origin of minor limestone shale cycles climatically induced or diagenetic the opinion is widely held that each limestone bed in minor limestone shale cyclic sequences however modified subsequently by diagenesis reflects an environmental event on the assumption that this is true attempts have recently been made to relate limestone shale cycles to climatic variations affected by milankovitch orbital cycles a test is proposed and illustrated mainly with reference to the blue lias formation of dorset england that appears to indicate that many limestones can be produced solely by rhythmic unmixing of caco 3 during diagenesis unless such limestones can be clearly distinguished from those that record genuine environmental signals orbital cycle analysis based on such sequences will give meaningless results

ichnological record of deep sea palaeoenvironmental changes around the oceanic anoxic event 2 cenomanian turonian boundary an example from the barnasiowka section polish outer carpathians abstract trace fossil assemblages including planolites thalassinoides palaeophycus taenidium chondrites and trichichnus have been determined in the upper cenomanian turbiditic deep water succession corresponding to the pre oae 2 interval in the barnasiowka ostra gora section the skole nappe polish outer carpathians these assemblages indicate that oxygenation within sediment fluctuated from oxic to dysoxic zone the overlying uppermost cenomanian organic rich facies bonarelli equivalent horizon corresponding to the main interval of the oae 2 contains black shale layers devoid of trace fossils representing anoxic euxinic events and green shales that include chondrites planolites and thalassinoides recording dysoxic intervals three subintervals phases within the oae 2 sediments have been distinguished in the barnasiowka section based on trace fossil diversity ichnofabrics and chemical redox indices these phases correspond to intervals with various intensity of sea floor oxygenation ichnofabrics within the oae 2 succession was also controlled by consistency of sediment post oae 2 interval recorded by the earliest turonian red and green siliceous manganiferous shales and two fe mn layers represent sediments deposited in well oxygenated conditions under extremely low sedimentation rate most of the shales are parallel laminated and they do not include any benthic microfossils and trace fossils however some of the red and green layers display cryptic bioturbation these features are related to food scarcity cessation of gravity flows unfavorable characteristics of bottom water and a low consistency of substrate

bioturbation in burgess shale type lagerstatten case study of trace fossil body fossil association from the kaili biota cambrian series 3 guizhou china abstract cruziana gordia planolites rusophycus and trichophycus are common ichnological elements of the kaili biota new discoveries based on the examination of 323 specimens include eldoniids echinoderms trilobites monoplacophorans and non biomineralizing arthropods that are associated with trace fossils based on the observed effects of bioturbation on the preservation of five different animal groups it is clear that infaunal scavengers deposit feeders were periodically active on the kaili sea floor and were able to reach historic layers yielding exceptionally preserved fossils in general the average level of infaunal activity is absent to moderate ichnofabric index i i 1 to 3 in the kaili substrate by contrast the phycodes beds are completely disturbed by infaunal activity i i 5 observed burrow diameter ranges from diminutive 0 2 mm to normal up to 4 2 mm computed tomography allows us to visualize the precise geometry of the trace fossil body fossil association in three dimensions we concluded that although some kaili infaunal animals could bore through the biomineralized echinoderms they did not appear to have scavenged upon these echinoderms based on our three dimensional reconstruction furthermore kaili burrowers can reach the historic layers containing exceptionally preserved fossils without altering soft tissue preservation on the other hand we used conventional techniques to reveal that one burrow is filled with fecal pellets 100 200 m at its terminal end because most burrows are filled with yellow brown coarse sediment and because there is no compositional difference between the fecal pellets and the surrounding coarse sediment within the burrow the origin of the yellow brown coarse sediment is interpreted here as parautochthonous and or autochthonous and as a result of sediment reworking by deposit or suspension feeders the relative scarcity of soft part preservation in the kaili biota compared to the older chengjiang biota and the younger burgess shale biota may be the result of post burial bio disturbance due to the relatively high intensity of local bioturbation it is plausible that oxygenation in deeper sediments facilitated by bioturbation allowed carcasses in the kaili biota to undergo a more extensive period of normal decay prior to final burial than in other cambrian konservat lagerstatten

