Sedimentary Structures Research Articles

Application of the reverberation waveform inversion for the high-resolution sediment layer structure in the Pacific

Summary This study presents the velocity structures of the seafloor sediment across the Pacific using teleseismic body waves recorded by arrays of ocean bottom seismometers (OBSs). We apply the method inverting the multi-component stacked autocorrelation functions and radial-component waveform of teleseismic body waves. We analyze OBSs distributed across the Pacific Ocean, through a wide range of water depths and equipped with various sensor types. The inverted models agreeing with measurements of seafloor drilling samples demonstrate the broad applicability of employing the inversion method to resolve sediment thicknesses ranging from <100 m to a few kilometers. In addition to various broadband OBSs, we confirm our method works also for short-period sensors, since our retrieved the model using data from a short-period sensor in Northeast Japan agrees with active-source seismic survey analyses. The constrained models show various numbers of intra-sediment discontinuities across different locations. Furthermore, the newly characterized sediment models have higher values of P-wave to S-wave velocity ratio (${V}_P$/${V}_S$) than the previously known scaling relationships, which suggests a larger effect of sediment-layer reverberation on the body-wave analysis than we have anticipated. The sediment velocity models obtained in this study produce Rayleigh-wave admittances that are consistent with those in previous studies but with much finer structural details within the sediment layer. The reverberation waveform inversion method will enable improved predictions of the impact of the water and sediment layers in body-wave records from existing and future OBS observations.

Bacterial Community Structure and Environmental Driving Factors in the Surface Sediments of Six Mangrove Sites from Guangxi, China

Mangroves, as blue carbon reservoirs, provide a unique habitat for supporting a variety of microorganisms. Among these, bacteria play crucial roles in the biogeochemical processes of mangrove sediments. However, little is known about their community composition, spatial distribution patterns, and environmental driving factors, particularly across the large geographical scales of mangrove wetlands. In this study, the composition and spatial distribution of the bacterial community structure and its response to fifteen physicochemical parameters (including temperature, pH, salinity, moisture, clay, silt, sand, organic carbon (OC), total nitrogen (TN), total phosphorus (TP), inorganic phosphorus (IP), organic phosphorus (OP), δ13C, δ15N, and carbon/nitrogen ratio (C/N ratio)) were characterized in 32 sampling locations of six different mangrove habitats from Guangxi, China, applying 16S rRNA gene high-throughput sequencing technology and correlation analysis. Our results indicated that the spatial distribution patterns in bacterial communities were significantly different among the six different mangrove sites, as evidenced by NMDS (non-metric multidimensional scaling), ANOSIM (analysis of similarity), and LDA (linear discriminant analysis) analysis. Composition analysis of bacterial communities showed that overall, Chloroflexi (8.3–31.6%), Proteobacteria (13.6–30.1%), Bacteroidota (5.0–24.6%), and Desulfobacterota (3.8–24.0%) were the most abundant bacterial phyla in the mangrove surface sediments. Redundancy analysis (RDA) further highlighted that salinity, δ13C, temperature, δ15N, and silt were the most critical environmental variables influencing the composition of bacterial communities across the whole mangrove samples. Notably, Chloroflexi, one of the most abundant bacterial phyla in the mangrove wetlands, displayed a significantly positive correlation with OC and a negative correlation with δ13C, suggesting its essential role in the degradation of terrestrial-derived organic carbon. These findings support the current understanding of the roles of the bacterial communities and their interactions with environmental factors in diverse mangrove ecosystems.

Seismic evidence for fluid migration on the outer Chukchi Shelf, Arctic Ocean

Methane released from shelf regions is considered to be an important contributor to climate change. Knowledge of potential emissions from specific regions is therefore critical for estimating expected climate change. To this end, we use multichannel seismic data acquired in 2011 to image the subsurface and identify upwelling gas migration structures along the outer Chukchi Shelf in the Arctic Ocean. We examine the upper 2500 ms TWT of sediments for distinct sedimentary structures and anomalous seismic velocities. The data show a wide distribution of bright spots as a consequence of the fluid migration from deeper sources throughout the study area. However, no bottom simulating reflector is identified in the study area to indicate the base of a likely gas hydrate zone. Furthermore, high seismic velocities and strong ground reflectors are not observed, suggesting an absence of submarine permafrost from the outer Chukchi Shelf.

