deep sea carbonates reading the carbon isotope signal the carbon isotope signal in deep sea sediments reflects a mix of 1 global changes in the rates of exchange of the ocean s carbon reservoir with biosphere soil and sediments 2 global and regional changes in surface water productivity 3 internal shifts in water mass structure and circulation basin basin fractionation oxygen minimum development and 4 organism specific fractionation effects due to changes in micro habitat and or ontogenic fractionation vital effects additional complications arise from differential preservation it is impossible to entirely isolate these various factors as a rule of thumb long period signals that are parallel for planktonic and benthic data reflect external global fractionation patterns whilst short period signals are more likely tied to internal patterns water mass fractionation the various approaches to interpretation are illustrated with three case studies the glacial holocene transition the messinian carbon shift and the miocene monterey excursion

reconstruction of palaeosalinity using carbon isotopes and benthic associations a comparison carbon and oxygen isotopes were determined on 40 recrystallized shells of late jurassic bivalves from the lusitanian basin of portugal in contrast with the oxygen isotopes which exhibited considerable diagenetic distortion the carbon isotopes are thought to preserve a record of the salinity of the jurassic marginal marine seas in which these bivalves lived the reconstructed palaeosalinities range from 35 o euhaline to 5 oligohaline comparing these values with the palaeosalinity reconstructed from a palaeoecological analysis of 17 stratigraphic levels within the basin the independently derived values agree in most cases strongly differing values are explained as being due to biotic factors and to diagenetic distortion of the isotopic signal they are less likely to be due to smallscale time averaging or insufficient microstratigraphic sampling on the whole the carbon isotope analyses are thought to produce reasonable palaeosalinity values although data from infaunal originally aragonitic bivalves appear to be less reliable than those from epifaunal bivalves with a predominantly or exclusively calcitic shell as diagenetic alteration of the carbon isotope signal is however unpredictable and biotic effects on the isotopic composition are insufficiently known palaeosalinity reconstructions based on stable isotope data should be supported by palaeoecological data

microfacies and geochemistry of middle jurassic algal limestones from scotland bathonian algal limestones from the duntulm formation of the great estuarine group western scotland are subdivided into three microfacies cryptalgal laminites thrombolites and porostromate micrites the distribution of early diagenetic carbonate spherulites and cement fringes is fabric selective spherulites are common in the thrombolitic microfacies whereas cement fringes were best developed upon algal nodules with porostromate microstructure syndiagenetic micrites spherulites and cement fringes have stable carbon isotopic ratios indicative of abundant organic derived hco3 incorporation d13c values of 0 05 to 15 enhanced magnesium concentrations in these fabrics relative to neomorphic and burial diagenetic spar cements suggest an original mg calcite mineralogy probably with precipitation under the influence of microbial systems brecciated early diagenetic fabrics and localized pockets of calcite pseudomorphs after gypsum were probably formed during subaerial exposure sulphate supply was probably from storm derived saline lagoon water washed on to supralittoral stromatolitic flats these inundations also allowed the temporary establishment of green algae and encrusting foraminifera the combined geological and geochemical information suggests a palaeoenvironment somewhat similar to recent sub tropical humid zone calcareous algal marshes

neotectonics geodesy and seismic hazard in the northern walker lane of western north america thirty kilometers of crustal shear and no strike slip article i nfo roughly 30 km of cumulative right lateral crustal displacement and 5 6 mm yr of the ongoing relative right lateral motion between the pacific and north american plates are observed in the northern walker lane the right lateral shear has been accommodated in large part by the development of a set of discontinuous en echelon normal fault bounded basins and perhaps significant vertical axis rotations of the intervening crust the observations provide an illustrative example of how large amounts of crustal shear may be accom modated in the absence of strike slip faults and point to difficulties attendant to melding geologic and geo detic observations in the analysis of seismic hazard in this particular case the assumption that all geodetically observed shear across the area will be recorded by earthquake displacements may be flawed