interpreting benthic oxygen levels in mudrocks a new approach quantified paleoecology and gamma ray spectrometry have been applied in the analysis of the kimmeridge clay a highly organic rich british jurassic mudrock decreasing benthic oxygen trends are reflected in decreasing species richness and dominance diversity values similarly the degree of fragmentation of the benthos reflects the benthic energy levels and covaries with benthic oxygen the calculation of authigenic uranium values from data gathered by gamma ray spectrometry shows enrichment in more oxygen deficient environments the good correlation between the independently derived paleoecological and authigenic u data indicates the importance of these techniques in environmental analysis of marine petroleum source rocks

trace fossil evidence for restoration of marine ecosystems following the end permian mass extinction in the lower yangtze region south china abstract unlike the high abundance low diversity macrofaunas that characterize many early triassic benthic palaeocommunities ichnofossils were relatively common in the aftermath of the end permian mass extinction worldwide ichnofossils therefore are a good proxy for ecosystem recovery after the end permian biotic crisis this paper documents 14 ichnogenera and one problematic form from lower triassic successions exposed in the lower yangtze region south china post extinction ichnodiversity remained rather low throughout the griesbachian early smithian period and abruptly increased in the late smithian however several lines of evidence including extent of bioturbation burrow size trace fossil complexity and tiering levels indicate that diversification of ichnotaxa in the late smithian did not signal full marine ecosystem recovery from the permian triassic p tr mass extinction marine ichnocoenoses did not recover until the late spathian in south china the marginal sea provided hospitable habitats for tracemakers to proliferate in the aftermath of the end permian mass extinction

ethology of the trace fossil chondrites form function and environment abstract the behaviour of the iconic ichnogenus chondrites is re evaluated based on review of existing literature and analysis of novel data macroscopic thin section and esem edx observations ct scans and resin peels of modern analogues computer controlled serial grinding morphometric analysis and theoretical morphology the bedding plane expression of chondrites is well constrained by morphometry 1 the angle of dichotomy formed by a pair of adjacent branches is typically between 30 and 56 interquartile range mean 47 2 branching order is between 1 and 9 3 branches are on average nine times longer than wide in the third dimension downward branching is dominant but bundled shafts and upward branching may be present the size of chondrites increased markedly from the late jurassic to the late cretaceous suggesting that the tracemakers became larger and larger microfabric analysis of chondrites shows active fill or alternatively passive fill of empty tunnels by currents or clay percolation a new mechanism proposed herein the tracemakers built chondrites to obtain food 1 vermiform deposit feeders produced chondrites for searching for food in the sediment fodinichnion 2 asymbiotic bivalves built chondrites for cultivating and directly ingesting bacteria agrichnion 3 chemosymbiotic bivalves produced chondrites to provide symbionts with chemical agents chondrites was modified through the life of the tracemaker or it represented a part of the producer s lifespan chondrites and its modern tracemaker s is associated with a range of marine settings including well oxygenated dysoxic and space limited nucleocave environments as such chondrites is regarded as an extremotolerant ichnotaxon

early jurassic mass extinction a global long term event the end pliensbachian extinction event 187 ma has been interpreted either as one of 10 global periodically recurring mass extinctions of the past 250 m y or as a minor localized european event elevated levels of family extinction spanned five ammonite zones during the late pliensbachian and the early toarcian an interval of 7 5 m y and were distributed unequally in the boreal tethyan and austral realms detailed sampling of invertebrate macrofaunas through complete expanded sequences in northwest europe shows that most species extinctions occurred in the early toarcian following a regional anoxic event the early jurassic mass extinction event took place over a long time scale and it was global in extent

