate depressions overland flow paths and watershed boundaries these methods were used to delineate watershed boundaries for a geochemical stream sediment survey to compare the results of extracting slope and flow paths from dems of varying resolutions and to examine the geomorphology of a martian dem

storm generated sedimentary structures in subtidal marine facies with examples from the middle and upper ordovician of southwestern virginia abstract the role of storms is of major importance in forming stratification in open shelf facies though storm processes on modern shelves are not fully understood enough is known to see how sediments are influenced storms transport sand away from coastal areas and suspend and transport sand and mud on the open shelf when wave base is lowered storms generate winnowed lag deposits of shells or gravel but coarser sediment and shells of megafauna are little transported except in very near shore environments the martinsburg formation middle and upper ordovician of southwestern virginia was deposited on a storm influenced open marine platform in contrast to the classic turbidite flysch basin of the martinsburg in the great valley of virginia to the northeast deposition of most martinsburg sediments was due to storms sediments deposited during single storm events occur in characteristic fining upward sequences 1 to 80 cm thick in both carbonate and terrigenous clastic dominated areas of the platform individual storm deposits consist of a sharp erosional base overlain by a couplet of whole fossil packstone and laminated calcarenite siltite or quartzose siltstone to very fine sandstone which is gradational to shale packstones are typically intraclastic have shells aligned parallel o bedding and convex up often with adhering or contained mud and have abundant infiltration textures such as cement filled shelter voids packstones grade abruptly to a laminated bed with horizontal plane lamination large and small scale in phase undulatory lamination and wave ripple cross lamination commonly occurring in ordered vertical sequences laminated beds contain escape burrows are weekly graded and have irregularly mounded upper bed surfaces laminated beds grade upward to shale containing horizontal branching burrows and marine fossils the faunal content of the fining upward sequences matches that of the interbedded shale storm generated beds vary greatly in thickness laterally and frequently pinch out completely fining upward sequences formed by scouring winnowing and suspension of the sand and mud during the height of the storm to form the erosional base and autochthonous lag of whole fossils storms preferentially preserve whole fossils rather than fragmenting them infiltration of finer sediment into the lag deposit and deposition of the laminated bed occurred as the storm waned laminated beds are molded into a distinctive suite of sedimentary structures by wave generated currents the shale represents deposition of finest storm suspended material and continued hemipelagic sedimentation similar sequences are widely distributed in the geologic record variability of storm generated sequences can be related to changing water depth thus storm deposits may be useful in basin analysis to estimate relative water depths and trace transgressive and regressive events

patterns in bioclastic accumulation through the phanerozoic changes in input or in destruction evolutionary changes in the ecology and diversity of organisms that produce and destroy calcareous skeletons suggest that bioclastic concentrations themselves might have changed in nature through the phanerozoic empirical data from marine siliciclastic records of ordovician silurian jurassic and neogene ages indicate a significant increase in the thickness of densely packed bioclastic concentrations over geologic time from a primarily thin bedded brachiopod dominated record in the ordovician silurian to a mollusk dominated record with many more and thicker shell beds in the neogene jurassic shell beds vary in thickness with the paleozoic or modern affinities of the chief constituents suggesting along with other evidence that the phanerozoic increase was determined neither by diagenesis nor by a shift in taphonomic conditions on the sea floor but rather by the evolution of bioclast producers namely groups with 1 more durable low organic skeletons 2 greater ecological success in high energy habitats and 3 on the basis of indirect evidence higher rates of carbonate production these results suggest that 1 reproductive and metabolic output has increased in benthic communities over time and 2 the scale of time averaging in benthic assemblages has increased owing to greater hard part durability of modern groups

the first half of tetrapod evolution sampling proxies and fossil record quality abstract the first half of tetrapod evolution witnessed substantial diversification of the clade and several major turnovers and mass extinctions in the time since their origin more than 380 myr ago to the beginning of the middle jurassic 175 myr ago tetrapods apparently diversified fitfully reaching their highest level in the middle permian and showing major diversity declines in the late moscovian early permian wordian lower wuchiapingian end permian lower anisian lower ladinian late triassic lower norian to upper rhaetian end triassic and early jurassic upper sinemurian lower pliensbachian of these diversity drops only the end permian and end triassic correspond to recognised mass extinctions and the late moscovian and early norian drops to other previously identified environmental crises the remainder could be real extinction or turnover events or partially artefacts of biased sampling there are strong correlations between formation counts and tetrapod palaeodiversity suggesting a sampling component in the raw data but the covariation is not uniform through the whole time span being poor from devonian to middle permian and better from late permian to early jurassic there is limited evidence for covariation between the tetrapod palaeodiversity time series and other putative sampling metrics such as specimen completeness numbers of publications map areas gap bounded sedimentary units rock volumes formations and fossil collections modelling by multiple correlations shows that formation count is generally the best explanatory model either on its own or combined with other sampling time series however it is not clear that formation count is independent of the palaeodiversity time series because rises and falls in both signals could reflect variations in original diversity or in preservation or in sampling