geographic and hydrodynamic characteristics of shallow coastal lagoons abstract coastal lagoons can be conveniently separated based on geomorphology into leaky restricted or choked systems this subdivision orders coastal lagoons hydrodynamically according to water exchange with the coastal ocean leaky systems have the shortest turnover time restricted lagoons have a longer turnover time and choked lagoons have the longest turnover time in leaky coastal lagoons oscillating tidal currents account for most of the flow variance and mask residual currents due to wind and freshwater induced density gradients in restricted lagoons tides winds and freshwater runoff drive components of the circulation winds are usually dominant in causing circulation and mixing in choked lagoons and water exchange with the coastal ocean largely depends on the hydrologic cycle coastal lagoons are usually well mixed but can exhibit significant horizontal gradients due to freshwater runoff and vertical gradients due to solar radiation or surface cooling in general river inflow and heat balance induce stratification in coastal lagoons whereas winds and tides cause mixing coastal lagoons usually trap inorganic sediment and organic matter thus serving as material sinks on a geologic time scale they are short lived landscape features with an existence intrinsically linked to filtering efficiency and relative sea level change as a result of glacioeustatic cycles and local tectonic activity

calcite cementation during bacterial manganese iron and sulphate reduction in jurassic shallow marine carbonates faunally restricted argillaceous wackestones from the middle jurassic of eastern england contain evidence of early diagenetic skeletal aragonite dissolution and stabilization of the carbonate matrix closely followed by precipitation of zoned calcite cements and precipitation of pyrite distinctive cathodoluminescence and trace element trends through the authigenic calcites their negative 13c compositions and the location of pyrite in the paragenetic sequence indicate that calcite precipitation took place during sequential bacterial mn fe and sulphate reduction calcite 18o values are compatible with cementation from essentially marine pore fluids although compositions vary owing to minor contamination with 18o depleted late cements mg and sr concentrations in the calcites are lower than those in recent marine calcite cements this may be a result of kinetic factors associated with the shallow burial cementation microenvironments bicarbonate for sustained precipitation of the authigenic calcites was derived largely from aragonite remobilization augmented by that produced through anaerobic organic matter oxidation in the metal and sulphate reduction environments aragonite dissolution is thought to have been induced by acidity generated during aerobic bacterial oxidation of organic matter distinction of post oxic metal reduction and anoxic sulphate reduction diagenetic environments in modern carbonate sediments is uncommon outside pelagic settings and early bacterially mediated diagenesis in modern platform carbonates is associated with extensive carbonate dissolution high detrital fe contents of the jurassic sediments and their restricted depositional environment were probably the critical factors promoting early cementation these precipitates constitute a unique example of calcite authigenesis in shallow water limestones during bacterial mn and fe reduction

the sedimentary facies of a late bathonian regressive episode the kilmaluag and skudiburgh formations of the great estuarine group inner hebrides scotland the kilmaluag and skudiburgh formations of the bathonian great estuarine group represent a unique palaeoenvironment within the british jurassic the kilmaluag formation is divided into two distinctive facies the clastic facies of n skye comprises calcareous mudstones and shales with subordinate sandstones and argillaceous limestones the depositional setting was one of shallow ephemeral lagoons closed from the sea sublittoral lagoon mudflat and channel environments are identified and fossil faunas indicate low salinity the argillaceous limestone facies of southern skye eigg and muck formed in a similar depositional setting although sandstones are not present alternations of carbonate rocks some dolomitic and calcareous shales probably reflect palaeoclimatic fluctuations between humid and arid periods the overlying skudiburgh formation is interpreted as being alluvial floodplain channel and overbank deposits are identified by the presence of mottled mudstones sands and lensoidal silts syndepositional carbonate nodules are interpreted as calcrete which occasionally coalesced to form extensive hardpan calcrete horizons the vertical facies sequence reflects a late bathonian regression the coastal plain terrestrial environments of the skudiburgh formation were transgressed during the callovian stage an event which ended paralic deposition in the hebridean jurassic