the ichnogenus rhizocorallium classification trace makers palaeoenvironments and evolution rhizocorallium is one of the oldest known trace fossils with wide distribution through the phanerozoic and all over the world originally introduced from the epicontinental triassic of central germany its high morphological lability gave reason for the subsequent erection of about twenty ichnospecies the study of newly collected material from the type area and many specimens from various collections permits the conclusion that rhizocorallium jenense and rhizocorallium commune are the only valid ichnospecies of rhizocorallium the type ichnospecies r jenense is a comparatively small inclined and heavily scratched firmground burrow with passive fill while r commune consists of extensive more or less horizontal burrows with occasionally scratched marginal tubes and an actively filled spreite between the tubes the faecal pellets coprulus oblongus are typically associated with r commune morphological variations of r commune are captured in ichnosubspecies and varieties of this ichnospecies and can aid a refined reconstruction of palaeoenvironments a review of more than 180 records from the literature reveals the common confusion of both ichnospecies which has consequences for the application of rhizocorallium in facies interpretations the end members of both ichnospecies may be linked by transitional forms which suggests the same kind of trace maker polychaetes are the most likely producers of marine rhizocorallium based on their long ranging occurrence morphological features appearance of faecal pellets associated soft body remains and modern analogues r commune occurs from early cambrian to holocene while r jenense just appears after the end permian mass extinction probably as a consequence of an adapted firmground burrowing lifestyle of its producer fluvial r jenense are probably produced by mayfly larvae in homology to marine polychaete burrows a consequent application of the newly established classification scheme allows for a more rigorous application of rhizocorallium in the reconstruction of palaeoenvironments thus palaeozoic and mesozoic r commune are restricted to the cruziana ichnofacies of shallow marine environments while in cenozoic time similar forms are also found in deep marine deposits r jenense on the other hand is a constituent of the widespread glossifungites ichnofacies and aside from continental environments occurs in peritidal to deep marine deposits several studies have demonstrated the value of rhizocorallium for interpreting sequence stratigraphical surfaces current directions and fluctuations in salinity and oxygen

integrated ichnology and ichthyology of the oligocene menilite formation skole and subsilesian nappes polish carpathians a proxy to oxygenation history the anoxic mostly black or brown fine grained sediments of the menilite formation oligocene early miocene in the skole and subsilesian nappes contain thin layers of bioturbated green or grey green mudstones some of which contain the trace fossils halimedides annulata multina isp palaeophycus tubularis planolites isp rhizocorallium isp trichichnus isp and zoophycos isp the trichichnus palaeophycus multina halimedides rhizocorallium suite indicates an increase in oxygenation of sediments the contribution of different ecological groups of fishes including epipelagic bathypelagic benthopelagic neritic and reef and demersal taxa changes significantly through the menilite formation the absence or reduction of bathypelagic fishes points to anoxia in the water column the combination of ichnological and ichthyological data and incorporation of data on benthic foraminifers allowed a reconstruction of oxygenation changes in the sediment and water column during deposition of the menilite formation total anoxia at the sea floor and in the water column attributed to a combination of thermo stratification and extremely high organic productivity occurred only during the period reflected by ichthyofaunal zone ipm2 middle part of the np23 zone anoxia restricted to the basin floor or upper slope related in part to upwelling occurred during sedimentation of the upper part of the menilite formation

unbioturbated sediments on a muddy shelf hypoxia or simply reduced oxygen saturation abstract x radiographs of sediment box cores acquired from the western gulf of mexico reveal limited bioturbation in sediment deposited in bathymetries greater than 35 m between 15 and 35 m sediments are thoroughly bioturbated with averaged bioturbation indices for all beds in a core between 2 1 and 5 6 and trace diversities between 2 and 9 distinct burrow forms below 35 m water depth box cores exhibit trace diversities of 1 3 and core averaged bioturbation indices range between 0 3 and 3 6 there is an overall decrease in trace diversity and bioturbation indices in the offshore direction cross shore ichnological trends are compared to dissolved oxygen contents of bottom waters above 35 m dissolved oxygen do contents show pronounced variability ranging from 100 do saturation through to hypoxia the data reveal a direct correlation between a the density of infauna and the diversity and density of burrows and b do of bottom water the degree of bioturbation is significantly reduced in waters that are oxic but below 80 do saturation based on these observations we suggest that it is inappropriate to link low bioturbation intensities and diversities to hypoxia