faunal composition and dynamics in unconsolidated sediments a case study from the middle ordovician of the east baltic the volkhov regional stage middle ordovician in the east baltic preserves diverse fossil assemblages dominated by epibenthic suspension feeders brachiopods ostracodes conodonts echinoderms and bryozoans are the main components of palaeocommunities obtained from clay horizons in the putilovo section st petersburg region russia whereas trilobites machaeridians hyolithids graptolites benthic foraminifers and gastropods are rare or occur sporadically brachiopod bioclasts volumetrically dominate the debris of the studied sediments quantitative faunal data are used to assess species diversity patterns as expressed by the species richness total number of species in the standardized sample size and by the evenness or equitability of the community the numerical abundance of particular taxa in each standardized sample was used to evaluate the density of the fauna the communities in the volkhov stage in putilovo quarry reveal a remarkable stability throughout the studied interval and are characterized by high density 1000 6000 specimens per 100 g relatively moderate species richness 10 15 species and a moderately variable equitability 0 3 0 7 for the dominant fossil groups conodonts ostracodes and brachiopods ostracodes significantly increase in numbers within the upper part of the section this confirms a shallowing of the basin during the late volkhov interval small scale variability of the diversity estimates does not correlate with the small scale sea level changes reconstructed for this part of the basin it may be connected with error in diversity measurement or the result of undetected environmental parameters variability estimates for different faunal groups are poorly correlated because particular groups have different environmental tolerances

marine diversity through the phanerozoic problems and prospects the fossil record provides direct evidence of how diversity has changed over time but cannot be taken at face value diversity curves constructed from counting taxa in the rock record are seriously biased by unevenness of geographical and stratigraphical sampling effort inequality in the rock record available for sampling and inconsistent taxonomic practice sample standardization removes some bias but does not overcome more general incompleteness problems modelling that accounts for potential biases is a newer approach but needs accurate estimates of rock record and consistent taxonomic data uncertainty remains over whether the steep rise in diversity over the last 100 ma is real or reflects sampling bias the repeated rise and fall of marine diversity over time correlates closely with the areal extent of sedimentary deposits and independent estimates of the quality of the fossil record implying a common driving factor namely tectonically mediated sea level change however whether changes in diversity are primarily biological in origin or reflect sampling artefact remains contentious there is a distinct possibility that many of the apparent rises and falls in diversity over the phanerozoic including most of the mass extinctions arise either partially or entirely from rock record bias

geospatial resources for the geology community the usgs national map abstractgeospatial data are a key component of investigating interpreting and communicating the geological sciences locating geospatial data can be time consuming which detracts from time spent on a study because these data are not obviously placed in central locations or are served from many disparate databases the national map of the us geological survey is a publicly available resource for accessing the geospatial base map data needs of the geological community from a central location the national map data are available through a viewer and download platform providing access to eight primary data themes plus the us topo and scanned historical topographic maps the eight themes are elevation orthoimagery hydrography geographic names boundaries transportation structures and land cover and they are being offered for download as predefined tiles in formats supported by leading geographic information system software data tiles are periodically refreshed to capture the most current content an

a comparison of lidar based dems and usgs sourced dems in terrain analysis for knowledge based digital soil mapping abstract while lidar based digital elevation models dem are more accurate and precise than the usgs sourced dem that are widely used in soil mapping in the us their high cost and other problems prohibit an easy decision of adopting them in service oriented soil mapping conducted by a government agency like usda nrcs this study compares the performances of lidar based dem and the usgs sourced dem in calculating slope gradient as an input for knowledge based digital soil mapping kbdsm aiming to provide scientific evidence and more importantly propose a scientific approach to evaluating the two types of dem for kbdsm we conducted the comparison by evaluating how closely the dem based slope gradient values match the field measured values for a small watershed in northern vermont us we prepared three dem including a 10 m dem interpolated from the 7 5 minute usgs topographic map a 1 m dem based on lidar points and a 5 m dem resampled from the 1 m dem when calculating slope gradient we applied two neighborhood sizes 10 m and 30 m two neighborhood shapes square and circular and three slope gradient algorithms evans young horn and modified zevenbergen thorne to the three dem we then compared the calculated slope gradient values with the values measured by soil scientists at 159 sample locations in the study area statistics show that across all the tested settings the lidar based dem perform significantly better than the usgs sourced dem we conclude that lidar based dem may considerably improve the quality of inputs for kbdsm we also find that the results from the 1 m lidar based dem and the resampled 5 m dem do not show considerable and consistent differences

