Intense Bioturbation Research Articles

Petrophysical Characterization of Bioturbated Sandstone Reservoir Facies In the Upper Jurassic Ula Formation, Norwegian North Sea, Europe

Abstract A petrophysical study of bioturbated shoreface sandstone rocks was completed by mapping local variations in air permeability measured in core samples acquired from the Upper Jurassic Ula Formation of the Norwegian Central Graben. Analysis of the shoreface fabrics resulting from bioturbation was studied using arithmetic, harmonic, and geometric numerical modeling and analytical techniques. The arithmetic mean best illustrates samples dominated by high volumes of horizontal or subvertical burrows (50–80%). The distal Skolithos to proximal Cruziana ichnofacies is an example of a rock fabric that can be characterized using the arithmetic mean. The harmonic mean best exemplifies samples dominated by low volumes of vertically oriented burrows (10–50%). The archetypal Skolithos ichnofacies is an example of a rock fabric that can be characterized using the harmonic mean. At extreme volumes of bioturbation (90–100%), isotropic flow units begin developing irrespective of burrow orientation. Numerical models of Ophiomorpha in two different sedimentary bedding features (laminated sandstone and massive sandstone) show that single-phase fluid flow is influenced by parameters such as burrow permeability, burrow connectivity, and bioturbation volume. Spot-permeametry measurements show that the Ophiomorpha burrows have higher permeability values than the host matrix. In the laminated-sandstone models, horizontal shale laminae and low bioturbation intensities promote lamination-parallel fluid flow. At higher volumes of bioturbation, fluid flow becomes increasingly isotropic due to greater amounts of burrow interpenetrations across mud laminae. In the massive-sandstone model, no flow barriers exist. As such, low volumes of bioturbation result in only minor influences on fluid flow. At higher volumes of bioturbation, greater burrow interconnectivity occurs and results in an enhancement of vertical and horizontal burrow permeabilities. Based on the results outlined above, this paper emphasizes the impact bioturbation plays in helping improve reservoir quality in the Ula Formation. Perhaps more importantly, the effectiveness of the numerical models presented to characterize fluid flow in different bioturbated fabrics (e.g., archetypal Skolithos ichnofacies) highlights the fact that the fluid-flow results outlined in this paper can also be applied more broadly to different shoreface successions using ichnofacies knowledge alone.

Ichnofabric characterization in cores: a method of digital image treatment

Ichnofabric analysis, as a relatively young ichnological approach, has witnessed rapid growth, showing its usefulness in basin analysis, with special attention to palaeoenvironmental interpretations. The ichnofabric approach has evolved from the description of trace composition and the intensity of bioturbation to integrate detailed information on numerous ichnofabric features, such as primary sedimentary structures, ichnological diversity, ichnological features, cross-cutting relation ships or tiering structures. This development has been associated with its application to the study of deep-sea sediments, especially in research on cores, which is not easy, owing to the particular features of cores. Here a method for improving ichnofabric characterization in modern marine cores is presented, on the basis of digital high-resolution image treatment, with special emphasis on the quantification of ichnofabric attributes. The proposed methodology is based on the modification of three image adjustments ( image adjustment ), the estimation of the percentage of the area occupied by bioturbation ( digital estimation ), the lateral and vertical quantification and comparison of pixel values for the infill of the trace fossils and the host sediment ( pixel counting ), and the integration of the information obtained in the visual representations of ichnofabrics (the ichnofabric representation ). The sequential application of these proposed steps allow, 1) better identification of trace fossils, together with cross-cutting relation ships and the characterization of trace-fossil assemblages, 2) estimation of the percent age of bioturbation associated to each ichnotaxon, the whole ichnocoenosis, or a complete ichnofabric, 3) differentiation between biodeformational structures and trace fossils, discrimination between ichnotaxa, distinction between passively and actively infilled structures, and 4) evaluation of the depth of penetration by particular tracemakers.