global permittivity mapping of the martian surface from sharad abstract sharad is a subsurface sounding radar aboard nasa s mars reconnaissance orbiter capable of detecting dielectric discontinuities in the subsurface caused by compositional and or structural changes echoes coming from the surface contain information on geometric properties at metre scale and on the permittivity of the upper layers of the martian crust a model has been developed to estimate the effect of surface roughness on echo power depending on statistical parameters such as rms height and topothesy such model is based on the assumption that topography can be characterized as a self affine fractal and its use allows the estimation of the dielectric properties of the first few metres of the martian soil a permittivity map of the surface of mars is obtained covering several large regions across the planet surface the most significant correspondence with geology is observed at the dichotomy boundary with high dielectric constant on the highlands side 7 to over 10 and lower on the lowlands side 3 to 7 other geological correlations are discussed

the geochemistry of early diagenetic dolostones from a low salinity jurassic lagoon in the middle jurassic great estuarine group of nw scotland argillaceous dolostones and dolomitic limestones occur interbedded with limestones and mudstones of low salinity 9closed lagoon9 palaeoenvironments these microcrystalline dolostones are imperfectly ordered and calcium rich and have escaped pervasive dissolution reprecipitation processes during burial diagenesis geochemical studies show that the dolostones retain a significant early diagenetic memory 18 o dol values 0 o are the heaviest reported from the great estuarine group and probably formed by evaporative fractionation from meteoric water 13 c dol values 2 o may represent precipitation from a standing body of meteoric water partially equilibrated with atmospheric co 2 calcite coexisting with the dolomite is lighter in both 13 c and 18 o by 0 8 and 3 7 o respectively it is suggested that the minerals were not co precipitates from one solution strontium is preferentially concentrated in the dolomite with 87 sr 86 sr values between 0 7095 and 0 7099 these values which are slightly more radiogenic than jurassic seawater may originate from the continental weathering of ancient limestones mixed with seawater derived from the compaction of underlying sediments sm nd model ages are consistent with weathering of dalradian rocks in the drainage hinterland dolomite formation proceeded by dolomitization of a caco 3 precursor in a warm seasonal climate during dry arid seasons the lagoon water evaporated concentrating 18 o raising mg ca and sr ca ratios and thereby promoting dolomitization during humid wet periods sr poor caco 3 precipitated with lighter 18 o and 13 c values

depletion of 13c in seawater σc02 on modern carbonate platforms significance for the carbon isotopic record of carbonates seawater co2 from modern carbonate platforms bahama banks and florida is depleted in 13c by as much as 4 relative to open ocean water depletion in 13c is caused by isotopically light co2 input from respiration of marine and terrestrial organic matter during water mass residence on the bank as such depletion in 13c is related to changes in water chemistry driven by evaporation freshwater discharge and caco3 withdrawal deviation of modern platform seawater 13c values away from those of surface seawater suggests that 13c values of ancient cratonic carbonate must be cautiously interpreted initial depositional environment as well as alteration during diagenesis must be considered in extracting valid secular trends for oceanic 13c variation

isotope paleontology growth and composition of extant calcareous species abstract isotope paleontology uses the isotopic composition of fossil remains of organisms to make inferences about the physical surroundings of growth of the organisms especially temperature and to obtain clues about life history and modes of growth in calcareous fossils oxygen isotopes are mainly used in the former and carbon isotopes in the latter however since physical surroundings and organism response are intimately associated both types of information are contained in each of the isotopic signals to explore the potential of isotope paleontology and to provide a basis for reconstruction a broad range of extant organisms has been studied taking the pioneering work of epstein and associates as a starting point results are summarized for a representative sampling of these studies with emphasis on work at the laboratories of the authors from the mid seventies to the present the organisms considered are nannoplankton benthic algae planktonic and benthic deep sea foraminifera larger foraminifera sponges corals bryozoans polychaetes arthropods bivalves gastropods cephalopods and vertebrates fish otoliths the survey broadly suggests that regarding oxygen isotopes materials tend to be precipitated close to equilibrium with the surrounding seawater as postulated by urey and that for carbon isotopes disequilibrium is the rule life spans growth rates differential seasonal growth and age of reproductive activity can be extracted under favorable circumstances from individual shells and skeletal parts in detail the interpretation of isotope records of individual shells is quite complicated and simple models will fail to give satisfactory results in many or most cases