did alpha diversity increase during the phanerozoic lifting the veils of taphonomic latitudinal and environmental biases abstract we estimate the effects of three biases on the observed alpha diversity of paleocommunities from the middle paleozoic and late cenozoic the first bias results from the preferential dissolution of aragonite relative to calcite this bias can lower the relative abundance and preserved diversity of aragonitic taxa potentially lowering the rarefied diversity of an entire fossil assemblage we model the effects of this bias by analytically reinserting aragonitic specimens and taxa into paleozoic assemblages that have been described in the literature the aragonitic specimens are inserted using a wide range of reasonable assumptions about the original local paleocommunity composition although the dissolution bias is probably not as severe as has been argued by some our analytical modeling indicates that the average paleozoic assemblage may have lost up to 29 of its total diversity the second bias results from the higher diversity of the tropics relative to temperate latitudes but the late cenozo

secular oscillations in the carbonate mineralogy of reef building and sediment producing organisms driven by tectonically forced shifts in seawater chemistry the primary mineralogy of oolites and early marine carbonate cements led sandberg nature 305 1983 19 22 to divide the phanerozoic eon into three intervals of aragonite seas and two intervals of calcite seas hardie geology 24 1996 279 283 has shown that these oscillations together with synchronous oscillations in the mineralogy of marine potash evaporites can be explained by secular shifts in the mg ca ratio of seawater driven by changes in spreading rates along mid ocean ridges the hardie model also predicts that high mg calcite should precipitate along with aragonite as it does in today s aragonite sea we have uncovered oscillations in the carbonate mineralogy of hypercalcifying organisms ones that have produced massive skeletons large reefs or voluminous bodies of sediment that correspond to sandberg s aragonite seas and calcite seas and that are predicted by the hardie model particular groups of corals sponges and algae appear to have been dominant reef builders only when favored by an appropriate mg ca ratio in seawater in early and middle paleozoic calcite seas calcite i reefs were dominated by calcitic tabulate heliolitid and rugose corals and calcitic stromatoporoids in contrast during the period of late paleozoic early mesozoic aragonite seas aragonite ii aragonitic groups of sponges scleractinian corals and phylloid algae as well as high mg calcitic red algae were principal reef builders during late cretaceous time at the acme of calcite ii massive rudists displaced aragonitic hermatypic corals in today s aragonite sea aragonite iii scleractinian corals are again dominant reef builders along with high mg calcitic coralline algae major sediment producing algae exhibit temporal patterns similar to those of reef builders calcitic receptaculitids flourished during calcite i whereas aragonitic dasycladaceans did not become dominant rock formers until aragonite ii during calcite ii calcitic nannoplankton formed massive coccolith chalks in warm shallow seas of the late cretaceous after the mg ca ratio of seawater had reached a very low value and calcium concentration a very high value as the mg ca ratio of seawater rose and calcium concentration fell during the cenozoic era individual coccoliths on average became less massive and encrusted cells less thickly by pliocene time during aragonite iii the prominent genus discoaster secreted only narrow rayed coccoliths that covered less than 25 of the cell surface also during aragonite iii the aragonitic green alga halimeda emerged as the dominant skeletal sediment producer in reef tracts the influence of seawater chemistry on skeletal secretion appears to have been especially strong for morphologically simple taxa that exert relatively weak control over their own calcification such groups include algae sponges corals and bryozoans morphological simplicity also permits these groups to adopt vegetative or colonial modes of growth that confer success in competition for space on reefs this linkage in addition to the basic chemical demands of hypercalcification has given the mg ca ratio of seawater strong control over the success of individual reef building taxa more generally this ratio appears to have strongly influenced evolutionary changes in the skeletal mineralogy of sponges and cheilostome bryozoans throughout their history we conclude that throughout phanerozoic time a chain of causation has extended from mid ocean ridge processes via seawater chemistry to the mineralogical and biological composition of reef communities and bioclastic carbonate deposits