Muddy Prodeltaic Hyperpycnites In the Lower Genesee Group of Central New York, USA: Implications For Mud Transport In Epicontinental Seas

Abstract Recent studies of marine continental shelves show that hyperpycnal flows are responsible for offshore transport of large volumes of sediment. Detailed facies analysis and petrography of the lower Genesee Group in the Northern Appalachian Basin (NAB) shows a wealth of sedimentary textures and fabrics that indicate mud deposition by lateral transport across and along the shelf under energetic conditions. Intervals of silt-rich mudstones and muddy siltstones with internal scours, diffuse stratification, soft-sediment deformation, normal and inverse lamina-set grading, and a reduced intensity and diversity of bioturbation occur in multiple facies types and “interrupt” what appears to be the overall background sedimentation. These intervals and their sedimentary features are interpreted as products of high-density fluvial discharge events, which generated turbulent flows that carried fine-grained clastics several tens of kilometers offshore from the paleoshoreline. Recognizing these sediments as products of river-flood- and storm-wave-generated offshore-directed underflows challenges previous depositional models for organic-rich mudstones in the lower Genesee succession, which call for clastic starvation and suspension settling of clay and silt in a deep stratified basin. Rapid deposition of fine-grained intervals from hyperpycnal plumes in a setting favoring preservation of organic-rich mudstones calls for a reappraisal of the depositional setting of not only the Genesee Group, but also of comparable mudstone successions in the Appalachian Basin and elsewhere.

Lithostratigraphy and depositional history of Upper Ordovician and lowermost Silurian sediments recovered from the Qusaiba-1 shallow core hole, Qasim region, central Saudi Arabia

Lithostratigraphy and depositional history of Upper Ordovician and lowermost Silurian sediments recovered from the Qusaiba-1 shallow core hole, Qasim region, central Saudi Arabia

Unearthing earthquakes and their tsunamis using multiple proxies: the 22 June 1932 event and a probable fourteenth-century predecessor on the Pacific coast of Mexico

Tsunami deposits have been widely studied in temperate latitudes, but the intrinsic difficulties associated with tropical coastal environments, and the intensity of bioturbation in these habitats, limit the possibilities of analysing these formations. Here, we investigate the deposits on the Colima coast of Mexico, which overlies the subducting Rivera and Cocos Plates, in order to reconstruct the tsunami inundation history and related hazard. We developed a multi-proxy study aimed to recognize and date historical and palaeotsunami deposits, including historical data on the effects of a known tsunami, geomorphological mapping, stratigraphic, grain size, organic matter content, diatoms, geochemical composition, magnetic susceptibility, and anisotropy of magnetic susceptibility, together with radiometric dating (210Pb and 14C). We identified two probable tsunami deposits at Palo Verde estuary including a historical event associated with the Mw 6.9 earthquake on 22 June 1932 and a palaeotsunami most likely generated by a similar event in the fourteenth century. This work shows that it is possible to identify both historical and palaeotsunamis in the tropical environment of Mexico’s Pacific coast. These data will serve to enhance our understanding of tsunami deposits in tropical environments and of the regional tsunami hazard.

Stabilization of the coupled oxygen and phosphorus cycles by the evolution of bioturbation