high northern palaeolatitude jurassic cretaceous palaeotemperature variation new data from kong karls land svalbard abstract the traditional view of warm equable global climates in the mesozoic with weakly developed latitudinal temperature gradients has been challenged by several recent studies however reliable high palaeolatitude palaeotemperature data is still rare in this study palaeotemperature data based on oxygen stable isotope palaeothermometry of well preserved mid jurassic to lower cretaceous belemnites from kong karls land svalbard are presented these data show cool high latitude marine isotopic palaeotemperatures for endemic species during the lower to mid valanginian 7 7 c which may be compatible with the formation of high latitude ice middle bathonian to kimmeridgian samples give warmer isotopic palaeotemperatures 9 4 c whilst samples from the aalenian to bajocian give isotopic palaeotemperatures which are warmer still 12 7 c comparison of the callovian temperatures with those from a mid latitude location shows them to be warmer than would be expected if previous latitudinal temperature gradients are correct this may be due to the ameliorating effect of oceanic heat transport pole wards from the panthalassa ocean via the south anyui ocean

ecological and paleoenvironmental information using stable isotope profiles from living and fossil molluscs stable oxygen and carbon isotopic profiles from recent specimens of spisula solidissima and placopecten magellanicus collected off the virginia coast are used to calibrate annual temperature controlled cycles in the 18o profile and shell growth characteristics the amplitude of the annual 18o cycles decreases with increasing water depth and may be used as a relative depth indicator the isotopic of profiles of two fossil specimens of pleistocene spisula are very similar in amplitude and character to modern analogues and verify the formation of annual growth increments in these fossil specimens offsets between the 18o profiles of the fossil and modern specimens are related to known temperature and ice volume effects during the late pleistocene and support the utility of mollusc isotope records as a stratigraphic tool carbon isotopic profiles from all specimens show light values in the spring and or fall associated with seasonal phytoplankton productivity offsets in the carbon isotopic composition between infaunal and epifaunal species may be related to sediment porewater geochemistry and oxidation of organic matter trends toward lighter 13c values through ontogeny suggest the effects of metabolic changes from a fast growing juvenile into a slower growing sexually mature adult anomalies in both the oxygen and carbon isotopic profiles identify the formation of minor shell growth increments as corresponding to hydrographic perturbations and to winter cold temperatures

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

oxygen and carbon isotope study of the holocene oyster reefs and paleoenvironmental reconstruction on the northwest coast of bohai bay china abstract the northwest coast of bohai bay supports large areas of holocene crassostrea gigas oyster reefs their huge size and the fact that the shells are composed of consecutive microgrowth layers consisting of primary calcite make them an ideal object for a stable isotope study the 18o and 13c compositions of the oysters reveal a number of interesting features the most important are annual cycles related to seasonal variations the regional 18o water versus salinity ratio and a stop in shell secretion below a water temperature of 11 5 1 5 c a positive correlation covariance between 18o and 13c values is present in most specimens the 18o and 13c values of single specimens are believed to be mainly controlled by water temperature and the occurrence of phytoplankton blooms and decomposition however correlation among oysters of different ages indicate that the 18o and 13c values are mainly dependant of three variables water temperature salinity and phytoplankton blooms decomposition between ca 6500 yrs cal b p and 2300 yrs cal b p the salinity in bohai bay increased from less than 20 to 30 based on geological data combined with ecological data from c gigas we suggest that precipitation decreased during this period while the input of fluvial sediments increased the sediment load of rivers increased markedly after ca 4000 yrs cal b p possibly due to a change in climate in our study area this resulted in the development of the oyster plain

