stromatolitic framework of carbonate mud mounds abstract mud mounds are an important phanerozoic reef type generally formed in deeper water on carbonate shelf to basin slopes they commonly contain radiaxial calcite spar filled depositional cavities stromatactis but lack abundant metazoan framebuilders mud mounds are often flanked by steeply dipping bioclastic grainstones and sometimes attained widths and depositional reliefs of over a hundred meters they occasionally occur as basal cores on which developed shallow water metazoan constructed reefs stromatactoid mud mounds are composed of a framework of submarine cemented crudely reticulate masses or a succession of laminar crusts surrounded by cement filled cavities these cavities developed when unconsolidated sediment around the framework was removed by winnowing mud mound frameworks of all ages show strong geometric and petrographic similarities to shallow water cryptalgal structures especially thrombolites unlaminated stromatolites which occur in many phanerozoic shallow water reefs both thrombolites and mud mound frameworks are largely composed of lime mudstone cemented on the sea floor and contain fenestral fabric and diagnostic cryptalgal microstructure their depositional surfaces were irregular not bored or burrowed and rarely encrusted stromatolitic lamina ion although commonly not well developed in the subtidal zone anyway occasionally occurs in mud mounds it is proposed that mud mounds accumulated from organic probably blue green algal binding of locally derived lime mud and bioclasts deposited relatively slowly from suspension patchy distribution of algal mats resulted in areas of both bound and unbound sediment sporadic floodlike episodes of rapid sedimentation however temporarily smothered the living algal mat and also left layers of unbound sediment periods of winnowing washed out unconsolidated unbound sediment and promoted synsedimentary cementation of resulting stromatactoid cavities and algal bound framework

palaeomagnetism of devonian ring complexes from the bayuda desert sudan new constraints on the apparent polar wander path for gondwanaland summary the approach of gondwana and or microplates derived from its northern margins towards laurentia and the closure of the intervening palaeo tethys is a controlling factor in the geodynamic evolution of the atlantic bordering continents during the early and mid palaeozoic however the currently available palaeomagnetic data for gondwana especially for the geodynamically critical silurian to devonian interval are still rather sparse and are compatible with a number of palaeogeographic scenarios in order to address this issue 114 samples 18 sites have been collected from a series of high level acidic intrusive rocks rhyolitic lavas and tuffs of mid devonian age 377 5 ma rb sr along the southern escarpment of the gilif hills sudan 17 83 n 32 67 e detailed demagnetization experiments revealed the presence of three directionally distinct components of magnetization labelled a b and c the overall mean direction of component a dec 0 9 inc 29 8 95 4 6 k 91 1 n 12 sites is indistinguishable from either the direction of the present field or the geocentric axial dipole field in northern sudan mixed polarities suggest a recent or pleistocene age of this component component b also displays mixed polarities the site mean directions for this component average to 6 5 declination 40 2 inclination with 95 7 0 and k 48 1 based on 10 sites and yield a palaeomagnetic pole at 48 8 s 23 5 e dp 5 1 1 dm 8 4 maghaemite and haematite have been identified to be the predominant magnetic phases in samples which carry component a and or b both a and b are the probable results of low temperature oxidation of magnetite although the age of component b can not be directly determined it is inferred by correlation with published data to reflect a late carboniferous overprint component c identified in 11 sites 39 samples is characterized by a very steep downward magnetization dec 296 2 inc 79 3 95 10 8 k 18 7 n 11 sites which corresponds to a palaeomagnetic pole position in the southwestern corner of libya 25 9 n 11 6 e dp 19 6 dm 20 6 maximum blocking temperatures of component c generally below 580 c are indicative of magnetite as carrier of this remanence the results of thermal remanent magnetization experiments demonstrate that component c is a thermo remanent magnetization probably primary and suggest this component to be representative for the direction of the earth s magnetic field during emplacement of the gilif hills volcanic rocks in mid devonian times the position of the resulting palaeomagnetic pole lends new support to geodynamic scenarios advocating the existence of a wide approx 5000 km pre hercynian ocean during devonian times the simplest reconciliation of component b with the reference apparent polar wander path is achieved by assuming it to be of carboniferous age and by invoking a westward cusp in the carboniferous part of the apparent polar wander path taken at face value the age of this cusp coincides with large scale post orogenic right lateral strike slip movements in hercynian europe and north africa this interpretation does not require the apparent polar wander apw loop and very high rates of plate motion implied by some alternative models however if it is valid it will require reinterpretation of the remanence age of one west african intrusive suite and of some eastern australian palaeozoic rocks

textural elemental and isotopic variations among constituents in middle devonian limestones north america abstract biogenic components brachiopods crinoids and corals in middle devonian limestones were manually separated from their enclosing matrix and analyzed by textural elemental and isotopic methods in order to assess the preservational state of these components in general brachiopods in these rocks are better preserved texturally than other constituents as determined by sem and cathodoluminescence observations elemental variations among the major components are compatible with a differential response to diagenesis governed by the relative stability of the original mineralogy of the component however among the most stable low mg calcite brachiopods the relationship between textural preservation and trace element content is complex this apparent complexity may be due at least par ially to our method of textural characterization of whole brachipod samples the nonluminescent portions of a set of well preserved brachipods have textural characteristics and elemental contents which are virtually indistinguishable from modern representatives consequently their 18o values 3 7 0 2 are assumed to represent primary biogenic calcite these 18o values are in the heavy end of the range of values from texturally well preserved whole brachiopods 6 6 to 2 5 texturally altered whole brachiopods 6 7 to 3 7 and texturally altered crinoids and cora s 6 7 to 2 7 this comparison suggests that marine derived pore waters were a predominant component during the principal phase of diagenesis the relatively low l8o values of well preserved brachiopods compared with modern representatives implies that middle devonian seawater was either warmer by about 10 c or depleted in 18o by about 2 3 relative to modern seawater

depth profiles of δ13c in bottom water and core top c wuellerstorfi on the ontong java plateau and emperor seamounts we have measured the carbon isotopic composition of dissolved inorganic carbon in bottom waters of the ontong java plateau western equatorial pacific and on the northern emperor seamounts northwest pacific each of these locations is several hundred miles from the nearest geochemical ocean sections study geosecs stations and the observed 13c values at each site differ substantially from regionally averaged geosecs 13c profiles we discuss the possible causes of these differences including horizontal variability near bottom effects and problems with the pacific geosecs 13c data we also measured the isotopic composition c and o of core top c wuellerstorfi from a depth transect of cores at each location the 18o data are used to verify that our samples are holocene comparison of foraminiferal and bottom water 13c values shows that this species faithfully records bottom water 13c at both sites and demonstrates that there is no depth related artifact in the dissolved inorganic carbon c wuellerstorfi 13c relationship at these sites

temporal spatial reconstruction of the early frasnian late devonian anoxia in nw africa new field data from the ahnet basin algeria anoxic conditions were widespread in nw africa during the early frasnian late devonian that resulted in deposition of organic rich shales and limestones with total organic carbon toc values of up to 14 organic richness and thickness of these sediments vary laterally and organic rich vs organic poor facies boundaries are likely to have been diachronous a precise temporal spatial reconstruction of this anoxic phase in nw africa is complicated because the organic matter in outcrops is largely oxidised and biostratigraphic resolution in boreholes is generally low due to the lack of recoverable conodonts this contribution is based on eight outcrop sections at the margin of the central algerian ahnet basin where detailed spectral gammaray measurements were carried out using a handheld instrument the pre weathering organic richness in frasnian outcrop sections is approximated using the characteristic uranium enrichment in the anoxic facies that based on well studies is positively correlated with the total organic carbon content conodont biostratigraphic results from these sections suggest that the uraniumenriched interval the anoxic interval at the basin margin is most common in the basal frasnian conodont biozones 1 2 confirming previous results from the anti atlas in morocco in three of the eight localities studied the basal frasnian has not been deposited and the frasnian here commences with distinctly younger uranium enriched intervals including zones 4 11 well data from the eastern algerian berkine basin is interpreted to indicate a significantly longer anoxic phase there million yearscale diachroneities of the frasnian anoxia therefore clearly exist across the north africa shelf it is assumed that the palaeorelief might have been a major factor in controlling the onset duration and intensity of anoxia in the region d 2003 elsevier b v all rights reserved