dissolution behaviour of bryozoan sediments taphonomic implications for nontropical shelf carbonates abstract diagenesis of carbonate sediments on temperate marine shelves is commonly destructive in contrast to a dominantly constructive regime in tropical waters in some cases dissolution can be important in temperate shelf waters particularly affecting the metastable carbonate minerals if dissolution does occur it can be an important taphonomic process reducing skeletal remains available for fossilization and biasing fossil assemblages the dissolution behaviour of bryozoans has been investigated in an acid bath laboratory experiment using nontropical carbonate sediments from the new zealand shelf bryozoans of various shapes growth forms and compositions low to high mg calcite and aragonite were extracted and their weight loss over time was measured results show that variations in stability of carbonate mineralogies are insufficient to predict relative dissolution rates if shelf dissolution occurs even if only periodically one might anticipate that the less stable aragonite and high mg calcite skeletons would dissolve first in fact this experiments shows that small delicate skeletons with non compact morphologies dissolve most rapidly apparently with little regard for mineralogy fossil bryozoan assemblages from temperate marine shelf environments may therefore not only reflect the preferential loss of certain aragonitic species but also the loss of small delicate species new zealand s nontropical tertiary bryomol limestones often lack aragonitic skeletons they may also be relatively depleted in small delicately branched bryozoans in contrast tropical shelf carbonate sediments can retain more delicate skeletons through early cementation and lack of dissolution a difference in character between tropical and temperate carbonate shelf deposits may therefore be in the proportion of small delicate bryozoan colonies

rhenium in the black sea comparison with molybdenum and uranium rhenium concentrations were measured in the black sea to determine the behavior of this redox sensitive element in an anoxic basin under reducing conditions re is removed from solution leading to its depletion from black sea deep waters compared with uranium water column re and mo profiles show evidence of removal from mid depths suggesting that these elements are scavenged either in the water column or slope sediments to a greater degree than u black sea surface waters are enriched in re reflecting re in rivers draining into the northern black sea that are enhanced up to 80 fold over other large world rivers this enrichment is most likely anthropogenic and may be due to coal burning in the region a simple two box model is used to demonstrate that re concentrations in the black sea are not in steady state with respect to current riverine inputs and authigenic fluxes riverine inputs of u and mo are not as significant perhaps reflecting the fractionation of re from the other elements during a high temperature combustion process re2o7 is volatile above 270 c

palaeoecological evidence for early aragonite dissolution in ancient calcite seas it has been proposed that shallow carbonate seas of upper ordovician and jurassic ages precipitated calcite as the primary calcium carbonate phase1 3 unlike such settings today it is expected that such seas were undersaturated with respect to the more soluble aragonite so that aragonitic invertebrate skeletons would have tended to dissolve here we describe examples in which this seems to have been the case our evidence consists of submarine cemented horizons in which aragonite fossils have been dissolved out to leave open moulds the walls of the moulds are encrusted by contemporaneous organisms confirming the early timing of the aragonite dissolution

experimental evaluation of the influences of biogenic reworking on carbonate preservation in nearshore sediments abstract laboratory experiments using two different species of marine infaunal deposit feeders have shown directly that macrofaunal activities influence sedimentary carbonate preservation feeding burrowing and irrigation increase solute transport and solid phase reaction rates simultaneously leading to more rapid carbonate dissolution than in sediments without fauna these findings help to explain previously reported regular temporal fluctuations of foraminifera assemblages and spatial patterns of carbonate dissolution in nearshore deposits and represent one more example of how bioturbation accelerates elemental cycles in marine sediments

sand pseudomorphs of dinosaur bones implications for non preservation of tetrapod skeletal material in the hartford basin usa abstract early jurassic tetrapod tracks are common in the hartford and adjacent deerfield basins newark supergroup of connecticut and massachusetts usa but skeletal material is rare among the few examples is a set of several bones preserved as natural casts on the base of a slab of arkosic sandstone that probably derived from the portland formation of middletown connecticut this mode of preservation is otherwise unknown in the hartford basin and examination of this specimen suggests a complex taphonomic history the bones were fluvially transported and arranged in a v shaped pattern as they were deposited on a muddy substrate longitudinal cracks gouging breakage rounding and invertebrate boring indicate significant alteration prior to preservation as casts furthermore the boring represents the first record of osteophagy in the hartford basin details of the cast preservation such as sediment rims that wrapped up and over some of the bones suggest that the bones were dissolved in the subsurface prior to lithification of the overlying sand bed the decrease in ph that led to bone dissolution probably resulted from the infiltration of naturally acidic rainwater root respiration and the decomposition of plants under oxidative conditions may have contributed as well the depauperate skeletal fossil record of the hartford basin is attributable to dissolution under acidic and oxidative sediment conditions these natural casts offer a unique view of the actions of bone dissolution as a taphonomic process the lack of body fossils in similar aged deposits elsewhere may also be due to groundwater acidity