a cored jurassic sequence from north lincolnshire england stratigraphy facies analysis and regional context a virtually complete jurassic sequence recently drilled in north lincolnshire is considered in a regional context liassic thicknesses and facies are largely unexceptional the frodingham and pecten ironstones are relatively thin and sandy but the marlstone rock bed is thicker and more ferruginous than elsewhere in the region the inferior oolite is exceptionally thick and exhibits strong deltaic influence equivalents of the gristhorpe member and scarborough formation of yorkshire may be present the great oolite was deposited in coastal lacustrine deltaic environments more proximal than to the south or east thin kellaways clay separates the argillaceous cornbrash from the kellaways sand the organic rich lower oxford clay is thin owing to a basal non sequence whilst the upper oxford clay is condensed and less organic than in the east midlands the regressive west walton beds are followed by very thick ampthill and kimmeridge clay sections beneath the spilsby sandstone regional thickness and facies analysis records the behaviour of the market weighton block and reveals the influence of the pennine high as both a positive tectonic feature and an occasionally active sediment source

evolution of early cenozoic marine temperatures the equator to high southern latitude sea surface and vertical temperature gradients are reconstructed from oxygen isotope values of planktonic and benthic foraminifers for the following five time intervals late paleocene early eocene early middle eocene late eocene and early oligocene paleotemperatures are calculated using standard oxygen isotope temperature equations with adjustments to account for 1 variations in sea water 18o related to changes in global ice volume over time and 2 latitudinal gradients in surface water 18o these reconstructions indicate that sea surface temperatures sst of the southern oceans in the early eocene were as high as 15 c whereas temperatures during the late paleocene and early middle eocene reached maximum levels of 10 12 c by the late eocene and early oligocene high latitude sst had declined to 6 and 4 c respectively for most of the early paleogene low latitude sub tropical temperatures remained constant and well within the range of holocene temperatures 24 ndash 25 c but by the late eocene and early oligocene declined to values in the range of 18 to 22 c the late paleogene apparent decline in tropical temperatures however might be artificial because of dissolution of near surface foraminifera tests which biased sediment assemblages toward deeper dwelling foraminifera moreover according to recent plate reconstructions it appears that the majority of sites upon which the late eocene and early oligocene tropical temperatures were previously established were located either in or near regions likely to have been influenced by upwelling global deepwater temperature on average paralleled southern ocean sst for most of the paleogene we speculate based on the overall timing and character of marine sea surface temperature variation during the paleogene that some combination of both higher levels of greenhouse gases and increased heat transport was responsible for the exceptional high latitude warmth of the early eocene

two stage neomorphism of jurassic aragonitic bivalves implications for early diagenesis abstract aragonitic bivalves in middle jurassic paralic limestones from lincolnshire eastern england have been neomorphically replaced by inclusion rich calcite spar textural features of neomorphic spar are analogous to examples from other calcitized jurassic molluscs but lincolnshire bivalves show evidence for two distinct stages of replacement staining cathodoluminescence and backscatter scanning electron microscopy reveal an initial volumetrically minor and frequently fabric selective replacement by nonferroan calcite with most of the remaining skeletal aragonite subsequently replaced by ferroan calcite spar stable isotope data from individual bivalves define a negative covariant mixing trend ranging from early relatively 13c depleted and 18o enriched nonferroan calcite d13c 2 pdb to ferroan calcite with composition similar to associated late pore filling cements d13c 2 pdb d1 o 7 pdb the early diagenetic end member precipitated from depositional pore fluids with bicarbonate supplied both from dissolution of skeletal aragonite and bacterial oxidation of organic material in contrast ferroan neomorphic spar precipitated during burial diagenesis and dewatering of shales surrounding the limestone bulk trace element signatures of the bivalves are dominated by late ferroan calcite but electron probe microanalysis and intrinsic blue cathodoluminescence shows that early neomorphic calcite contains negligible amounts of fe mn and mg together with isotopic and fabric data this indicates that early neomorphism occurred at shallow depths and may even have begun at the sediment water interface in the absence of through flowing meteoric waters early replacement did not proceed via an inward migrating neomorphic front instead calcitization was initiated at numerous individual reaction centers throughout the shells and was principally controlled by the distribution of the skeletal organic matrix this contrasts strongly with most previous reports of aragonite neomorphism during arly diagenesis