sedimentary evolution of a palaeozoic basin and ridge system the middle and upper devonian of the ahnet and mouydir algerian sahara the ahnet and mouydir regions of southern algeria are part of one of the world s largest almost undeformed exposures of palaeozoic rocks which exemplify a hitherto poorly known early variscan development of a devonian basin and ridge system this area includes a series of intracratonic basins along the northern margin of the west african craton which consists from w to e of the reggane basin azel matti ridge ahnet basin foum belrem ridge and mouydir basin the depositional and palaeogeographic interpretation is based on 71 sections in this region which for the first time were biostratigraphically calibrated by means of conodonts goniatites and brachiopods the structural evolution during devonian times was probably controlled by reactivation of ancient n s to nw se running faults in the precambrian basement which caused differential subsidence and uplift of a previously largely unstructured siliciclastic shelf a hiatus during emsian times indicates widespread emergence during this interval the entire area was flooded during the earliest eifelian when the first vestiges of the azel matti ridge become evident by stratigraphic condensation the palaeogeographic differentiation is most apparent during the givetian when a shoal with reduced carbonate sedimentation was established on the azel matti ridge passing towards the west and east into basinal environments of the reggane and ahnet basins respectively the foum belrem ridge is distinguished by increased subsidence during the early givetian and by revived uplift during the late givetian in the mouydir basin further east up to 1000 m of shales were deposited during the givetian the early frasnian is marked by the ubiquitous sedimentation of black shales and bituminous styliolinites these lithologies occur repeatedly already during the middle devonian and document intermittent anoxic conditions the basin and ridge topography is levelled by the shallowing up sequence of up to 1400 m thick upper frasnian and famennian shales which grade into a deltaic sequence of uppermost famennian tournaisian sandstones the up to now only vaguely discriminated lithostratigraphic formations of the devonian have been biostratigraphically defined in suitable type sections

new architectures of deep water carbonate buildups evolution of mud mounds into mud ridges middle devonian algerian sahara spectacular totally exhumed carbonate mud buildups of givetian age can be found in the southern ahnet basin of the central sahara their geometries range from isolated mud mounds to mud ridges up to 8 km long the latter are the result of lateral coalescence of closely spaced individual mounds mounds up to 40 m high and ridges up to 85 m high show original slopes of 25 to 65 frame builders in the massive core of the buildups are restricted to scattered small tabulate and solitary rugose corals the total absence of stromatoporoids colonial rugose corals and algae as well as the lack of debris at the toes of the buildups shows that they were constructed in deep water below the photic zone and storm wave base the arrangement of the buildups in north to northwest trending clusters and ridges suggests that their distribution was controlled by early variscan extensional movements inherited from precambrian patterns

the relationship between pore water carbon isotopic composition and bottom water oxygen concentration abstract we present pore water 13c profiles from the western north atlantic and the eastern pacific oceans and interpret them using a stoichiometric model of the relationship between carbon isotopic composition and oxygen concentration pore water 13c at the depth in the sediments where oxygen approaches zero is largely determined by the oxygen concentration of the overlying bottom water although sulfate reduction adds a significant amount of isotopically light carbon at some locations the 13c difference between bottom water and pore water at o2 0 13c increases from 1 in sediments of three low bottom water oxygen basins in the southern california borderlands to 4 in sediments underlying well oxygenated water in the western north atlantic we suggest that the relationship between bottom water oxygen and 13c could form the basis for a new way to use the benthic foraminifera 13c record to estimate bottom water oxygen concentration paleo oxygen values derived from a pair of benthic foraminifera species which record the bottom water pore water o2 0 13c difference will be independent of the preformed 13c of the deep water mass

on the structure and origin of major glaciation cycles 1 linear responses to milankovitch forcing time series of ocean properties provide a measure of global ice volume and monitor key features of the wind driven and density driven circulations over the past 400 000 years cycles with periods near 23 000 41 000 and 100 000 years dominate this climatic narrative when the narrative is examined in a geographic array of time series the phase of each climatic oscillation is seen to progress through the system in essentially the same geographic sequence in all three cycles we argue that the 23 000 and 41 000 year cycles of glaciation are continuous linear responses to orbitally driven changes in the arctic radiation budget and we use the phase progression in each climatic cycle to identify the main pathways along which the initial local responses to radiation are propagated by the atmosphere and ocean early in this progression deep waters of the southern ocean appear to act as a carbon trap to stimulate new observations and modeling efforts we offer a process model that gives a synoptic view of climate at the four end member states needed to describe the system s evolution and we propose a dynamic system model that explains the phase progression along causal pathways by specifying inertial constants in a chain of four subsystems solutions to problems involving systems of such complexity are not born full grown like athena from the head of zeus rather they evolve slowly in stages each of which requires a pause to examine data at great lengths in order to guarantee a sure footing and to properly choose the next step victor p starr

deep sea benthic foraminifera and biogenic opal glacial to postglacial productivity changes in the western equatorial pacific the accumulation rate of benthic foraminifera in deep sea sediments is strongly correlated with the productivity of the surface ocean core tops from diverse deep sea settings straddling different export production levels are used in this study for the calibration between the organic carbon flux to the seafloor jsf flux and the benthic foraminifera accumulation rate bfar the correlation can be used to reconstruct organic carbon flux to the seafloor in the past and hence the productivity of past oceans these results are compared with the accumulation rates of carbonate and biogenic opal in the sediments it is found that biogenic opal is more sensitive to productivity changes in the modern ocean than is carbonate the appropriate equations are derived for the equatorial pacific using core material fromeurydice erdc indomed inmd andpleiades plds expeditions these equations are then applied to two selected time horizons in sediments from the ontong java plateau a western equatorial pacific pelagic depth transect the last transition lt and the last glacial lg it is found that productivity was higher during the last glacial relative to the late holocene by a factor between 1 6 and 2 during the last transition productivity was intermediate between glacial and recent values we do not see any corresponding increase in the rates of accumulation of carbonate or biogenic opal during the last glacial this observation questions the importance of ocean productivity as the main controlling factor of carbonate and biogenic opal accumulation rates back through geological time

the influence of microhabitats on the carbon isotopic composition of deep sea benthic foraminifera we report 13c and 18o values for rose bengal stained benthic foraminifera from a set of box cores from continental margin environments in the atlantic and pacific oceans these isotopic results are compared with foraminiferal distribution data and pore water 13c profiles to evaluate the importance of environmental microhabitat effects on the carbon isotopic composition of benthic foraminifera the 13c values of infaunal taxa are consistently lower than those of epifaunal taxa suggesting that microhabitat effects on test composition do exist the 13c differences between foraminiferal carbonate and bottom water dissolved inorganic carbon are not correlated with the 18o differences between benthic foraminifera tests and equilibrium calcite but they do correlate with variations in the chemistry of sediment pore waters however interspecific 18o differences as well as 13c differences between species with similar vertical distributions in the sediments indicate that taxon specific vital effects also influence test composition