trace fossil model for reconstructing oxygenation histories of ancient marine bottom waters application to upper cretaceous niobrara formation colorado abstract trends of decreasing trace fossil diversity burrow size and penetration depth with diminishing oxygen availability in contemporary marine settings have been incorporated with the concept of trace fossil tiering to develop a model for reconstructing the oxygenation history of ancient basin bottom waters more sensitive than traditionally employed criteria the model permits detailed evaluation of relative degree of oxygenation through the recognition of oxygen related ichnocoenosis ori units or units of strata deposited under similar levels of bottom water oxygenation when employed in detailed vertical sequence analyses the model may also be used to construct interpreted oxygenation curves that reflect rates and magnitudes of temporal change in redox conditions the utility of the model is demonstrated by its application to rhythmically bedded calcareous strata of the niobrara formation upper cretaceous exposed in colorado non laminated strata of the niobrara are characterized by four recurring trace fossil suites referred to as the 1 chondrites 2 zoophycos teichichnus 3 planolites and 4 thalassinoides assemblages on the basis of trends in trace fossil diversity burrow diameter and penetration depth these assemblages are recognized as oxygen related ichnocoenoses interpreted oxygenation curves for the basal fort hays member reflect high magnitude redox cycles characterized by rapid deoxygenation events followed by gradual reoxygenation phases curves for the overlying smoky hill member reflect an overall lower degree of oxygenation with more irregular lower magnitude redox fluctuations significant correlation between interpreted levels of paleooxygenation and organic carbon content illustrate the model s potential for evaluating the distribution of organic rich fine grained strata co interpretation of these variables with carbonate content supports the premise that deposition was controlled by paleoceanographic perturbations modulated by milankovitch like climatic cycles comparison with traditionally emphasized macroinvertebrate body fossil criteria clearly demonstrates the greater utility of the trace fossil approach for evaluating paleo oxygenation

a smithian early triassic ichnoassemblage from lichuan hubei province south china implications for biotic recovery after the latest permian mass extinction abstract a late smithian ichnoassemblage is reported from the lower triassic succession from the lichuan section western hubei province south china this ichnoassemblage comprises 13 ichnogenera which include simple horizontal traces cochlichnus cosmorhaphe didymaulichnus gordia circulichnis and planolites vertical traces arenicolites laevicyclus oblique or horizontal branching traces palaeophycus slightly complex burrow networks thalassinoides and treptichnus and grazing traces cosmorhaphe and gyrochorte these traces are also categorized into fodinichnia domichnia pascichnia and repichnia ethologic types suggesting a fairly high degree of behavioral complexity of these the fodinichnia traces are most abundant the ichnoassemblage horizons usually have rather high ichnofabric indices ii reaching ii 4 5 bedding plane coverage is up to 70 90 resulting in high bedding plane bioturbation index as well bpbi 4 5 the lichuan ichnoassemblage traces have mean and maximum diameters of 4 8 mm and 9 mm respectively n 342 the traces penetrate to a depth of up to 30 mm into the sediment with an average depth of penetration of 15 mm indicating third to fourth tiering levels 3 4 overall the lichuan ichnofauna shows an abrupt increase in ichnodiversity burrow width behavioral complexity and ecologic tiering level which are indicative of recovery stages of 2 3 in the late smithian when compared to five coeval ichnoassemblages from other sections in south china yashan susong daxiakou gaimao and tianshengqiao and coeval ichnoassemblages from elsewhere northern italy western australia western us the lichuan trace fossil assemblage reaches similar level of diversity during this time interval paleoenvironmental analysis indicates that the late smithian recovery pulse is typically confined to the offshore transition setting whereas more distal zones record much lower levels of recovery therefore time and environmental conditions were the two crucial factors controlling the rate and degree of recovery of trace making organisms in the aftermath of the latest permian mass extinction

redox and paleoenvironmental conditions of the devonian carboniferous sappington formation southwestern montana and comparison to the bakken formation williston basin abstract the sappington formation outcrops in southwestern montana and spans the upper devonian lower mississippian transition it consists of two carbonaceous shale intervals one upper and one lower and a middle dolomitic siltstone and silty sandstone it was deposited in a relatively small basin with the central montana uplift separating the sappington basin from the williston basin to the east which contains the economically important bakken formation previous biostratigraphic and lithostratigraphic comparisons have identified the sappington and bakken formations as depositionally contemporaneous and due to the subsurface nature of the bakken the sappington has emerged as a useful outcrop analogue this study presents new geochemical data from the sappington formation providing a comparison to the bakken and insight into paleo redox conditions during the devonian carboniferous on the western laurentian margin geochemical signals in the sappington formation outcrop samples are partially obscured by oxidative weathering but similar to the lower bakken shale the lower sappington shale appears to record dominantly euxinic conditions albeit with intervals when the water column was suboxic to ferruginous and with an absence of organic or inorganic geochemical evidence for photic zone euxinia the upper sappington shale appears to have been deposited in ferruginous to dysoxic conditions thus the sappington appears to be an excellent analogue to the bakken on a lithostratigraphic level but the geochemical environment for black shales source rock deposition appears less reducing closer proximity to the open ocean and consequent lower levels of restriction may have been responsible for these redox differences however sappington core samples and an increased focus on integrating geochemical data into the regional stratigraphic record will be necessary to further understand paleoceanographic and biogeochemical drivers