Animal burrowing and sediment-mixing (bioturbation) began during the run up to the Ediacaran/Cambrian boundary1, 2, 3, initiating a transition4, 5 between the stratified Precambrian6 and more well-mixed Phanerozoic7 sedimentary records, against the backdrop of a variable8, 9 global oxygen reservoir probably smaller in size than present10, 11. Phosphorus is the long-term12 limiting nutrient for oxygen production via burial of organic carbon13, and its retention (relative to carbon) within organic matter in marine sediments is enhanced by bioturbation14, 15, 16, 17, 18. Here we explore the biogeochemical implications of a bioturbation-induced organic phosphorus sink in a simple model. We show that increased bioturbation robustly triggers a net decrease in the size of the global oxygen reservoir—the magnitude of which is contingent upon the prescribed difference in carbon to phosphorus ratios between bioturbated and laminated sediments. Bioturbation also reduces steady-state marine phosphate levels, but this effect is offset by the decline in iron-adsorbed phosphate burial that results from a decrease in oxygen concentrations. The introduction of oxygen-sensitive bioturbation to dynamical model runs is sufficient to trigger a negative feedback loop: the intensity of bioturbation is limited by the oxygen decrease it initially causes. The onset of this feedback is consistent with redox variations observed during the early Cambrian rise of bioturbation, leading us to suggest that bioturbation helped to regulate early oxygen and phosphorus cycles.

English

Mangrove soils are anoxic, sulphidic and variable since their chemistry is regulated by a variety of factors such as texture, tidal range and elevation, redox state, bioturbation intensity, forest type, temperature and rainfall. Sulphur-oxidizing bacteria such as photoautotrophs, chemolithotrophs and heterotrophs play an important role in the mangrove environment for the oxidation of the toxic sulphide produced by sulphur reducing bacteria and act as a key driving force behind all sulphur transformations in the mangrove ecosystem which is most essential to maintain the sulphur cycle as well as eco health. These overviews summarizes the current state of knowledge of diversity and important biotechnological contributions of these microorganisms in agriculture, bio fertility, reduction of environmental pollution, maintenance of the productivity of ecosystems and also highlight areas in which further research is needed to increase our basic understanding of physiology, genomics and proteomics of these microorganisms which is most essential. Key words: Mangrove habitat, sulphur oxidising bacteria, sulphur cycle, sulphide oxidase.

A quantitative assessment of the neoichnology and biology of a delta front and prodelta, and implications for delta ichnology

The Fraser River delta front exhibits asymmetry with respect to its neoichnological, biological, and sedimentological characteristics on opposing sides of the main distributary channel, as well as the submarine canyon seaward of the channel. This phenomenon is attributed to asymmetric tidal flow, wherein there is a net northward tidal flow along the delta front. The southern delta front lying updrift of the Main Channel, is primarily composed of sandy beds and bedsets that exhibit low bioturbation intensities (BI 0–3) and limited biodiversity (H` 0.9–1.4). Tracemakers in the updrift delta front predominantly consist of polychaetes and bivalves that produce suites dominated by simple vertical and horizontal traces attributable to the Skolithos Ichnofacies. The suites consist of traces comparable to Thalassinoides, Planolites, Skolithos, rare Conichnus, and Cylindrichnus. The northern delta front lying downdrift of the Main Channel, and the prodelta consist of muddy beds and bedsets that display high bioturbation intensities (BI 3–6) and diverse infaunal populations (H` 1.4–2.3). Polychaetes, bivalves, and echinoderms comprise the main tracemakers, and these infauna produce complex and robust ichnological suites typical of the Cruziana Ichnofacies. The downdrift delta front and prodelta trace assemblage includes traces comparable to Skolithos, Planolites, Thalassinoides, Artichnus, Palaeophycus, Conichnus, Asterosoma, Rosselia, Cylindrichnus, Gyrolithes, Teichichnus, Phycosiphon, Arenicolites, Polykladichnus, and Scolicia. The spatial distribution (area and volume) of biogenically induced sediment disruption indicates that the updrift delta front is significantly less bioturbated than its downdrift counterpart. Additionally, the prodelta does not exhibit evidence of asymmetry with respect to its neoichnological character; all spatial variability is recorded in delta front positions.