13c record of benthic foraminifera in the last interglacial ocean implications for the carbon cycle and the global deep water circulation the 13c 12c ratios of upper holocene benthic foraminiferal tests genera cibicides and uvigerina of deep sea cores from the various world ocean basins have been compared with those of the modern total carbon dioxide tco2 measured during the geosecs program the 13c difference between benthic foraminifera and tco2 is 0 07 0 04 for cibicides and 0 83 0 07 for uvigerina at the 95 confidence level 13c analyses of the benthic foraminifera that lived during the last interglaciation isotopic substage 5e about 120 000 yr ago show that the bulk of the tco2 in the world ocean had a 13c value 0 15 0 12 lower than the modern one at the 95 confidence level reflecting a depletion compared to the present value of the global organic carbon reservoir regional differences in 13c between the various oceanic basins are explained by a pattern of deep water circulation different from the modern one the antarctic bottom water production was higher than today during the last interglaciation but the eastward transport in the circumpolar deep water was lower

conodont thermal alteration patterns in devonian and carboniferous rocks of the ahnet and mouydir basins southern algeria abstract conodont colour alteration indices cai have been used to recognize the thermal maturation of the middle devonian to late carboniferous sedimentary infill of the ahnet and mouydir basins in southern algeria the study is based on a large set of conodont samples obtained from 69 sections and outcrops in the ahnet basin the middle and upper devonian rocks at the surface show a uniform thermal maturity level characterized by the cai values of 2 which suggests maximum burial temperatures about 50 c in contrast the carboniferous exposures yield a wide range of cai values from 1 to 3 5 besides the general tendency in maturity decreases upward in the sequence there are anomalies related to heating by small local magmatic bodies in the mouydir basin the cai values in the middle and upper devonian outcrops range from 2 to 3 5 the pattern indicates that the majority of middle devonian rocks have experienced burial temperatures of about 120 c while thermal maturation of the upper devonian strata is clearly lower with maximum heating temperatures about 50 c at the edges of the basin the observed thermal maturity has been mostly achieved during late carboniferous time prior to the variscan inversion and only locally during the mesozoic it is a product of loading by carboniferous overburden succession of which part is now removed approx 200 1300 m in the ahnet basin and up to 1200 m in the mouydir basin the analysis of palaeotemperatures suggests an average geothermal gradient of 30 c km during carboniferous time in the ahnet mouydir area in terms of hydrocarbon exploration the studied devonian succession is predominantly mature for oil and gas in contrary the carboniferous rocks are immature higher maturities are likely to occur in the subsurface

coccolithophorids on the continental slope of the bay of biscay production transport and contribution to mass fluxes abstract coccoliths collected by sediment traps deployed on the slope of the bay of biscay northeastern atlantic from june 1990 to august 1991 were examined to determine their contribution to the transport of carbonate on a mid latitude continental margin they also were used as tracers of particle transfer processes on this slope two traps located at 1900 m respectively at 2300 mooring site 1 and 3000 m mooring site 2 water depths provided high resolution 4 7 days time series samples covering a 14 month period at ms2 and a 3 month period at ms1 coccoliths from 28 species were identified over the course of the experiment among which emiliania huxleyi was always dominant relative abundance range 59 93 total coccoliths number fluxes were high but variable ranging from 390 106 to 1610 106 coccoliths m 2 day 1 at ms1 and from 58 106 to 1500 106 coccoliths m 2 day 1 at ms2 the time weighted mean flux calculated for the whole experiment at ms2 was 499 106 coccoliths m 2 day 1 estimate of coccoliths minimal contribution to total carbonate flux at 1900 m depth averaged 12 which represented a weighted mean flux of 7 3 mg m 2 day 1 2 7 g m 2 yr 1 lateral transport of coccoliths resuspended from shelf and or upper slope sediments seems to be the dominant transfer process to depth on this northeastern atlantic slope nevertheless the clear seasonal succession observed in the species composition implies that the deposition resuspension transport sequence is rapid presumably less than a few months several short and unsmoothed signals directly issued from coccoliths bloom events also were recorded in our traps a result that indicates rapid settling rates the overall coccolith sedimentation processes appear as being quite diversified but quantitative and qualitative analyses of aggregates collected by the traps suggest that they are important carriers of coccoliths in this margin environment

environmental and biological effects on the stable isotope composition of recent deep sea benthic foraminifera from the western mediterranean sea abstract the influence of microhabitat organic matter flux and metabolism on the stable oxygen and carbon isotope composition of live rose bengal stained and dead empty tests deep sea benthic foraminifera from the gulf of lions western mediterranean sea have been studied the total range of observed foraminiferal isotope values exceeds 1 0 for 18o and 2 2 for 13c demonstrating a wide range of coexisting disequilibria relative to 18o of equilibrium calcite 18oeq and 13c of bottom water dissolved inorganic carbon 13cdic the mean 18o values reveal strongest disequilibria for the studied epifaunal to shallow infaunal species cibicidoides pachydermus uvigerina mediterranea uvigerina peregrina while values approach equilibrium in deep infaunal species globobulimina affinis globobulimina pseudospinescens the mean 13c values decrease with increasing average living depths of the different species thus reflecting a dominant microhabitat pore water signal at the axis of the lacaze duthier canyon a minimum 13cdic pore water gradient of approximately 2 1 is assessed for the upper 6 cm of the surface sediment although live individuals of u mediterranea were found in different depth intervals their mean 13c values are consistent with calcification at an average living depth around 1 cm the deep infaunal occurrence of u mediterranea specimens suggests association with macrofaunal burrows creating a microenvironment with geochemical characteristics similar to the topmost centimeter this also explains the excellent agreement between stable isotope signals of live and dead individuals the ontogenetic enrichment in both 18o and 13c values of u mediterranea suggests a slow down of metabolic rates during test growth similar to that previously observed in planktic foraminifera enhanced organic carbon fluxes and higher proportion of resuspended terrestrial organic material at the canyon axis are reflected by 13c values of u mediterranea on average 0 58 lower than those from the open slope these results demonstrate the general applicability of the 13c signal of this species for the reconstruction of past organic matter fluxes in the mediterranean sea further studies on live specimens are needed for a more quantitative paleoceanographic approach

relationships between the distribution and stable isotopic composition of living benthic foraminifera and cold methane seep biogeochemistry in monterey bay california 1 as part of an ongoing effort to explore the use of foraminifera as a means to assess modern and ancient methane release we compared ambient pore water chemistry with the distribution and stable isotopic composition of living rose bengal stained foraminifera in mbari rov ventana tube cores taken from modern seepage areas about 1000 m water depth in monterey bay california benthic foraminiferal isotopic differences between sites clearly indicate that methane influenced pore waters affect foraminiferal distributions and carbonate isotope geochemistry carbon isotope signatures of living benthic foraminifera did not conform to the very negative 30 to 48 methane influenced carbon isotope values of the pore waters they live in instead the influence of methane seep pore waters was reflected in the greater range and carbon isotopic variability of living seep foraminifera compared with published 13c values of foraminifera living in nonseep habitats it is not clear what relative influences biological ecological and physical factors have on the carbon isotopic signatures observed in seep foraminifera substantial carbon isotope differences can exist between individuals of the same seep species for instance 13c values of living globobulimina pacifica varied by as much as 2 9 between seeps within 8 km of each other whereas 13c values of living uvigerina peregrina varied by as much as 1 95 within the same seep provided there is no diagenetic alteration of the test carbonate isotopic results of individual seep foraminifera support the hypothesis that foraminifera can be used to assess past and present methane seepage