chondrites a trace fossil indicator of anoxia in sediments the trace fossil chondrites a highly branched burrow system of unknown endobenthic deposit feeders occurs in all types of sediment including those deposited under anaerobic conditions in some cases such as the jurassic posidonienschiefer formation of germany chondrites occurs in black laminated carbonaceous sediment that was deposited in chemically reducing conditions in other cases such as numerous oxic clastic and carbonate units throughout the geologic column chondrites typically represents the last trace fossil in a biotutbation sequence this indicates that the burrow system was produced deep within the sediment in the anaerobic zone below the surficial oxidized zone that was characterized by freely circulating and oxidizing pore waters

characteristic trace fossil associations in oxygen poor sedimentary environments the sedimentologic record of oxygen poor depositional environments typically includes trace fossils produced by deposit feeding organisms we propose an oxygen controlled trace fossil model in which increasing oxygen concentration of interstitial water in sediment parallels a transition from fodinichnia dominated through pascichnia dominated to domichnia dominated trace fossil associations in certain places such as parts of the late paleozoic oquirrh basin in utah this model provides a reasonable alternative to the more traditional depth controlled model of trace fossil distribution

palaeoecological patterns within the dysaerobic biofacies examples from devonian black shales of new york state abstract two distinct body fossil distribution patterns are recognized within devonian dysaerobic strata of the appalachian basin 1 taxa that are adapted to the lowest bottom water oxygen conditions and 2 taxa that are adapted to fully oxygenated conditions with distinct ranges of tolerance to low oxygen conditions the latter group taxa demonstrate a nested pattern of distribution in association with relative bottom water oxygen levels as a result rather than discrete unique taxonomic assemblages within the dysaerobic biofacies the successive loss of species in a predictable order in association with decreasing oxygen serves to delineate relative bottom water oxygen levels throughout the range of dysaerobic conditions rhynchonelliform brachiopods of the super family camarotoechioidea are overwhelmingly dominant but species within this clade are ecologically variable valve size of eumetabolotoechia multicostata with a broad range of oxygen tolerance varies in association with relative oxygen levels further variation in life habit distribution is recognized through the dysaerobic zone epifaunal filter feeders dominate under extremely reduced oxygen levels and specific oxygen thresholds are recognized by the addition and or dominance of specific life habits this nested pattern of taxonomic distribution through the dysaerobic zone is unexpected based on observations in modern and other post palaeozoic low oxygen communities and is likely associated with frequent and pervasive periods of dysoxia in shallow epeiric seaways

neoichnology of a barrier island system the mula di muggia grado lagoon italy abstract barrier islands are common landforms and biodiverse habitats yet they received scarce neoichnological attention this gap is tackled by studying the mula di muggia barrier island system grado lagoon italy focusing on morphology ecology and ethology of individual traces the following incipient ichnotaxa are identified archaeonassa arenicolites bergaueria diverging shafts helminthoidichnites lockeia macanopsis monocraterion nereites parmaichnus polykladichnus skolithos thalassinoides and squat burrows vertebrate avipeda ardeipeda like canipeda and invertebrate tracks parallel furrows are also described for each ichnotaxon tracemaker and behavior are discussed together with their position with respect to sediment barriers results suggest that sediment barriers impose a sharp contrast in terms of ichnological composition back barrier is dominated by branched burrows i e thalassinoides parmaichnus while the fore barrier presents vertical and u shaped burrows arenicolites skolithos the environmental conditions of the back barrier show that low oxygen substrates favor intense bioturbation provided that the water column is sufficiently oxygenated