Ichnofabric characterization of a deep‐marine clastic system: a subsurface study of the Middle Eocene Ainsa System, Spanish Pyrenees

AbstractThis paper documents a subsurface trace fossil and ichnofabric study of the proximal parts of a structurally confined and channelized sand‐rich, lower slope and proximal basin‐floor deep‐marine system in the Middle Eocene Ainsa basin, Spanish Pyrenees. Five depositional environments are recognized based on sedimentary facies associations, depositional architecture and stratigraphic context (channel axis, channel off‐axis, channel margin, leveé‐overbank and interfan), as well as a channel abandonment phase. Each environment is characterized by distinct and recurring ichnofabrics. Ichnological measurements and observations were recorded from six cores recovered from six wells drilled at a spacing of between 400 m and 500 m at outcrop, and totalling 1213 m in length. From channel axis to levée‐overbank environments, there is a trend of increasing bioturbation intensity and ichnodiversity. Ichnofabrics in channel axis and channel off‐axis environments are characterized by low bioturbation intensity and low ichnodiversity. Thalassinoides‐dominated firmground ichnofabrics associated with erosive sediment gravity flows are common in these environments. In contrast, channel margin and levée‐overbank environments are characterized by ichnofabrics associated with high bioturbation intensity and ichnodiversity. Sediments of the interfan are characterized by the highest bioturbation intensity, associated with burrow mottling and an absence of primary sedimentary structures. This paper demonstrates that in core‐based studies, ichnofabric analysis is an important and valuable tool in discriminating between different environments in channelized deep‐marine siliciclastic systems. The results of this study should find wide applicability in reservoir characterization studies in the petroleum industry, in field‐based analogue ichnofabric studies and other core‐based studies in deep‐water siliciclastic systems worldwide such as the Integrated Ocean Drilling Program.

Variability In Sedimentological and Ichnological Signatures Across A River-Dominated Delta Deposit: Peay Sandstone Member (Cenomanian) of the Northern Bighorn Basin, Wyoming, U.S.A

Abstract Incorporation of ichnological (trace fossil) data into facies models for deltaic systems and their deposits has yielded useful new criteria for discriminating between alternative interpretations. Nevertheless, despite the recognition that trace-fossil suites may vary spatially across an individual delta system, few spatially resolved datasets are currently available to inform these interpretations. We present a new sedimentological and ichnological analysis of the Upper Cretaceous (Cenomanian) Peay Sandstone Member of the Frontier Formation in the northern Bighorn Basin, Wyoming, U.S.A., based on three-dimensional mapping of surface exposures and subsurface drilling data. Both down-depositional-dip and across-depositional-strike trends can be determined and quantified prima facie from this dataset. The Peay Sandstone Member is a crudely coarsening-upward sandstone body that has an elongate planform shape, projecting southeastward into the basin. Based on a facies analysis, the vertical facies succession in the core of the body represents, from base to top, deposits of prodelta (stratified siltstone), distal delta front (thinly interbedded siltstone and sandstone), through medial–proximal delta front (thickly interbedded siltstone and sandstone to bioturbated fine-grained sandstone), to mouth bar (amalgamated, largely unbioturbated, fine- to medium-grained sandstone) environments. This complete progradational succession persists for at least 40 km in the depositional dip direction, thinning and fining abruptly over a few kilometers south of the town of Greybull, Wyoming. Transects from the delta axial core to the peripheral flanks reveal changes in facies reflecting a transition from a high-energy, river-dominated mouth-bar environment to a tide- and wave-influenced medial–proximal delta flank, and ultimately, to a low-energy, storm- and wave-influenced distal delta flank setting on the peripheries of the delta lobe. The trace assemblage in the axial core is interpreted as a highly stressed expression of the Skolithos Ichnofacies. This passes gradationally down depositional dip into stressed expressions of the Cruziana Ichnofacies, and along depositional strike into an archetypal (unstressed) expression of the Cruziana Ichnofacies on the delta flanks. This spatial variation in bioturbation intensities and trace-fossil assemblages is interpreted to reflect dissipation of physico-chemical stresses from the axial core to peripheral flanks. The outcomes of this study emphasize the point that data acquired on an ancient deltaic deposit in isolation of their paleogeographic context may give rise to a misleading impression of that system, and potentially a less than parsimonious interpretation of its process balance and planform geometry.