microhabitat preferences and stable carbon isotopes of endobenthic foraminifera clue to quantitative reconstruction of oceanic new production seventeen surface sediment samples from the north atlantic ocean off ne greenland between 76 and 81 n and nine samples from the south atlantic ocean close to bouvet island between 48 and 55 s were taken with the aid of a multiple corer and investigated for their live rose bengal stained benthic foraminiferal content within the upper 15 cm of sediment preferentially endobenthic melonis barleeanum m zaandami and bulimina aculeata as well as preferentially epibenthic lobatula lobatula were counted from 1 cm thick sediment slices each and analyzed for stable carbon and oxygen isotopic compositions of their calcareous tests live and dead specimens were counted and measured separately the carbon isotopic composition of the foraminifera was compared to that of the dissolved inorganic carbon dic of simultaneously sampled bottom water during a period of one month one station off ne greenland was replicately sampled once every week and samples were processed as above live specimens of lobatula lobatula are confined to the uppermost two centimeters of sediment live specimens of melonis spp are found down to eight centimeters within the sediment but with a distinct sub surface maximum between two and five centimeters the down core distribution of live bulimina aculeata shows a distinct surface maximum in the top centimeter and constant but low numbers down to 11 cm subbottom depth the average stable carbon isotopic composition d13c per mil vpdb of live l lobatula off ne greenland is by 0 4 0 1 per mil higher than the d13cdic of the ambient bottom water at the time of sampling there is evidence that this species calcify before the ice free season when bottom water d13cdic is supposed to be higher this would reconfirm the one to one relationship between d13c of ambient water dic and cibicids widely used by paleoceanographers live m barleeanum show a negative offset from bottom water dic of 1 7 0 6 per mil in the uppermost sediment and of 2 2 0 5 per mil in three to four centimeters subbottom depth all d13c values of live melonis spp decrease within the upper four centimeters regardless of the time of sampling and site investigated the offset of live b aculeata from bottom water d13cdic values of eight stations rather constantly amounts 0 6 0 1 per mil no matter what subbottom depth the specimens are from at one station however where is strong indication of elevated organic carbon flux the negative offset averaged over all sub bottom depths increases to 1 5 0 2 per mil buliminids actively move within the sediment and by this either record an average isotope signal of the pore water or the signal of one specific calcification depth the recorded signal however depends on the organic carbon flux and reflects general but site specific pore water d13cdic values if compared with epibenthic d13c values from the same site not influenced by pore water and related phytodetritus layer effects bulimina d13c values bear some potential as a paleoproductivity proxy specimens of melonis spp seem to prefer a more static way of life and calcify at different but individually fix depths within the sediment although live specimens thus record a stratified pore water d13c signal there is no means yet to correct for bioturbational and early diagenetic effects in fossil faunas

slope transport of suspended particulate matter on the aquitanian margin of the bay of biscay abstract spatial and temporal characteristics of the water masses and the dispersion of the suspended particulate matter were investigated using current meter hydrographic and nephelometric observations gathered during the ecofer experiment 1989 1991 in the cap ferret canyon on the aquitanian margin of the bay of biscay while characteristics of the deep water masses were stable from one year to another large hydrographic change in the upper 500 m related to winter renewal induced by poleward advection of warm and saline water along the continental slope the slope circulation and seasonal eddy activity appear as important dynamical mechanisms that control the entrainment and the dispersion of the suspended particulate matter from the neritic domain to the deep ocean a predominantly northward along slope current with occasional reversal characterizes this circulation the nephelometric structures also showed seasonal changes in the overall suspended particulate matter content but recurrent features such as the presence of intermediate nepheloid layers at the shelf break depth and various depths along the slope 500 1000 and 2000 m were observed these nepheloid layers extended off the slope to about 10 30 km but especially laterally along the slope their presence indicated that suspended particulate matter exchanges between the shelf and the slope occurred mainly in the head of the canyon and along the southern open slope the intermediate nepheloid layers around 500 m depth detached from the slope particularly in regions where the bottom slope is close to critical for the m2 internal tide

carbonate ramp depositional systems abstract the classification tectonic settings stratigraphy and early diagenesis of carbonate ramp systems are reviewed carbonate ramps are common in all geological periods but were dominant at times when reef constructing organisms were absent or inhibited ramps can be subdivided into inner mid and outer ramp environments the mid ramp zone extends from fair weather wave base to normal storm wave base although the water depths which these boundaries represent vary an additional outer slope environment occurs on distally steepened ramps as with siliciclastic shelves a range of wave storm and tide dominated ramps can be recognised and this forms the most convenient basis for ramp classification the carbonate productivity profile of ramps differs from that of rimmed shelves with the inner ramp showing lower production rates than comparable shallow water facies on rimmed shelves the zone of greatest organic carbonate sediment production appears to have shifted from the mid ramp to the inner ramp since the late jurassic carbonate ramps occur in most types of sedimentary basin but are best developed where subsidence is flexural and gradients are slight over large areas as in foreland and cratonic interior basins and along passive margins the featureless depositional profiles of many ramps means that sequence geometries are best observed on regional seismic lines some show low angle sigmoidal or shingled clinoforms suggesting that ramps may seldom be homoclinal but possess subtle slope geometries which reflect depositional environments because of their low angle slopes ramps respond differently to rimmed shelves during relative sea level changes although results seem to be strongly dependent on the rate of relative sea level change during a minor fall shallow ramp facies belts will simply shift basinwards in a forced regression in contrast the whole surface of a steep sloped flat topped rimmed shelf may be exposed so that sediment production ceases or is drastically reduced during a major fall shallow ramp bounded basins may empty completely conversely ramps also flood gradually whereas rimmed shelves do so more rapidly homoclinal ramps develop no resedimented lowstand deposits rimmed shelves and distally steepened ramps in contrast may develop lowstand talus or turbiditic wedges distally steepened ramps may behave more like homoclinal ramps during minor base level falls and like rimmed shelves during major base level falls many ramps consist of layered successions of several ramp sequences stacked one upon the other ramp stacks of this sort may show gross vertical accretion but individual ramp sequences seldom appear to develop in a keep up style apart from minor organic buildups as with many rimmed shelves steepening of the outer ramp margin due to tectonism slope inheritance or differential sedimentation may promote the development of a distally steepened ramp or rimmed shelf a wide variety of organisms have constructed buildups in mid and outer ramp environments isolated buildups may seed early in ramp development accrete to wave base or sea level and continue growth by stacking through successive ramp sequences so that depositional and diagenetic features within them are in concert with those of the shallow ramp the location of isolated buildups on ramps is governed by tectonism halokinesis antecedent topography or by the subtle slope geometry of the previous ramp sequence diagenesis on ramps shows some major variations compared with diagenesis on steep sloped flat topped carbonate platforms ramp bounded basins may form prolific petroleum sourcing and reservoiring systems and offer a range of subtle stratigraphic play types and lateral facies variations which determine reservoir quality and distribution isolated buildups in the mid and outer ramp environments represent one of the commonest petroleum reservoirs in ramp systems and tend to have their foundations in transgressive systems tracts grainstone and packstone reservoirs are widespread and range from shoreline carbonate sandbodies to major detached shoal complexes or shoals over offshore highs grainstone sandbodies occur in both highstand systems tracts and in prograding lowstand wedges