are oxygen and carbon isotopes of mollusc shells reliable palaeosalinity indicators in marginal marine environments a case study from the middle jurassic of england stable isotope analyses of non luminescent texturally pristine oyster calcites praexogyra hebridica are presented from bathonian inner ramp limestones in central eastern england a relatively large range of values 18 o 2 0 to 2 3 pdb 13 c 0 9 to 3 7 pdb define a crude positive covariance which can be regionally correlated with an offshore tonearshore increase in faunal restriction this palaeoenvironmental stress gradient has previously been interpreted as a consequence of fluctuating nearshore palaeo salinities due to episodic or seasonal runoff from surrounding hinterlands covariant 18 o 13 c trends from euryhaline bivalves are commonly ascribed to regional salinity gradients the most isotopically depleted examples representing the greatest influence of fresh water however bathonian palaeogeography indictates that the most isotopically depleted oysters inhabited relatively offshore settings assuming an average growth temperature of 25 c these shells were secreted in near normal sea water 18 o 0 4 smow in contrast isotopically heavy oyster calcites were produced in highly evaporated nearshore lagoonal waters 18 o up to 4 8 smow modelling the oxygen isotope and chemical evolution of residual waters during evaporation suggests that up to 65 of the nearshore water mass was lost probably in a complex of hydrodynamically choked lagoons evaporites were not precipitated in the sediment and geochemical modelling suggests that the lagoon waters may have been brackish rather than marine in origin the regional 13 c trend is contrary to that predicted on the basis of palaeosalinity and may have resulted from variations in organic carbon production decay and fixation in early diagenesis

palaeoecology of benthic associations in salinity controlled marginal marine environments examples from the lower bathonian jurassic of the causses southern france abstract lower bathonian sediments of the causses southern france largely represent very shallow low energy carbonates deposited on an extensive carbonate platform environments range from distal oolite shoals to shallow lagoons interto supratidal mudflats brackish bays coastal swamps and freshwater ponds associated benthic macrofauna occurs often in high abundance but low diversity and is dominated by bivalves and gastropods stenohaline elements are rare to absent the fauna can be grouped in 13 associations some of them monospecific and several assemblages some of the associations are clearly substrate controlled especially by grain size and substrate consistency however the main controlling factor appears to have been salinity which fluctuated from zero freshwater to more than 40 hypersaline an analysis integrating sedimentological data microfauna microflora and ecological data of the macrobenthos made it possible to assign the benthic associations to various salinity regimes accordingly the naricopsina matheroni association is restricted to freshwater the eomiodon angulatus myrene raristriata neomiodon ruthenensis and neomiodon ruthenensis placunopsis socialis associations occupied the oligohaline mesohaline regimes the placunopsis socialis association ranged from the upper mesohaline to the lower brachyhaline regime the protocardia stricklandi protocardia buckmani ostrea sp protocardia buckmani bakevellia waltoni and bakevellia waltoni protocardia buckmani associations lived in brachyhaline environments only two associations the pholadomya lirata and the ceratomya striata associations occurred in normal marine water the opportunistic myrene raristriata apparently managed to thrive also in hypersaline environments as is shown by its close association with pseudomorphs after autigenic gypsum and with algal layers the lower bathonian macrofauna of southern france resembles that of other middle jurassic salinity controlled benthic faunas from scotland northern africa western india and tibet except for its scarcity of mytilid bivalves and lack of corbulids