Dynamics of late Cenozoic aeolian deposition along the South African coast: a record of evolving climate and ecosystems

Abstract Dune systems along the South African coast are sensitive barometers of fluctuations in palaeoenvironments, as archived in their orientation, geometry, internal architecture, composition, granulometry, diagenesis, palaeontology and archaeological content. Presently, the pronounced climatic/oceanographic gradients around the southern African coastline, including the west coast Mediterranean climate type, with cold upwelling to progressively warmer in terms of climate and sea temperatures eastwards, are mirrored by variations in these parameters. Here, we review and contribute new information concerning their fluctuations from the Miocene to the present to track changes in the bio-, hydro- and geospheres through time. West coast dunes take the form of dune plumes, which have an orientation since the Miocene that mirrors the southerlies of the South Atlantic Anticyclone (SAA), muted during the warm Pliocene, as reflected by intense bioturbation. Shoreline-parallel, vertically aggraded dune cordons dominate along the southern and eastern coasts, formed by (winter) polar westerlies since the Miocene. The contrasting dunefield morphologies relate to seasonality of wind strength and precipitation. Subtropical east coast dunes are profoundly weathered – on the shelf, glacial-period dunes indicate different atmospheric circulations. The long-term stability of the warm Agulhas Current contrasts with variability in the Benguela. The aeolianites host a rich human and faunal archive, including human ichnofossils. Supplementary material: Tables S1 and S2, which provide details of the data and data sources for Figure 9 in the text, are available at http://www.geolsoc.org.uk/SUP18711 .

Interactions between Ediacaran animals and microbial mats: Insights from Lamonte trevallis, a new trace fossil from the Dengying Formation of South China

A new ichnogenus and ichnospecies, Lamonte trevallis, is formally described from the Shibantan Member limestone of the upper Ediacaran Dengying Formation, Yangtze Gorges area, South China. It is characterized by horizontal tunnels connected with short vertical burrows and surface trails. The horizontal burrows are elliptical or bilobed in transverse cross-section, preserved in full relief, and filled with carbonate intraclasts, micrites, as well as calcite and silica cements. They occur exclusively in silty, crinkled, and microlaminated layers that are interpreted as amalgamated cyanobacterial microbial mats; no burrows have been found in intraclastic layers adjacent to the microlaminated layers. The vertical traces are filled with the same material as the burrows, but they typically project through the crinkled microlaminae and are exposed on the bedding surface. The surface tracks are always preserved in negative epirelief or positive hyporelief and consist of two parallel series of either sharp scratch marks or small knobs. The burrow infill has δ18Ocarb and δ13Ccarb values distinct from, but intermediate between, microlaminated and intraclastic layers, consistent with petrographic observation that burrow infill consists of a mixture of early carbonate cements, intraclasts, and micrites. Bedding plane bioturbation intensity (20–40%)—measured as percentage of bedding plane area covered by L. trevallis traces—is comparable to similar measurements in pre-trilobite Cambrian carbonates. The exclusive occurrence of L. trevallis within microbial mats may have both taphonomic and ecological significance. These mats may have provided firm substrates and localized geochemical conditions that contributed to the structural integrity of the burrows, and they may have also facilitated early diagenetic cementation of burrow infill, thus facilitating burrow preservation. The close association of these burrows with microbial mats implies that the trace producers actively mined cyanobacterial mats to exploit oxygen or nutrient resources. The trace makers of L. trevallis were better able to utilize the resources around them than many other Ediacaran trace makers and provide an ichnological record of a flourishing benthic ecology in late Ediacaran oceans at the dawn of the agronomic revolution.