glacial water mass geometry and the distribution of δ13c of σco2 in the western atlantic ocean 1 oxygen and carbon isotopic data were produced on the benthic foraminiferal taxa cibicidoides and planulina from 25 new piston cores gravity cores and multicores from the brazil margin the cores span water depths from about 400 to 3000 m and intersect the major water masses in this region these new data fill a critical gap in the south atlantic ocean and provide the motivation for updating the classic glacial western atlantic 13c transect of duplessy et al 1988 the distribution of 13c of co2 requires the presence of three distinct water masses in the glacial atlantic ocean a shallow 1000 m southern source water mass with an end member 13c value of about 0 3 0 5 vpdb a middepth 1500 m northern source water mass with an end member value of about 1 5 and a deep 2000 m southern source water with an end member value of less than 0 2 and perhaps as low as the 0 9 values observed in the south atlantic sector of the southern ocean ninnemann and charles 2002 the origins of the water masses are supported by the meridional gradients in benthic foraminiferal 18o a revised glacial section of deep water 13c documents the positions and gradients among these end member intermediate and deep water masses the large property gradients in the presence of strong vertical mixing can only be maintained by a vigorous overturning circulation

carbon and oxygen isotopic disequilibria of recent deep sea benthic foraminifera abstract the carbon and oxygen isotopic compositions of 149 samples of benthic foraminifera from deep sea core tops indicate that none of the nine species studied secrete calcium carbonate in isotopic equilibrium with ambient bottom water uvigerina pyrgo murrhina and oridorsalis tener are the closest to 18o equilibrium with average deviations about 0 4 while planulina wuellerstorfi and p murrhina are the closest to 13c equilibrium with average deviations about 1 p wuellerstorfi shows the most systematic relationship between 13c and bottom water apparent oxygen utilization the intraspecific variabilities in 18o and 13c suggest that estimates of bottom water paleotemperatures can be made to a precision of 0 7 c while estimates of past apparent oxygen utilization aou can be made to 35 mol kg based on intraspecific comparisons of the recent samples with fossils no temporal changes in the degree of either 18o or 13c disequilibrium have been detected for planulina wuellerstorfi uvigerina oridorsalis tener and globocassidulina subglobosa

reconstructing benthic carbon oxidation rates using δ13c of benthic foraminifers abstract in non bioturbated organic rich marine sediments the concentration and carbon isotope gradient of total dissolved carbon dioxide co2 primarily reflects the rate of carbon oxidation and co2 diffusion within the sediment pore waters here we investigate whether the calcium carbonate secreted by benthic foraminifers that inhabit discrete depths within the sediments record the pore water 13c co2 gradient and hence can be used as proxies to reconstruct a history of carbon oxidation change the distribution and live abundance of bolivina argentea s l bolivina subadvena s l and buliminella tenuata inhabiting the dysoxic marine sediments along the california margin is similar among sites that differ in co2 gradient by up to a factor of three bolivina argentea inhabits the sediments at the sediment water interface whereas b subadvena s l is most abundant at depths of between 2 and 4 mm buliminella tenuata is most abundant at deeper depths of between 4 and 6 mm the 13c of calcite precipitated by these species matches the 13c of the pore water co2 at the depth of their maximum live abundance indicating that each species has a preferred depth for calcification the 13c difference between the species increases as the rate of carbon oxidation increases these results indicate the carbon isotopic difference between these species in fossil assemblages can be used to estimate changes in the rate of carbon oxidation

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

a model for the origin and petrogenesis of the red stromatactis limestone of paleozoic carbonate mounds abstract from study of the devonian recifs rouges of belgium and the silurian mounds of the quebec appalachians we propose that deposition of the red stromatactis limestone facies of cambrian devonian carbonate mounds was controlled by sponges and that the red color and stromatactis result from early diagenesis within a few meters below the substrate water interface in the deep marine environment the common presence of sponge body fossils as well as various stages of sponge preservation ranging from easily delineated to indiscernible bodies in the belgian recifs rouges support the conclusion that a large part of the red finely crystalline limestone originated from early cementation of sponge communities or spicule rich organic mats derived from degradation of sponge communities the role of bacteria and other microbes although widely advocated as the main primary builders of carbonate mounds is difficult to assess microbial communities may have contributed to mound accretion as primary builders and or mud producers but their presence cannot be documented we suggest that they may have acted as agents for concurrent sponge putrefaction and early cementation biodiagene is during the sulfate reduction phase under alkaline ph and anoxic conditions we interpret stromatactis as a spar body that resulted from early marine cementation of a cavity network created by excavation of uncemented material in partly indurated decaying sponges and spicule rich organic mats derived from degradation of sponge communities through circulation of interstitial water in the uppermost few meters of the sediment this cavity network was connected to sea floor and flushed by oxic waters that changed the diagenetic environment from anoxic to oxic converting amorphous iron sulfide to hematite and giving the facies its red color recognition of the dominant role of sponge communities in the construction of red stromatactis limestone mounds of the cambrian devonian time interval has important implications for the geological record this community was the main deep water mound building community during this time but is difficult to recognize because of its variable but often poor preservation

shell directions as a tool in palaeocurrent analysis abstract conical shells mostly orthoconic nautiloids locally gastropods and rugose corals were used to determine current directions in ludlovian to upper famennian cephalopod limestones in the eastern anti atlas of morocco ougarta aulacogen and ahnet basin both algeria data plots established on 50 413 measurements from 217 localities document rather consistent current patterns which show only minor variations through subsequent intervals a conspicuous feature are currents derived from pelagic platforms and directed towards adjacent basins shell accumulations decrease markedly towards platform margins yielding less distinctive information on current directions which due to lack of shells cannot be established in the basins proper orientation patterns of styliolinids show such a puzzling variation in adjacent samples that their use for current analysis is doubtful the same is true for the presumed down stream position of goniatite apertures which shows a highly variable pattern which is rarely consistent with that of concomitant orthoconic nautiloids the direction of orthocones in cephalopod limestones onlapping lower givetian mud mounds and ridges in the ahnet basin of algeria shows a radial pattern which is the result of a mere gravitational deposition of shells on the steep slopes of these buildups apart from this exception the applicability of conical shells for current analysis is confirmed

genesis of stromatactis cavities between submarine crusts in palaeozoic carbonate mud buildups it is concluded that the sedimentary structure stromatactis is a calcite cement fill of a swarm of labyrinthine cavities developed in marine carbonate muds of palaeozoic bioherms commonly with geopetal sediment field and petrographic evidence is examined from the ordovician of dalarna sweden the devonian of the canning basin australia and of belgium and the carboniferous of lancashire u k in all four occurrences the host microspar encloses masses of radiaxial fibrous calcite with sub horizontal floors following other workers radiaxial fibrous calcite is regarded as an altered marine cement a conclusion supported by the presence of overlying geopetal sediments with marine fossils widespread evidence of early submarine cementation in palaeozoic mud mounds isotopic analyses and the presence of microdolomites the stromatactis masses are separated from each other by laminae of host biomicrospar which show discontinuities resembling fractures which are healed with the early marine cement or filled with marine internal sediments the fractures are therefore of submarine origin the evidence points to episodic submarine lithification of the carbonate muds yielding successive crusts between which cavities were eroded and then filled the crusts were broken and rotated during bioturbation differential compaction or shearing during burial