The occurrences, characteristics, and implications of chaotic rocks (accretionary rocks) within the Trans-Saharan orogenic belt in Nigeria

The present research paper reveals the occurrences of metachaotic rocks/accretionary sediments (mélange) in Igarra, southwest Nigeria. These metachaotic rocks are associated with schistose, marble, pegmatite, granitoid, and gneissose units. The mineral assemblage of the surrounding rocks shows garnet, biotite, staurolite, quartz, and chlorite assemblage, which suggest that the rocks were metamorphosed to the epidote–amphibolite facies grade. The chaotic rocks were classified into four groups based on major differences in color, grain size distribution, and stratigraphic and structural characteristics of observed blocks and matrices surrounding them. Group I metachaotic rocks are melanocratic in color, and form pockets of coarse lineation. They are highly deformed with mullions, rodding structures, and burrow-like cavities and show quartzite veins. Group II occurs as a leucocratic to melanocratic mixture of meta-siltstone and metapelite with greenish rock fragments trapped within the meta-siltstone layers. This unit has a cover of meta-siltstone layers occurring in pockets as striations forming mullion and rodding structures as well as ripple-like lineation. Group III metachaotic rocks occur as multiple benches of different strata tilted between 70 and 80º, with mild folding and burrow-like cavities at the base and well-preserved sedimentary structures. In Group IV, there are evidences of pre-metamorphic soft sediment deformational structures that are well-preserved in the metachaotic rocks, with discontinuous blocks of different units trapped within mixtures of metapelite and meta-siltstone. In this group, micro-faults and quartzite veins are scattered within the rocks. Meta-siltstone occurs as horizontal and vertical layers and web-like structures showing dendritic patterns. Overall, the field and petrographic characteristics suggest the Igarra metachaotic rocks are tectonic in origin and may have been formed within the trench basin of an outer accretionary complex.

Prediction of Thin Pre-Salt Reservoir in PL Gas Field, Sichuan Basin

In this paper, the reservoir characteristics and favorable exploration zones of Jialingjiang Formation in Triassic Jialingjiang Formation in PL gas field, Sichuan Basin are studied in depth. By means of geophysical processing and prediction techniques such as seismic attribute analysis, inversion and structural interpretation, this paper focuses on the sedimentary facies, structure, fractures, reservoir thickness and physical properties of the porous reservoir in Jialingjiang Formation, in order to define the spatial distribution of the reservoir and the favorable exploration zones. The research results show that the development of Baiyun rock beach facies reservoir in Jiawumember is mainly concentrated in well area JT1-PS7-PY1, where the local deposition thickness increases and it is a favorable tidal beach facies zone. Based on multi-attribute analysis and phase-controlled p-wave velocity inversion, this paper predicts the lithology and gas distribution of carbonate and gypsum rocks in the Jiawu Member, revealing that the reservoir thickness of the Jiawu 1 submember is between 5 and 22 meters, and gradually thinning in the southeast direction. Based on paleogeomorphology analysis and fracture development characteristics, six favorable exploration areas with a total area of 48 square kilometers are selected, and three proposed well locations are proposed, which provide guidance for future exploration evaluation and deployment.

True substrates from a Mississippian wetland: windows into the biogeomorphology of Visean tetrapod habitats (Tyne Limestone Formation, UK)

Records of Carboniferous biogeomorphology and tetrapod distribution are drawn predominantly from Pennsylvanian strata, with Mississippian counterparts more sparsely known. In this paper we describe trace fossils and vegetation-induced sedimentary structures from a Mississippian (Visean) fluvio-deltaic succession (Yoredale Group, UK). These structures are archived on true substrates and as sampled topography, providing a high-resolution glimpse into an early tetrapod habitat. A diverse ichnoassemblage comprises 18 invertebrate ichnospecies in addition to what are likely Britain's earliest tetrapod trackways. Vegetation-induced sedimentary structures include upturned laminae, scour-and-mound bedding and mudstone-filled hollows, which developed as plants mediated sedimentation and erosion. The distribution of both the trace fossil and VISS record is controlled by the abundance of true substrates. We contend that sedimentary landscapes that were primarily stabilized by vegetation promoted true substrate preservation, such that a complex interplay between biogeomorphology and palaeoenvironmental records exists in lower Carboniferous terrestrial successions. This case study demonstrates that an improved consideration of true substrate distribution within sedimentary successions can strengthen understanding of faunal habitats and biogeomorphic interaction.