effects of digital elevation model resolution on derived stream network positions 1 this study examines the effect of digital elevation model dem resolution on the positional accuracy of derived hydrologic networks and quantitatively confirms that dem resolution should be greater than the average hillslope length when used for hydrologic modeling seven hundred kilometers of mapped stream networks are compared to stream networks derived from 17 dems with resolutions ranging from 30 m to 3 km comparison between predicted and mapped streams reveals that accuracy of predicted stream locations decays quickly beyond a dem resolution of 180 m a new application of gyasi agyei et al s 1995 dem resolution suitability test based on average slope and vertical resolution indicates an average hillslope length of between 150 and 180 m tarboton et al s 1991 method of determining average hillslope length based on slope and accumulation areas reconfirms the length to be 150 m and verifies the link between network accuracy and hillslope scale two algorithms are used to derive stream networks the d algorithm tarboton 1997 which allows for flow dispersion and the d8 algorithm o callaghan and mark 1984 which does not a comparison between the d8 and d algorithms shows that modeling flow dispersion is not necessary in steep terrain as both algorithms performed equally well however the d algorithm is found to be less susceptible to modeling erroneous hillside flow convergence making it preferable to the d8 algorithm when used by tarboton et al s 1991 method of determining the average hillslope lengths finally field observation indicates that predicted first and second order streams tend to exist when predicted however mapped streams have a higher positional accuracy

shallow burial diagenesis of skeletal carbonates selective loss of aragonite shell material miocene to recent queensland plateau and queensland trough ne australia implications for shallow cool water carbonates in burial environments carbonate sediments undergo mineralogical stabilization and increasing lithification with depth as yet however little knowledge exists with respect to the corresponding effects on fossil preservation and taphonomic modification of the original sediment composition countings of particles 63 m in miocene to recent periplatform sediments odp leg 133 ne australia exhibit a clear trend of reduction of skeletal aragonite downcore low and high mg calcite grains occur in a continuous order of magnitude over the studied interval 600 m sub bottom depth original microtextures are retained in high mg calcite biota although converted to low mg calcite thus the conversion to low mg calcite appears to occur without introducing a significant quantitative bias aragonite skeletons pelagic gastropods however exhibit a successive exposure of deeper crystal layers and a chalky preservation with burial depth which we interpret to result from dissolution hints for the originally more numerous existence of aragonite biota exist in soft sediments and chalks by the presence of internal moulds shells replaced by microspar and mouldic porosity in early cemented hardgrounds in deep sections barren of aragonite the number of casts and replaced shells remains unchanged and is insignificant as compared to aragonitic biota probably originally present within the sediment 2 in ooze chalk vs 30 50 of grains in modern periplatform sediments therefore palaeontological information must be significantly biased through selective removal of aragonite rates of preservation and destruction depend on external factors during sediment accretion sedimentation rates clay content total organic carbon content and rates of fluid flow within the sediment these observations are relevant with respect to the diagenetic potential and patterns of fossil preservation in little cemented calcitic cool water carbonates because they may originally have contained more aragonite biota as important constituents of an ecosystem than is commonly suspected and calcite aragonite ratios in ancient carbonate sediments may not necessarily reflect original input signals climate or sea level

missing molluscs field testing taphonomic loss in the mesozoic through early large scale aragonite dissolution as a field test for cherns and wright9s previously published hypothesis based on paleozoic faunas that the paucity of certain molluscan groups in the fossil record may be largely a taphonomic not ecologic effect a typical mesozoic molluscan dominated fauna is compared with its early silicified lagerstatten equivalent from a similar depositional setting a lower jurassic offshore carbonate ramp facies in south wales comprises alternating thin limestones and organic rich shales the former are largely diagenetic and carry marine isotopic signatures their typical fauna is mainly well preserved calcitic and bimineralic calcite aragonite epifaunal and semiinfaunal bivalves evidence of aragonitic shells is normally restricted to molds common in inner ramp settings in high energy skeletal grainstones but mostly poorly preserved and restricted to deep burrowing bivalves in offshore deposits however locally early silicification in offshore facies has preserved much more diverse faunas with abundant formerly aragonitic molluscs including many more infaunal burrowers it appears that major aragonite dissolution normally distorted both apparent diversity 65 decrease in bivalve diversity and the trophic structure of the offshore facies providing aragonite that probably sourced the diagenetic carbonates we suggest that aragonitic shells were selectively dissolved in the upper sediment column in lower energy settings where high organic contents favored microbial decay and acidity such early dissolution was absent from the higher energy facies that originally had low organic contents taphonomic loss through early skeletal aragonite dissolution was an equally important process in mesozoic offshore shelf environments and although still leaving depleted molluscan dominated faunas resulted in a massive distortion of diversity