the role of internal erosion and sedimentation in the formation of stromatactis mudstones and associated lithologies abstract a distinctive suite of lithologies occurs in the lower devonian rocky camp mound of eastern victoria lime mudstones contain abundant stromatactis cavities and wackestones contain shelter cavities beneath skeletal constituents together with unsheltered stromatactis cavities irregularly distributed muddy masses occur in grain supported lithologies together with abundant shelter cavities and perched internal sediment these lithologies are completely intergradational and clast content is the principal variant controlling the fabric developed it is proposed that the fabrics in this lithological suite resulted from a process of internal erosion and sedimentation internal reworking which occurred directly below the sediment water interface each stromatactis cavity was subject to erosion of carbonate mud from the roof and redeposition of this material on the floor either directly below the point of erosion or downstream from it in this way cavities migrated upwards leaving a trail of internal sediment beneath in wackestones the migrating cavities were trapped beneath skeletal constituents forming diagenetic shelter cavities the fabrics of grain supported lithologies can be similarly explained a precursor cavity system of organic or inorganic origin is required to initiate the process of internal erosion there is no evidence relating to the origin of this cavity system in the rocky camp mound

widespread venting of methane rich fluids in late cretaceous campanian submarine springs tepee buttes western interior seaway u s a from new mexico to montana campanian shales contain abundant submarine spring and seep deposits aligned along early laramide basement faults from spring conduit to sea floor characteristic lithofacies are a a vuggy pelletoid micrite core to 30 m height 5 m width b lucinoid bivalve coquina in pelletoid micrite or grainstone c carbonate slump breccias in shale d concretion bearing shale and e organic rich shale stable isotope analyses of early zoned marine core rock cements suggest a an oxidized methane source for the carbonate carbon b venting of methane charged fluids along active fracture zones and c methanogenesis in older and coeval organic carbon rich muds six consistently zoned foraminifer and macrofaunal communities suggest a strong environmental stress gradient over a few metres from spring vents to the adjacent sea floor these methane springs and their biotas were dynamic and episodically active over a 1 25 m y time span

stable oxygen and carbon isotopes of live benthic foraminifera from the bay of biscay microhabitat impact and seasonal variability we determined the stable oxygen and carbon isotopic composition of live rose bengal stained benthic foraminifera n150 am size fraction of seven taxa sampled along a downslope transect between 140 to 2000 m water depth in the bay of biscay at the five stations hoeglundina elegans cibicidoides pachydermus uvigerina peregrina uvigerina mediterranea preferentially occupy shallow infaunal niches whereas melonis barleeanus and uvigerina elongatastriata occupy an intermediate infaunal microhabitat and globobulimina spp live in a deep infaunal niche close to the zero oxygen boundary when compared with d 18 o values of calcite formed in equilibrium with bottom waters u peregrina forms its test in close equilibrium with bottom water d 18 o all other foraminiferal taxa calcify with a constant offset to calculated equilibrium calcite there is no systematic relationship between the foraminiferal microhabitat depth and the dd 18 o between foraminiferal and equilibrium calcite we calculated correcting factors for the various taxa which are needed for constructing multispecies based oxygen isotope records in paleoceanographic studies of the study area the d 13 c values of foraminiferal taxa investigated in this study do neither record bottom water d 13 cdic in a 1 1 relationship nor with a constant offset but appear to be mainly controlled by microhabitat effects the increase of d 13 c values of shallow infaunal taxa with increasing water depth reflects the decrease of the exported flux of organic carbon along the bathymetric transect and early diagenetic processes in the surface sediment this is particularly the case for the shallow infaunal u peregrina the d 13 c values of deep infaunal globobulimina spp are much less dependent on the exported organic matter flux we suggest that the dd 13 c between u peregrina and globobulimina spp can shed light on the various pathways of past degradation of organic detritus in the benthic environments at a station in 550 m water depth where periodic eutrophication of sediment surface niches was demonstrated previously we performed a two year seasonal survey of the isotopic composition of foraminiferal faunas no marked seasonal changes of the stable carbon isotopic composition of shallow intermediate and deep infaunal foraminiferal taxa were observed thus the d 13 c values of foraminiferal individuals belonging to the n150 am fraction may result from rather long term calcification processes lasting for several weeks or months which limit the impact of ephemeral 12 c enrichment of shallow infaunal niches on the isotope chemistry of adult individuals during eutrophic periods only highly opportunistic taxa reproducing or calcifying during

comparison of cryoconite organic matter composition from arctic and antarctic glaciers at the molecular level abstract glacier surfaces are reservoirs that contain organic and inorganic debris referred to as cryoconite solar heating of this material results in the formation of water filled depressions that are colonized by a variety of microbes and are hypothesized to play a role in carbon cycling in glacier ecosystems recent studies on cryoconite deposits have focused on their contribution to carbon fluxes to determine whether they are a net source or sink for atmospheric co 2 to better understand carbon cycling in these unique ecosystems the molecular constituents of cryoconite organic matter com require further elucidation com samples from four glaciers were analyzed by targeted extraction of plant and microbial derived biomarkers in conjunction with non targeted nmr experiments to determine the com composition and potential sources several molecular proxies were applied to assess com degradation and microbial activity using samples from greenland the canadian arctic and antarctica com from canadian john evans glacier and greenlandic leverett glacier locations was more chemically heterogeneous than that from the antarctic likely due to inputs from higher plants mosses and sphagnum as suggested by the solvent extractable alkyl lipids and sterols and the detection of lignin and sphagnum derived phenols after cupric oxide chemolysis solid state 13 c nuclear magnetic resonance nmr experiments highlighted the bulk chemical functional groups of com allowing for a general assessment of its degradation stage from the alkyl o alkyl proxy whereas solution state 1 h nmr highlighted both microbial and plant contributions to base soluble extracts from these com samples the dominance of 1 h nmr signals from microbial protein peptides in base soluble extracts of com from antarctica joyce glacier and garwood glacier phospholipid fatty acid plfa biomarker detection and the absence of plant derived biomarkers in both the solvent and cupric oxide extracts suggests that this com is dominated by microbial derived material these results indicate that com carbon composition is dependent on the local glacier environment which may have a profound impact on carbon cycling and sequestration on glacier surfaces

living benthic foraminifers from the central arctic ocean faunal composition standing stock and diversity abstract fifty short sediment cores collected with a multiple corer and five box cores from the central arctic ocean were analysed to study the ecology and distribution of benthic foraminifers to work out living faunal associations standing stock and diversity separate analyses of living rose bengal stained and dead foraminifers were carried out for the sediment surface the size fractions between 63 and 125 m and 125 m were counted separately to allow comparison with former arctic studies and with studies from the adjacent norwegian greenland sea barents sea and the north atlantic ocean benthic foraminiferal associations are mainly controlled by the availability of food and competition for food while water mass characteristics bottom current activity substrate composition and water depth are of minor importance off spitsbergen in seasonally ice free areas high primary production rates are reflected by high standing stocks high diversities and foraminiferal associations 125 m that are similar to those of the norwegian greenland sea generally in seasonally ice free areas standing stock and diversity increase with increasing food supply in the central arctic ocean the oligotrophic permanently ice covered areas are dominated by epibenthic species the limited food availability is reflected by very low standing stocks and low diversities most of these foraminiferal associations do not correspond to those of the norwegian greenland sea the dominant associations include simple agglutinated species such as sorosphaerae placopsilinellae komokiacea and aschemonellae as well as small calcareous species such as stetsonia horvathi and epistominella arctica those of the foraminiferal species that usually thrive under seasonally ice free conditions in middle bathyal to lower bathyal water depth are found under permanently ice covered conditions in water depths about 1000 m shallower if present at all