joining forces combined biological and geochemical proxies reveal a complex but refined high resolution palaeo oxygen history in devonian epeiric seas abstract two devonian black shale intervals were analysed using a carefully integrated palaeontological geochemical approach to interpret relative bottom water oxygen levels at a resolution and sensitivity possible only through the combined approach biological data presented as both trace and body fossil abundance and size suggest generally low bottom water oxygen conditions the frequently laminated sediments would be viewed traditionally as recording appreciable intervals of anoxic and or euxinic anoxic and h 2 s containing bottom waters the inclusion of parallel geochemical results however refines this perspective well established iron proxies for depositional redox tell us that anoxic and euxinic conditions were intermittent and short lived if present at all manganese and molybdenum data point to conditions with a likelihood of very low oxygen in bottom waters during intervals with high pore water concentrations of hydrogen sulphide near the sediment water interface these elevated sulphide concentrations under iron limited diagenetic conditions varied with organic content and may have further inhibited benthic colonization and an active infaunal community more specifically the combined palaeontological and geochemical data sets point to depleted bottom water oxygen contents with evidence for hydrogen sulphide limited dominantly or exclusively to the pore waters anoxia and euxinia if present occurred only intermittently and lasted only briefly perhaps on seasonal time scales tied to primary productivity cycles or climatically induced variation in water column stratification the time scales and mechanisms of rapid redox variation remain to be studied the details of such transitional and dynamic settings are best viewed from a multi proxy biogeochemical and palaeoecological perspective as an important byproduct geochemical methods refined in multi proxy studies of phanerozoic shales can then be extended with greater utility to precambrian strata without trace and body fossils

a new approach to an absolute timescale from measurements of orbital cycles and sedimentary microrhythms when the sedimentary microrhythms of two groups of jurassic zones forming part of the chronostratigraphical scale are analysed against recent radiometric age scales there is good agreement with present day cycles resulting from the obliquity of the ecliptic sedimentary microrhythms resulting from orbital forcing can be used to establish either the relative or the true length of chronostratigraphical zones and thus provide a new approach towards an absolute timescale such a scale will be powerful in the analysis of other cycles and perturbations in the geological record

trace fossil model for reconstruction of paleo oxygenation in bottom waters recognition of fluctuations in the degree of paleo oxygenation of bottom waters recorded in fine grained pelagic strata is important for interpretation of paleoceanographic and paleoclimatologic conditions general sedimentary fabric composition of trace fossil assemblages and burrow size and crosscutting relationships have been incorporated into a trace fossil tiering model that permits detailed reconstruction of changes in paleo oxygenation of bottom waters applications of this model to the miocene monterey formation california and the cretaceous niobrara formation colorado indicate that the ichnologic approach is more sensitive to both magnitude and rates of change in oxygenation levels compared to macrobenthic body fossil information

mineral magnetic properties of a weathering sequence of soils derived from basalt in eastern china abstract the magnetic properties and magnetic mineralogy of a weathering sequence of soils developed on basalt parent material from eastern china were studied by rock magnetism x ray diffraction and soil chemical analyses to establish the connection between mineral magnetic properties and pedogenic development in a subtropical region the magnetic susceptibility of soils formed on basalt varied greatly and did not increase with the degree of pedogenic development the frequency dependent susceptibility fd values of soils ranged from 1 0 to 11 1 and increased with the pedogenic development highly significant linear relationship was found between the frequency dependent susceptibility and the fed content r2 0 683 and fed fet ratio r2 0 780 in soils indicating that pedogenic sp ferrimagnetic grains were associated with enrichment of the secondary iron oxide minerals in the weathering process of soil rock magnetism analysis showed that the major magnetic carriers in the weakly weathered soil profiles are magnetite and or maghemite and the highly developed soil profiles are generally enriched in magnetite maghemite grains of pedogenic origin and the magnetically hard haematite indicating that the magnetic component was transformed from a ferrimagnetic phase magnetite to antiferromagnetic phase hematite during pedogenic development results indicated that some of the magnetic parameters of soils in this case fd can be useful for pedogenic comparisons and age correlations in the weathering sequence of soil it is thus suggested that multiparameter rock magnetic investigations represent a more powerful approach for pedogenesis