assessment of drainage network extractions in a low relief area of the cuvelai basin namibia from multiple sources lidar topographic maps and digital aerial orthophotographs abstract accurate delineation of drainage networks dns is crucial for hydrological or hydraulic modelling and the comprehension of fluvial processes this task presents challenging aspects in complex lowland terrains with subtle relief and particularly for data poor areas like the cuvelai river basin crb namibia where the present study takes place in the crb standard methods of drainage network extraction from low resolution gridded digital elevation models dems are unsuitable hence airborne light detection and ranging lidar solutions have been utilized however lidar also presents challenges to large areal applications especially with a surface roughness exceeding the capacity of numerous algorithms indeed lidar based dems 2 and 50 m resolutions need to be hydrologically corrected and smoothed to enable the extraction of scale relevant geomorphologic features such as dns in the present contribution channels from topographic maps blue lines were compared to those from hydrologically corrected and uncorrected lidar dems heads up digitized channels from high resolution digital aerial orthophotographs field mapped channels and auxiliary data the maximum gradient deterministic eight d8 gis algorithm was applied to the corrected and uncorrected lidar dems using two network extraction methods area threshold support and curvature drop analysis different progressive flow accumulation threshold values 12 were used to delineate channels with these methods validation was performed between the field mapped channels the modelled channels and those derived from multiple sources additionally spatial and quantitative analyses were performed on geomorphologic parameters and indices the results have shown that hydrologically corrected lidar dems offer useful details for identifying low order stream segments in headwaters while blue lines derived from the national hydrography datasets for watersheds located in elevated and low lying areas of the study area underestimated total stream lengths for field mapped channels by 15 3 and 88 5 respectively this study also confirmed that dns can be extracted from complex low terrain areas with standard gis algorithms and accurate field data the results will aid national mapping agencies in data poor regions to modernize their national hydrography datasets and to account for changing land surface conditions that can affect channel spatial arrangements over time

the dissolution kinetics of major sedimentary carbonate minerals abstract among the most important set of chemical reactions occurring under near earth surface conditions are those involved in the dissolution of sedimentary carbonate minerals these minerals comprise about 20 of phanerozoic sedimentary rocks calcite and to a significantly lesser extent dolomite are the major carbonate minerals in sedimentary rocks in modern sediments aragonite and high magnesian calcites dominate in shallow water environments however calcite is by far the most abundant carbonate mineral in deep sea sediments an understanding of the factors that control their dissolution rates is important for modeling of geochemical cycles and the impact of fossil fuel co 2 on climate diagenesis of sediments and sedimentary rocks it also has practical application for areas such as the behavior of carbonates in petroleum and natural gas reservoirs and the preservation of buildings and monuments constructed from limestone and marble in this paper we summarize important findings from the hundreds of papers constituting the large literature on this topic that has steadily evolved over the last half century our primary focus is the chemical kinetics controlling the rates of reaction between sedimentary carbonate minerals and solutions we will not attempt to address the many applications of these results to such topics as mass transport of carbonate components in the subsurface or the accumulation of calcium carbonate in deep sea sediments such complex topics are clearly worthy of review papers on their own merits calcite has been by far the most studied mineral over a wide range of conditions and solution compositions in recent years there has been a substantial shift in emphasis from measuring changes in solution composition to determine batch reaction rates to the direct observation of processes occurring on mineral surfaces using techniques such as atomic force microscopy afm however there remain major challenges in integrating these two very different approaches a general theory of surface dissolution mechanisms currently lacking although see lasaga and luttge science 291 2001 2400 is required to satisfactorily relate observations of mineral surfaces and the concentration of dissolved components studies of aragonite high magnesian calcites magnesite and dolomite dissolution kinetics are much more limited in number and scope than those for calcite and provide at best a rather rudimentary understanding of how these minerals are likely to behave in natural systems although the influences of a limited number of reaction inhibitors have been studied probably the greatest weakness in application of experimental results to natural systems is understanding the often profound influences of foreign ions and organic matter on the near equilibrium dissolution kinetics of carbonate minerals