Assessing Early Triassic paleoceanographic conditions via unusual sedimentary fabrics and features

Analysis of the Lower Triassic sedimentary rock record has revealed a variety of unusual sedimentary fabrics and features, many of which were more common in the Proterozoic than the Phanerozoic. These so-called “anachronistic facies” developed across Lower Triassic carbonate platforms from shallow subtidal environments to deep, basinal environments and include features associated with the enhanced precipitation of calcium carbonate (e.g., synsedimentary seafloor cements), microbialites, and fabrics that formed due to a reduction in the intensity and depth of bioturbation (e.g., vermicular limestones). Three interacting factors controlled the development of anachronistic facies during the Early Triassic including: 1) environmental conditions (i.e., the unusual chemistry of Early Triassic oceans); 2) biotic factors (e.g., bioturbation and grazing pressures); and, 3) actualistic sedimentologic processes (e.g., waves and currents, sedimentation). Because these three factors typically acted in concert with each other, it is important to consider the role of each when using anachronistic facies to reconstruct Early Triassic paleoceanographic conditions. Overall, anachronistic facies are indicative of enhanced calcium carbonate precipitation during the Early Triassic, but do not necessarily indicate the former presence of anoxic or suboxic conditions where they were deposited. The influence of seawater chemistry on the deposition of anachronistic facies was most pronounced in deeper water environments, where anomalous paleoceanographic conditions heightened calcium carbonate precipitation and caused a decrease in bioturbation and grazing pressures via the upwelling of anoxic, alkaline waters. The degree of environmental influence lessened shorewards, where biotic factors and actualistic processes were more important, and slight modification of seawater by microbial activity, shallow burial, or CO2-degassing was necessary for calcium carbonate precipitation to occur. Anachronistic facies are a useful tool for reconstructing paleoceanographic conditions and examining shifts in the distribution of environmental stress during the recovery from the Permian–Triassic mass extinction, but only when the biologic, sedimentologic, and geochemical factors that led to their growth are carefully considered.

A cost-effective method to quantify biological surface sediment reworking

We propose a simple and inexpensive method to determine the rate and pattern of surface sediment reworking by benthic organisms. Unlike many existing methods commonly used in bioturbation studies, which usually require sediment sampling, our approach is fully non-destructive and is well suited for investigating non-cohesive fine sediments in streams and rivers. Optical tracer (e.g., luminophores or coloured sand) disappearance or appearance is assessed through time based on optical quantification of surfaces occupied by tracers. Data are used to calculate surface sediment reworking (SSR) coefficients depicting bioturbation intensities. Using this method, we evaluated reworking activity of stream organisms (three benthic invertebrates and a fish) in laboratory microcosms mimicking pool habitats or directly in the field within arenas set in depositional zones. Our method was sensitive enough to measure SSR as low as 0.2 cm2.d-1, such as triggered by intermediate density (774 m-2) of Gammarus fossarum (Amphipoda) in microcosms. In contrast, complex invertebrate community in the field and a fish (Barbatula barabatula) in laboratory microcosms were found to yield to excessively high SSR (>60 cm2.d-1). Lastly, we suggest that images acquired during experiments can be used for qualitative evaluation of species-specific effects on sediment distribution.

Unmixing of stable isotope signals using single specimen δ18O analyses

AbstractThe resolution at which foraminiferal stable isotopes are applied in paleo‐environmental studies is ever increasing, resulting in continuous sampling of sediment cores. The resolution of such continuously sampled records depends on the rate of sedimentation of foraminiferal shells in its relation to the intensity of bioturbation. Bioturbation essentially mixes sediment layers of different age, altering the primary climate signal, thereby impacting the accuracy of both the timing and magnitude of reconstructed climate changes. A new approach to assess and correct the impact of bioturbation is investigated here, based on the δ18O of individual specimens of planktonic foraminifera Globorotalia inflata from a series of boxcore samples in the Eastern North Atlantic. Average δ18O values decrease southward from 1.62 to 1.07‰ with the exception of site T86‐11 (1.35‰). The δ18O distribution of each station can be fitted with a uni‐ to polymodal distribution. A nonunimodal distribution strongly suggests admixing of bioturbated individuals. Quantification of these distributions allows deconvolving the original and bioturbated signals and subsequently provides a correction for bioturbation.