the early toarcian jurassic anoxic event stratigraphic sedimentary and geochemical evidence detailed paleogeographic studies in the southern alps suggest that an oxygen minimum model is appropriate for interpreting conditions on tethyan continental margins advection of manganese in this low oxygen layer may explain the local occurrence of carbonate ores of this element carbon sulfur and iron sulfur ratios although showing considerable scatter suggest that bottom waters at this time were locally euxinic containing free hydrogen sulfide this oceanic anoxic event was preceded by significant faunal turnover of ammonites in tethys and accompanied by widespread extinction of benthos in northern europe in response to the lateral spread of anoxic bottom waters during transgression similar changes may be recognized in other parts of the world other anoxic events may have taken place during the jurassic but documentation is as yet meagre furthermore models for such phenomena remain largely speculative although upwelling and increased planktonic productivity commencing in pre toarcian time are favored for the falciferum zone event documented here from author

life in near anoxic conditions a case study of the ichnology and infaunal ecology of silurian graptolitic black shales from sardinia italy abstract a trace fossil assemblage from the silurian llandovery black shales of the genna muxerru formation is reported for the first time the ichnofossil record is abundant well preserved and comprises alcyonidiopsis cochlichnus phymatoderma planolites large morphotype planolites rough morphotype and a small chondritid sedimentological and ichnological evidence indicates that the bioturbated black shales were deposited in outer shelf or deeper settings with severe depletion in oxygen the seafloor provided a high food supply for the infauna the palaeoenvironment of the genna muxerru formation is an ancient analogue of modern oxygen minimum zones burrow fills are apparently massive in macroscopic view whereas observations under the petrographic and stereoscopic microscope show that a lifestyle dominated i e that of pellet filling sediment feeders as such the genna muxerru formation provides not only a new ethologic reference for low oxygen settings but also a methodological benchmark for ichnological analysis of black shales bioturbation in black shales may be commoner than expected but its recognition requires microscopic observation

ichnological analysis the common ground between ichnofacies workers and ichnofabric analysts abstract this work compares the approaches taken by the two schools of applied ichnology ichnofacies and ichnofabric analysts and proposes an integrated approach for future improvement there are many similarities between the two approaches which mainly differ in the resolution at which ichnological analysis is undertaken ichnofabric work is by necessity performed on a bed by bed basis and interpreted at a scale consistent with that of the sedimentologist s facies ichnofacies studies in contrast seldom report bed by bed changes in ichnological content instead highlighting bedset to facies association scale changes in ichnology through comparison with summary models of ichnological trends this work highlights the fact that workers from both schools utilize the same tools with slightly different emphases to reach their ichnological conclusions contention between the two schools has in recent years centred upon which is the better method for studying trace fossils an artificial construct that detracts from the fact that both are extremely useful to sedimentological and palaeoenvironmental studies ichnofabric analysis is in addition a highly effective means of documenting bioturbated sedimentary fabrics as the basis for petrophysical studies ichnofacies and ichnofabric analysts use the same toolbox of ichnological methods to determine ancient palaeoenvironments the scale of observation is generally different but the results closely comparable the recognition of key stratal surfaces on ichnological grounds is also possible with both methodologies ichnofacies analysts look to recognise glossifungites ichnofacies firmgrounds in offshore mudstones ichnofabric studies would look to recognise overprinting of fabrics created by groups of organisms from different palaeoenvironments but associated with the same key surface it is proposed that the terms ichnofacies method and ichnofabric analysis are abandoned in favour of a neutral ichnological analysis method encompassing the approaches and terminology of both to produce a unified scientifically rigorous user friendly methodology

primary versus diagenetic origin of blue lias rhythms dorset uk evidence from sulphur geochemistry the question of a primary versus diagenetic origin for the limestone shale rhythms of the blue lias has been addressed through a study of pyrite abundance and isotopic composition pyrite is relatively abundant and isotopically light in the central portions of the bioturbated limestones as compared to adjacent less calcareous sediment the abundance of pyrite shows that the limestones were a focus for prolonged sulphate reduction and pyrite formation the isotopic data indicate that bioturbation oxidized some pyrite to produce isotopically light sulphate part of which was subsequently reduced back to pyrite before preservation by burial acidity generated by pyrite oxidation was buffered in the limestones by carbonate dissolution hence supersaturation of sulphides could be maintained by contrast in adjacent less calcareous sediments carbonate dissolution was unable to buffer acidity and bioturbational oxidation of pyrite formed iron rich pore solutions continued sulphate reduction in the limestones acted as a sink for iron from the adjacent sediments and with burial below the zone of bioturbation the alkalinity so generated caused cementation of the limestones diagenetic cementation would be enhanced during an hiatus in sedimentation an event which might be related to a milankovitch forcing mechanism but which would not be recorded in bioturbated less calcareous sediment thus leaving an imperfect record only cyclicity in pre diagenetic sedimentation patterns may be safely related to a milankovitch forcing mechanism as proposed by weedon