taphonomy on the continental shelf and slope two year trends gulf of mexico and bahamas abstract the shelf and slope experimental taphonomy initiative was established to measure taphonomic rates in a range of continental shelf and slope environments of deposition eods over a multiyear period we deployed experiments on the forereef slope off lee stocking island bahamas and on the continental shelf and slope of the gulf of mexico for 2 yr in 18 distinctive eods at depths from 15 to 530 m overall most shells deployed at most sites had relatively minor changes in shell condition most eods generated relatively similar taphonomic signatures a few sites did produce taphonomic signatures clearly distinguishable from the central group and these sites were characterized by one or more of the following high rates of oxidation of reduced compounds presence in the photic zone and significant burial and exhumation events thus unique taphonomic signatures are created by unique combinations of environmental conditions that include variables associated with regional gradients such as depth and light and variables associated with edaphic processes such as the seepage of brine or petroleum or the resuspension and redeposition of sediment most sites however showed similar taphonomic signatures despite the variety of eod characteristics present suggesting that insufficient time had elapsed over 2 yr to generate a more diverse array of taphonomic signatures discoloration and dissolution were by far the dominant processes over the 2 yr deployment period periostracum breakdown loss of shell weight and chipping and breakage was less noticeable eods were chosen based on the expectation that the process of burial and the influence of depth and sediment type should play the greatest roles in determining between eod differences in taphonomic signature eod specific edaphic factors often overrode the influence of geographic scale environmental gradients taphonomic alteration was greater on hardgrounds and in brine exposed sites than on terrigenous muds dissolution was less effective at sites where burial was greatest discoloration occurred most rapidly at shallower sites and on hardgrounds water depth was less influential in determining taphonomic signature than burial state or sediment type the limited influence of water depth is likely due to the presence of shallow sites that for one reason or another were protected from certain taphonomic processes and deeper sites that were characterized by unusually strong taphonomic signals

precise re os ages of organic rich mudrocks and the os isotope composition of jurassic seawater rhenium and osmium isotope and abundance data have been obtained on precisely located samples from three suites of immature organic rich mudrocks from jurassic coastal outcrops in england the data provide accurate whole rock ages of 207 12 ma 181 13 ma and 155 4 3 ma for suites of hettangian toarcian exaratum subzone and kimmeridgian sensu anglico wheatleyensis subzone samples these new re os ages are indistinguishable within the assigned analytical uncertainties from interpolated depositional ages estimated from published geological timescales and establish the importance of the re os dating technique for chronostratigraphic studies early diagenetic pyrite nodules possess levels of re and os which are 1 2 orders of magnitude lower than in the enclosing organic rich mudrocks indicating that these elements had already been removed from sediment pore waters at the time of nodule formation thus the re os isotope system in these organic rich mudrocks has been closed since or from very soon after the time of sediment deposition because most of the re 98 and os 95 99 8 in the mudrocks is shown to be hydrogenous the 187os 188os i of the samples is interpreted to be that of contemporaneous seawater the data thereby provide the first estimates of the os isotope composition of jurassic seawater during the earliest jurassic hettangian the seawater 187os 188os ratio was extremely unradiogenic 0 15 it had increased to 0 8 at the end of the early jurassic toarcian 20 ma later while in the late jurassic kimmeridgian the seawater 187os 188os ratio was 0 59 the most likely explanation for the unradiogenic os isotope composition of hettangian seawater is that the contribution of unradiogenic os to the oceans from the hydrothermal alteration of oceanic crust greatly exceeded the input of radiogenic os from the continents at that time this interpretation is in line with observations suggesting that global weathering rates were low in the hettangian and that increased hydrothermal and volcanic activity preceded the break up of pangea the re os ratios of hettangian mudrocks and by inference of contemporaneous seawater are similar to those of mudrocks deposited at later times during the jurassic and argues against the unradiogenic os in hettangian seawater being derived from extraterrestrial meteoritic sources

the prediction of hillslope flow paths for distributed hydrological modelling using digital terrain models the accuracy of the predictions of distributed hydrological models must depend in part on the proper specification of flow pathways this paper examines some of the problems of deriving flow pathways from raster digital terrain data in the context of hydrological predictions using topmodel distributed moisture status is predicted in topmodel on the basis of spatial indices that depend on flow path definition the sensitivity of this index to flow path algorithm and grid size is examined for the case where the surface topography is a good indicator of local hydraulic gradients a strategy for the case where downslope subsurface flow pathways may deviate from those indicated by the surface topography is described with an example application