metallogenesis of cratonic oolitic ironstone deposits in the bled el mass azzel matti ahnet and mouydir basins central sahara algeria the bled el mass azzel matti ahnet and mouydir areas are located in the northwest of the touareg shield central sahara algeria within the devonian sedimentary formations nine oolitic ironstone occurrences of exid type extensive ironstone deposition are interbedded

benthic carbonate factories of the phanerozoic marine carbonate precipitation occurs in three basic modes abiotic or quasi abiotic biotically induced and biotically controlled on a geologic scale these precipitation modes combine to form three carbonate production systems or factories in the benthic environment 1 tropical shallow water factory dominated by biotically controlled mainly photo autotrophic and abiotic precipitates 2 cool water factory dominated by biotically controlled mainly heterotrophic precipitates and 3 mud mound factory dominated by biotically induced mainly microbial and abiotic precipitates sediment accumulations of the factories differ in composition geometry and facies patterns and some of these differences appear prominently in seismic data thus facilitating subsurface prediction the characteristic accumulation of the tropical factory is the flat topped often reef rimmed platform in cool water systems reefs in high energy settings are scarce and hydrodynamic influence dominates producing seaward sloping shelves and deep water sediment drifts often armored by skeletal framework the typical accumulation of the mud mound factory is groups of mounds in deeper water where the mud mound factory expands into shallow water it forms rimmed platforms similar to the tropical factory the tropical factory is most productive the mud mound factory reaches 80 90 and the cool water factory 20 30 of the tropical growth rate the three factories represent end members connected by transitions in space transitions in time are linked to biotic evolution

genesis and geometry of the meiklejohn peak lime mud mound bare mountain quadrangle nevada usa ordovician limestone with submarine frost heave structures a possible response to gas clathrate hydrate evolution abstract during the early middle ordovician early whiterockian the meiklejohn peak lime mud mound a large whaleback or dolphin back dome grew on a carbonate ramp tens to hundreds of kilometres offshore this ramp extended from the northwest margin of laurentia into the open waters of the ancestral pacific ocean to the north the mound developed in an outer ramp environment in relatively deep and cold water a steep northern margin with a slope that exceeds 55 characterizes the mound this margin is split by a 14 m long vertical fracture that separates a zone of slumped drag folded and brecciated rocks from the main mass of the mound failure along this fracture occurred subcutaneously as highlighted by covering beds that are folded next to the mound brecciated blocks and clasts contain zebra and stromatactis structures indicating that these rocks and structures were lithified early in the history of the mound the southern end of the mound is less steep and is characterized by large echinodermal grainstone cross beds these deposits are part of a large subaqueous dune that grew northwards and preceded the main development of the mound southward dipping and downlapping layers of mud mound mudstone and wackestone overlie the dune these muddy limestone layers are cut in several places by injection dykes and are pierced near the contact with the underlying dune by a 25 m long pipe filled with rotated nodular and brecciated mud mound clasts this long pipe extends to the edge of the mound and appears to have been a conduit where fluidized materials that came from the mound s interior were vented the interior of the mound is typified by light grey limestone with zebra bands and stromatactis structures both structures represent former cavity systems that are filled with fibrous and bladed calcite and pelleted and laminated geopetal mudstone spar bands of zebra limestone often extend for several metres and appear to have been unsupported over these distances zebra banded rocks are also accompanied by snout and socket structures and in some instances are folded and sheared by curving kink bands zebra and stromatactis limestone structures found throughout the mud mound resemble frost heave and cryoturbation structures identified in both holocene and pleistocene cryosols and in laboratory experiments with advancing freezing fronts in clay size sediment significantly modern occurrences of methane clathrate hydrate methane charged ice display parallel and digitate layering similar in depositional appearance to that of zebra and stromatactis limestone from meiklejohn peak early carbonate cements are also commonly associated with these modern clathrate hydrate deposits consequently gas clathrate hydrates may have been the propping agent for zebra and stromatactis structures observed in the mud mound in this scenario carbonate cements would have precipitated and stabilized these structures both with the consolidation and dissociation of gas clathrate hydrates and with the oxidation and reduction of associated gases stable 13 c and 18 o isotope ratios collected from mudstone and spar of zebra and stromatactis structures indicate that they were lithified in equilibrium with ordovician seawater the 13 c isotope ratios recorded at meiklejohn peak are similar to 13 c isotopic ratios obtained from co 2 evolving from modern seafloor these isotopic ratios may indicate that frost heave structures in the meiklejohn peak mud mound are the result of consolidation and dissociation of carbon dioxide clathrate hydrates even though the bulk of gas clathrate hydrates identified to date in modern ocean floors are composed of methane carbon dioxide clathrate hydrates are known from the modern seafloor of the okinawa trough they may also be common in areas of abundant carbonate sediment accumulation as suggested by recent observations from the great australian bight

the hydrocarbon systems of north africa abstract north africa s hydrocarbon reserves are spread over a wide range of reservoirs and play types but derive from only a few key source rock intervals particularly those in the silurian late cretacous and eocene source rock maturity together with the timing of the generation relative to trap forming and destructive events seem to be the key controlling factors on success or failure at a basin level in most cases generation clearly post dates trap formation although a more complex relationship is evident in various parts of algeria where some traps did not form until gas was being generated source rock deposition and maturation can be tied to two main extensional tectonic cycles which ended in the compressional and destructive events of the hercynian and alpine orogenies remaining exploration potential in mature provinces may be tied largely to subtle and stratigraphic traps in addition significant frontier potential can be identified in two main fairways corresponding to regions where silurian devonian and cretaceous eocene sources respectively are likely to have generated petroleum critical factors in these areas are timing of generation and preservation the gas discoveries in the ahnet basin provide analogues for palaeozoic generation preservation models in the interior silurian devonian source fairway similarly the key to success for the mesozoic basins of the atlas region lies in demonstrating a favourable relationship between trap formation destruction and generation from late cretaceous and eocene source rocks

the world s most spectacular carbonate mud mounds middle devonian algerian sahara abstract in the southern ahnet basin of algeria a unique middle devonian underwater scenery is revealed which consists of 191 mud mounds 15 mud ridges and 2 mud atolls distributed over nine discrete areas of a shallow pelagic carbonate shelf all the buildups are totally exhumed thus perfectly exhibiting their original morphologies slope angles elevations and the pattern of the onlapping strata as well as their relationship to the surrounding off buildup sequences types of buildups include 1 small mud mounds 1 5 m high 10 20 m diameter 2 large mud mounds up to 40 m high up to 200 m diameter 3 lens shaped mounds 1 30 m thick 10 15 slopes 4 mud ridges up to 100 m high 150 8500 m long and 5 semicircular mud atolls 2500 2800 m long 20 30 m high the buil ups were constructed during a short time span in the early givetian part of the lower varcus zone probably not exceeding a few hundred thousand years sedimentation rates in coeval off buildup areas were at least 3 10 times lower the lithology of the buildups is a massive boundstone with a high proportion of irregular spar filled cavities stromatactis but only very scattered potential frame builders the carbonate mud is considered to be an in situ cyanobacterial precipitate although evidence of such is poor organic components of the buildups are not significantly different from those of the off buildup lithologies and include numerous crinoid ossicles some tabulate and solitary rugose corals brachiopods mollusc debris trilobites and very rare sponge spicule and bryozoans the total absence of stromatoporoids large tabulate and colonial rugose corals green algae and micritic envelopes around grains suggests growth of the buildups in the lower part of the photic zone 100 200 m they are onlapped by a discontinuous sequence a few meters thick of skeletal packstones and grainstones and locally orthoceratid floatstones that were deposited later in the givetian a widespread rise of sea level in the early frasnian buried both buildups and adjacent areas under shales and siltstones postdepositional alteration of the mud buildups includes 1 formation of stromatactis cavities probably by dewatering and internal erosion 2 cementation of stromatactis cavities and interparticle pore spaces by early high mg calcite and late blocky low mg calcite 3 neomorphic transformation of mg calcitic mud and aragonitic skeletal debris 4 occasional formation of neptunian dikes 5 slight erosion of buildup surfaces 6 compaction and slumping of onlapping capping beds and 7 local dolomitization the formation and growth of the mud build ups is envisaged as a sequence of stages starting with patchy coral thickets evolving into small mud mounds that coalesced into larger mounds by continuous vertical and lateral growth closely spaced mounds amalgamated into mud ridges and mud atolls regular nw se and n s patterns of some mound rows and ridges show a relationship of buildup formation to rejuvenated late precambrian tectonic lineaments which reappear in the hercynian orogeny the isotopic signature of matrix and early ements of mound rocks however does not support the idea of fluid migration and hydrothermal co2 supply during buildup growth