Neoichnology of a barrier-island system: The Mula di Muggia (Grado lagoon, Italy)

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.

Palaeoenvironmental significance of iron carbonate concretions from the Bathonian (Middle Jurassic) ore-bearing clays at Gnaszyn, Kraków-Silesia Homocline, Poland

ABSTRACT Witkowska, M. 2012. Palaeoenvironmental significance of iron carbonate concretions from the Bathonian (Middle Jurassic) ore-bearing clays at Gnaszyn, Krakow-Silesia Homocline, Poland. Acta Geologica Polonica, 62 (3), 307-324. Warszawa. Iron carbonate concretion horizons are characteristic features of the Bathonian (Middle Jurassic) claystone-mudstone succession at Gnaszyn. They occur in single horizons, which generally represent the same genetic type. The siderite concretions are the main type of iron carbonate concretions at Gnaszyn; a second type is represented by phosphate-siderite concretions. On the basis of the fieldwork, and their petrographical and mineralogical characteristics, the genesis of the concretions and their palaeoenvironmental significance is discussed. The results of this study (based on the localization, mode of occurrence, mineralogy of iron carbonate concretions and also the textural relationship between the concretions and host sediment layers) suggest an early diagenetic origin of the concretions. The preferential occurrence of the concretion horizons in single layers in the ambient sediments was associated with particular conditions of their deposition and early diagenesis, favored by a slower sedimentation rate and more intense bioturbation, and related primarily to the greater availability of reactive iron ions. From the viewpoint of physicochemical conditions the horizons with iron carbonate concretions in the study area reflect the redox boundary between oxic/bioturbated and anoxic/non-bioturbated zones. The conditions favoring the formation of such horizons was possibly due to longer periods of diminished sedimentation rate when the redox boundary remained in the same position within the sediment.

Biogenically enhanced reservoir properties in the Medicine Hat gas field, Alberta, Canada

Spot-minipermeametry and micro-CT analytical techniques were employed to evaluate the effect(s) of bioturbation on porosity and permeability distributions in reservoir facies of the highly prolific Medicine Hat gas field of Alberta, Canada. This study focuses on five cm-scale samples that occur at different stratigraphic positions and intersect discrete bioturbated horizons within the Medicine Hat Member. Spot-minipermeability results demonstrate that permeability in bioturbated sandstones is up to two orders of magnitude greater than those of the muddy matrix, thus corresponding to a dual-porosity fluid flow system. Graphing bioturbation intensity versus measured permeability indicates that burrows are well connected horizontally and provide flow conduits. Micro-CT models reveal that burrowed sandstones are isolated and planiform in character; rare interpenetrations from vertically oriented trace fossils serve to interconnect these hydraulically isolated strata. Results show that bioturbated sandstones possess reservoir properties comparable to those associated with laminated sandstones, yielding an anisotropic porous medium. Understanding the impact of trace fossils on porosity–permeability distributions in the Medicine Hat gas field can be used to identify potential reservoir from previously interpreted non-reservoir rock, and, ultimately, improve reserve estimations.