basinal restriction black shales re os dating and the early toarcian jurassic oceanic anoxic event 1 profiles of mo total organic carbon toc through the lower toarcian black shales of the cleveland basin yorkshire united kingdom and the posidonia shale of germany and switzerland reveal water mass restriction during the interval from late tenuicostatum zone times to early bifrons zone times times which include that of the putative early toarcian oceanic anoxic event the degree of restriction is revealed by crossplots of mo and toc concentrations for the cleveland basin which define two linear arrays with regression slopes ppm of 0 5 and 17 the slope of 0 5 applies to sediment from the upper semicelatum and exaratum subzones this value which is one tenth of that for modern sediments from the black sea mo toc regression slope 4 5 reveals that water mass restriction during this interval was around 10 times more severe than in the modern black sea the renewal frequency of the water mass was between 4 and 40 ka the mo toc regression slope of 17 applies to the overlying falciferum and commune subzones the value shows that restriction in this interval was less severe and that the renewal frequency of the water mass was between 10 and 130 years the more restricted of the two intervals has been termed the early toarcian oceanic anoxic event but is shown to be an event caused by basin restriction local to nw europe crossplots of re os and mo against toc show similar trends of increasing element concentration with increase in toc but with differing slopes together with modeling of 187 os 188 os and d 98 mo the element toc trends show that drawdown of re os and mo was essentially complete during upper semicelatum and exaratum subzone times mo toc regression slope of 0 5 drawdown sensitized the restricted water mass to isotopic change forced by freshwater mixing so that continental inputs of re os and mo via a low salinity surface layer created isotopic excursions of up to 1 3 in d 98 mo and up to 0 6 for 187 os 188 os restriction thereby compromises attempts to date toarcian black shales and possibly all black shales using re os chronology and introduces a confounding influence in the attempts to use d 98 mo and initial 187 os 188 os for palaeo oceanographic interpretation

review and new aspects concerning the formation of eastern mediterranean sapropels abstract in this paper it is proposed that the formation of eastern mediterranean sapropels occurred in an anti estuarine type of circulation which was to some degree weakened relative to the present in response to reduction of the eastern mediterranean excess of evaporation over freshwater input this reduction of excess evaporation would have been imposed by intensifications of 1 the indian ocean summer sw monsoon influencing the eastern mediterranean via increased nile discharge and 2 the system of mediterranean depressions an element of the westerly atlantic system causing increased precipitation and decreased evaporation both the indian ocean summer monsoon and the westerly atlantic system would be intensified in response to the occurrence of distinct minima in the cycle of precession it is demonstrated that reduced excess evaporation whether or not coinciding with global phases of deglaciation would lead to reduction of surface water salinities in the eastern mediterranean causing isolation of previously formed high salinity cooler deep water thus mixing was severely reduced possibly even restricted to eddy diffusion with only occasional convective events that because of the existing density gradient hardly ever would reach the deepest parts of the basin the consequently diminished oxygen advection down from a few hundred meters in the water column favoured preservation of the sinking organic matter this would however suffice only to enable the formation of sapropels with low organic carbon contents the high organic carbon contents observed in various sapropels are argued to reflect superimposed increases of export production the described scenario accounts for previously reported 1 increases of organic carbon content with increasing depth of deposition within individual sapropels and 2 asymmetrical sequences of sapropel deposition characterized by gradual build up and rather abrupt ending it is furthermore in agreement with 3 isotopic and faunal reconstructions of the history of exchange transports through both the strait of sicily and the strait of gibraltar and 4 faunal and floral reflections of the presence or absence of a distinct deep chlorophyll maximum with its associated increases in export production indicating the presence or absence of a shallow pycnocline within the euphotic layer moreover the described scenario 5 is in no way conflicting with the reports of sapropels in the western mediterranean and 6 seems to be endorsed by occasionally intercalated intervals suggestive of somewhat improved oxygenation amidst anoxic benthic desert levels a situation that has been observed in a few quaternary and pliocene sapropels

molybdenum isotope evidence for global ocean anoxia coupled with perturbations to the carbon cycle during the early jurassic relatively brief periods of severe paleoenvironmental change during the jurassic and cretaceous were associated with the widespread accumulation of organic rich marine deposits termed oceanic anoxic events oaes these intervals involved abrupt global warming of 5 10 c higher rates of continental weathering elevated extinction rates and large scale perturbations to the global carbon cycle the major oaes also overlapped temporally the emplacement of large igneous provinces however despite being known as oaes the extent of seawater anoxia at those times is undefined and the causative processes remain unclear here we show how changes in seawater molybdenum isotope ratios a proxy for seawater anoxia during the toarcian early jurassic oae define the onset and expansion of oxygen deficient conditions our data also place constraints on the areal extent of marine anoxia during the event and demonstrate that anoxia expanded and contracted periodically broadly in line with precession driven changes in 13 corg despite their intermittent occurrence over geological history oaes have an important contemporary relevance because the magnitude and high rates of environmental change then were broadly similar to those occurring at the present day