core top evaluation of benthic foraminiferal isotopic ratios for paleo oceanographic interpretations abstract the benthic foraminifera cibicides wuellerstrofi oridforsalis tener and pyrgo rotalaria from twelve core tops in the norwegian greenland sea and c wuellerstorfi o tener and o umbonatus from four core tops on the rio grande rise were analysed for oxygen and carbon isotopes the norwegian greenland sea data yield statistically discrete means for both 18o and 13c for each species the interregional comparison of our data to published data from the east pacific rise shows that c wuellerstorfi 18o and 13c variations reflect oceanographic differences between sites whereas the data on oridorsalis and pyrgo do not we conclude that c wuellerstorfi is a reliable sensor of bottom water conditions whereas other taxa are of unproven utility supportive biological studies and the lack of positive covariance of 18o to 13c among deep sea benthic taxa lead us to reject models calling for incorporation of metabolic co2 to explain the isotopic variation among deep sea benthic foraminifera we propose a within sediment microhabitat hypothesis to explain the 13c variations among taxa

a time series study of the carbon isotopic composition of deep sea benthic foraminifera 1 variation of the 13c of living rose bengal stained deep sea benthic foraminifera is documented from two deep water sites 2430 and 3010 m from a northwest atlantic ocean study area 275 km south of nantucket island the carbon isotopic data of hoeglundina elegans and uvigerina peregrina from five sets of multicorer and soutar box core samples taken over a 10 month interval march may july and october 1996 and january 1997 are compared with an 11 5 month time series of organic carbon flux to assess the effect of organic carbon flux on the carbon isotopic composition of dominant taxa carbon isotopic data of hoeglundina elegans at 3010 m show 0 3 lower mean values following an organic carbon flux maximum resulting from a spring phytoplankton bloom this 13c change following the spring bloom is suggested to be due to the presence of a phytodetritus layer on the seafloor and the subsequent depletion of 13c in the pore waters within the phytodetritus and overlying the sediment surface carbon isotopic data of h elegans from the 2430 m site show an opposite pattern to that found at 3010 m with a 13c enrichment following the spring bloom this different pattern may be due to spatial variation in phytodetritus deposition and resuspension or to a limited number of specimens recovered from the march 1996 cruise the 13c of uvigerina peregrina at 2430 m shows variation over the 10 month interval but an analysis of variance shows that the variability is more consistent with core and subcore variability than with seasonal changes the isotopic analyses are grouped into 100 m size classes on the basis of length measurements of individual specimens to evaluate 13c ontogenetic changes of each species the data show no consistent patterns between size classes in the 13c of either h elegans or u peregrina these results suggest that variation in organic carbon flux does not preferentially affect particular size classes nor do 13c ontogenetic changes exist within the 250 to 750 m size range for these species at this locality on the basis of the lack of ontogenetic changes a range of sizes of specimens from a sample can be used to reconstruct 13c in paleoceanographic studies the prediction standard deviation which is composed of cruise core subcore and residual replicate variability provides an estimate of the magnitude of variability in fossil 13c data it is 0 27 for h elegans at 3010 m and 0 4 for u peregrina at the 2430 m site since these standard deviations are based on living specimens they should be regarded as minimum estimates of variability for fossil data based on single specimen analyses most paleoceanographic reconstructions are based on the analysis of multiple specimens and as a result the standard error would be expected to be reduced for any particular sample the reduced standard error resulting from the analysis of multiple specimens would result in the seasonal and spatial variability observed in this study having little impact on carbon isotopic records

stable carbon isotopes in marine porewaters abstract we present centimeter scale porewater 13 c profiles from four sites in the central equatorial pacific ocean which reveal steep porewater 13 c gradients up to 1 0 cm 1 at the deep sea sediment water interface we describe the processes controlling porewater carbon isotopic composition and incorporate them into a model of the deep sea benthic carbon cycle our measured 13 c profiles are then used to estimate carbon rain rates at two of these sites manop sites c and s the 13 c data yield organic carbon rain rates of 10 mol c cm 2 yr 1 at site c and 3 mol c cm 2 yr 1 at site s these carbon rain rates agree with recent carbon rain estimates based on sediment trap data and porewater nitrate profiles but are a factor of 2 lower than estimates based on millimeter scale porewater oxygen profiles our model results indicate that changes of the same magnitude in organic carbon rain rate and bottom water composition o 2 co 2 and 13 c have comparable effects on the carbon isotopic composition of surficial porewaters this suggests the need to consider the effects of benthic carbon cycling on the stable carbon isotope record preserved in deep sea benthic foraminifera tests

early cenozoic benthic foraminiferal isotopes species reliability and interspecies correction factors 1 oxygen and carbon isotope records are important tools used to reconstruct past ocean and climate conditions with those of benthic foraminifera providing information on the deep oceans reconstructions are complicatedby interspecies isotopic offsets that result from microhabitat preferences carbonate precipitation in isotopically distinct environments and vital effects species specific metabolic variation in isotopic fractionation we provide correction factors for early cenozoic benthic foraminifera commonly used for isotopic measurements cibicidoides spp nuttallides truempyi oridorsalis spp stensioina beccariiformis hanzawaia ammophila and bulimina spp showing that most yield reliable isotopic proxies of environmental change the statistical methods and larger data sets used in this study provide more robust correction factors than do previous studies interspecies isotopic offsets appear to have changed through the cenozoic either 1 as a result of evolutionary changes or 2 as an artifact of different statistical methods and data set sizes used to determine the offsets in different studies regardless of the reason the assumption that isotopic offsets have remained constant through the cenozoic has introduced an 1 2 c uncertainty into deep sea paleotemperature calculations in addition we compare multiple species isotopic data from a western north atlantic section that includes the paleocene eocene thermal maximum to determine the most reliable isotopic indicator for this event we propose that oridorsalis spp was the most reliable deepwater isotopic recorder at this location because it was best able to withstand the harsh water conditions that existed at this time it may be the best recorder at other locations and for other extreme events also

benthic foraminiferal δ13c and accumulation rates of organic carbon uvigerina peregrina group and cibicidoides wuellerstorfi the glacial to interglacial 13c records of the benthic foraminifera cibicidoides wuellerstorfi and the uvigerina peregrina group from deep sea cores cannot be adjusted by a generally valid constant the 13c values of the u peregrina group largely correlate with the accumulation rates of organic carbon suggesting a local habitat effect those of c wuellerstorfi vary independently with respect to the carbon flux and record fluctuations in the 13c of the ambient bottom water isotopic composition