Earthen mound formation in the Uruguayan lowlands (South America): micromorphological analyses of the Pago Lindo archaeological complex

The stratigraphic excavation of the Pago Lindo archaeological complex, in central-eastern Uruguay (La Plata basin), helped to propose an alternative model for mound formation that expresses the intrinsic complexity of prehistoric earthen architecture. This model, known as the spatial–temporal discontinuous model, sees mound complexes as multi-functional areas, with diverse earth works occupied and abandoned intermittently. Since earthen mound sediments are homogeneous, resemble natural soils and show evidences of intense bioturbation, soil micromorphology was used to confirm, refute and further investigate issues raised during field work, related with the prime material used for mound construction, detection of major episodes of mound building, identification of activity areas and taphonomic processes. In this paper, we present the results of the micromorphological analyses of two different earth works from the Pago Lindo archaeological complex (a mound and a micro-relief). Analyses proved the recurrent use of surface horizons for mound and micro-relief building throughout the entire period of site occupation. It also demonstrated the difficulty in identifying discrete depositional episodes and occupation surfaces, because of the intense bioturbation. Two activity areas where recognized: a domestic hut built over a platform, ca. 1600 yrs. BP; and an area of plant residue accumulation over a platform, raised almost 800 hundred year after the domestic hut. The practice of cleaning the occupation surfaces was interpreted from the complete absence of bioarchaeological remains (bones and micro-charcoal). The use of micromorphology as a complementary tool in the stratigraphic excavation of Pago Lindo unraveled evidences that corroborate a newly proposed model for mound growth.

Late Quaternary spatial and temporal variability in Arctic deep-sea bioturbation and its relation to Mn cycles

Changes in intensity and composition of bioturbation and trace fossils in deep-sea settings are directly related to changes in environmental parameters such as food availability, bottom water oxygenation, or substrate consistency. Because trace fossils are practically always preserved in situ, and are often present in environments where other environmental indicators are scarce or may have been compromised or removed by diagenetic processes, the trace fossils provide an important source of paleoenvironmental information in regions such as the deep Arctic Ocean. Detailed analysis of X-ray radiographs from 12 piston and gravity cores from a transect spanning from the Makarov Basin to the Yermak Plateau via the Lomonosov Ridge, the Morris Jesup Rise, and the Gakkel Ridge reveal both spatial and temporal variations in an ichnofauna consisting of Chondrites, Nereites, Phycosiphon, Planolites, Scolicia, Trichichnus, Zoophycos, as well as deformational biogenic structures. The spatial variability in abundance and diversity is in close correspondence to observed patterns in the distribution of modern benthos, suggesting that food availability and food flux to the sea floor are the most important parameters controlling variations in bioturbation in the Arctic Ocean. The most diverse ichnofaunas were observed at sites on the central Lomonosov Ridge that today have partially ice free conditions and relatively high summer productivity. In contrast, the most sparse ichnofauna was observed in the ice-infested region on the Lomonosov Ridge north of Greenland. Since primary productivity, and therefore also the food flux at a certain location, is ultimately controlled by the geographical position in relation to ice margin and the continental shelves, temporal variations in abundance and diversity of trace fossils have the potential to reveal changes in food flux, and consequently sea ice conditions on glacial–interglacial time scales. Down core analysis reveal clearly increased abundance and diversity during interglacial/interstadial intervals that were identified through strongly enhanced Mn levels and the presence of micro- and nannofossils. Warm stages are characterized by larger trace fossils such as Scolicia, Planolites or Nereites, while cold stages typically display an ichnofauna dominated by small deep penetrating trace fossils such as Chondrites or Trichichnus. The presence of biogenic structures in glacial intervals clearly show that the Arctic deep waters must have remained fairly well ventilated also during glacials, thereby lending support to the hypothesis that the conspicuous brown layers rich in Mn which are found ubiquitously over the Arctic basins are related to input from rivers and coastal erosion during sea level high-stands rather than redox processes in the water column and on the sea floor. However, the X-ray radiograph study also revealed the presence of apparently post-sedimentary, diagenetically formed Mn-layers which are not directly related to Mn input from rivers and shelves. These observations thus bolster the hypothesis that the bioturbated, brownish Mn-rich layers can be used for stratigraphic correlation over large distances in the Arctic Ocean, but only if post sedimentary diagenetic layers can be identified and accounted for in the Mn-cycle stratigraphy.