variations in bioturbation across the oxygen minimum zone in the northwest arabian sea abstract oxygen minimum zones are expected to alter substantially the nature rates and depths of bioturbation along continental margins yet these effects remain poorly studied using excess 210 pb profiles sediment x radiography and box core samples for macrofauna we examined bioturbation processes at six stations 400 700 850 1000 1250 and 3400 m deep along a transect across the oxygen minimum zone omz on the oman margin bottom water oxygen concentrations ranged from 0 13 ml l 1 at 400 m to 2 99 ml l 1 at 3400 m 210 pb mixed layer depth and bioturbation intensity db exhibited high within station variance and means did not differ significantly among stations however the mean mixed layer depth 4 6 cm for pooled omz stations 400 1000 m depths 0 13 0 27 ml l 1 bottom water oxygen was half that for stations from similar water depths along well oxygenated atlantic and pacific slopes 11 1 cm suggesting that oxygen stress reduced 210 pb mixing depth on the oman margin modal burrow diameter and the diversity of burrow types at a station were highly correlated with bottom water oxygen concentration from the edge to the core of the oman omz spearman s rho 0 89 p 0 02 suggesting that these parameters are useful proxies for bottom water oxygen concentrations under dysaerobic conditions in contrast neither the maximum diameter and nor the maximum penetration depth of open burrows exhibited oxygen related patterns along the transect reduced 210 pb mixing depth within the oman margin omz appeared to result from a predominance of surface deposit feeders and tube builders within this zone rather than from simple changes in horizontal or vertical distributions of macrofaunal abundance or biomass the number of burrow types per station was highly correlated with macrofaunal species diversity suggesting that burrow diversity may be a good proxy for species diversity in paleo dysaerobic assemblages we conclude that bottom water oxygen concentrations of 0 13 0 27 ml l 1 substantially alter a number of bioturbation parameters of importance to diagenetic and biofacies models for continental margins

composite ichnofabrics and tiering of burrows infaunal communities in marine environments typically are tiered that is different taxa live at different depths within the sediment tiered suites of biogenic structures yield complex biogenic sedimentary fabrics ichnofabrics with the traces of deep burrowing organisms overprinted on those of shallow burrowing organisms careful analysis of crosscutting relationships of burrows in such composite ichnofabrics allows reconstruction of the tiered nature of fossil endobenthic communities it is important to recognize that the best preserved and most prominently displayed trace fossils in most assemblages usually represent the deepest tier thus they were farther removed from the sea floor and therefore less indicative of actual sea floor conditions than the more poorly preserved traces of the shallower tiers on which the deeper traces are juxtaposed

provenance of upper devonian clastic meta sediments of the bollstein odenwald mid german crystalline zone variscides detrital zircons separated from paragneisses of the northern bollstein odenwald mid german crystalline zone variscides yielded cambrian to upper devonian u pb ages the age of the youngest detrital zircon population 371 3 ma and the lower carboniferous metamorphic overprint indicate an upper devonian depositional age of the paragneiss protolith the age spectra of detrital zircons suggest the latter to be derived from 1 ordovician igneous rocks of the saxothuringian zone 2 a late cadomian magmatic arc tepla barrandian unit and 3 a silurian devonian magmatic arc cadomian igneous activity is also documented by cambrian zircon cores in lower to middle devonian detrital zircons meso and paleoproterozoic detrital zircons which are typical for the old red continent and the rhenohercynian zone are entirely lacking the restricted palaeozoic detrital zircon age spectrum is attributed to a silurian devonian intra arc or trench setting both the lack of mesoproterozoic detrital zircons and the striking similarity of the u pb ages of the detrital zircon obtained from the bollstein odenwald with u pb ages from crystalline rocks of the saxothuringian basement rules out that the bollstein odenwald is forming a tectonic window rhenohercynian lower plate inside the mid german crystalline zone saxothuringian upper plate