Bayesian statistical analysis reveals spatial heterogeneity in Cyclone Thane deposits from Southeast India

Modern and geological records of storm sedimentary deposits preserved on siliciclastic coastlines are important archives to evaluate the past and current magnitude and impacts of.storms. Examination of modern storm deposits also offers the opportunity to evaluate the similarities and differences between storm and other coastal overwash processes and hazards.We examined the stratigraphy and sedimentary characteristics of the 31st December 2012 Cyclone Thane and underlying coastal units from 14 pits from six sites from the coastal zone of Tamil Nadu Province, southeast India. We analysed the grain size parameters, grain shape, and heavy mineral proportions of each deposit in high resolution and examined the sedimentary structures of each unit. For the first time, we use Bayesian factors to quantitatively evaluate the similarities and differences between the storm sedimentary deposits and other co-located coastal sedimentary deposits. At several sites, the storm deposits differ in several parameters from the underlying coastal deposits, but at some locations, distinguishing between different depositional units cannot be achieved. In comparing the storm deposits from the different sites, mean grain size results in the most coherent pattern with closely located sites having similar mean grain size, and more southerly sites being finer grained. The other measured parameters show a far less coherent pattern with adjacent sites often preserving larger differences than more distal sites attesting to very local hydrodynamic variations during sediment deposition. As with the sedimentary parameters, the sedimentary structures formed during sediment deposition preserved at each site are highly variable. To date, the presence of terminal foresets at the landward edge of washover fans remains the only diagnostic feature of storm deposition, but that this feature is not ubiquitous across all storm deposits. Our findings demonstrate the spatially heterogeneous nature of storm sediment deposition and the challenges of identifying storm deposits in coastal siliciclastic sequences. The use of Bayesian statistical approaches also offers a robust method for evaluating and discriminating between coastal sediment deposits that has many advantages over traditional frequentist approaches. This method can easily be applied to other sedimentary depositional environments.

Anisotropy study based on uniaxial compression experiments

Abstract Due to the striated sedimentary structure of laminated shale and the weak cementation between the layers, the damage patterns of laminated shale with different laminar angles have obvious anisotropy after being damaged by uniaxial compression experiments. In this paper, numerical simulations of uniaxial compression experiments are carried out based on GDEM-PDyna on laminated shale with laminar angles (0°, 15°, 30°, 45°, 60°, 75°, 90°) to explore the anisotropy of compression parameters of laminated shale. The results show that: (1) the uniaxial compressive strength of laminated shale with the increase of the angle of the laminate shows a “U” type change rule which decreases first and then increases, and the uniaxial compressive strength is the smallest when the angle of the laminate reaches 60°; (2) the Poisson’s ratio of the specimen of laminated shale with the increase of the angle of the laminate shows a process which decreases first and then increases, and the change of Poisson’s ratio with the angle of the laminate has the same relationship with the elasticity of the specimen, and it is the same as the change of the compression parameters. The change of Poisson’s ratio with the increase of lamination angle and the change of modulus of elasticity show a similar law, all of them have experienced the “U” type change rule of decreasing first and then increasing, and all of them reach the minimum value when the lamination angle is 30°. (3) The brittleness index of the laminated shale samples showed a “stepwise” decrease with the increase of the laminated angle.

Rheological characterization and modeling of ultra-high-velocity fluidized submarine landslides

Submarine landslides are critical phenomena due to their potential to reshape seabed topography, trigger tsunamis, and compromise offshore infrastructure. Understanding the rheological properties, particularly shear stress and viscosity under high shear rates, is essential for comprehending the dynamics of these landslides, a topic often underexplored in previous research. This study explores the rheological behavior of fluidized submarine landslides, with a focus on in-site sediments from the South China Sea and the Western Pacific Ocean. Samples prepared with varying densities were subjected to extensive rheological testing in the laboratory and analyzed under shear rates of up to 2000 s−1. Results indicated that all samples exhibited non-Newtonian fluid characteristics, showing shear-thinning behavior at low shear rates and shear-thickening behavior at higher shear rates. This transition is attributed to the breakdown of internal sediment structures, leading to changes in viscosity. This study also found that higher water content generally results in lower yield stress and consistency coefficients, while increasing the shear rate reduces the nonlinearity of the fluid's behavior. To model this complex behavior, a piecewise rheological model based on the Herschel-Bulkley framework was proposed. This model effectively captures the variations in rheological properties across different shear rate stages, with critical shear rates influenced by the sediment type and water content. These findings contribute to a deeper understanding of submarine landslides under extreme conditions, and the proposed model offers a more accurate tool for predicting the behavior of fluidized submarine landslides.

Evidence for subsidence rate variation in the Newark Basin and its influence on the deposition of fluvial sediments of the Stockton Formation

AbstractThe Stockton Formation of the Newark Basin represents an exciting time in Earth history during the early stages of rifting of the North Atlantic, with rapid subsidence and sediment deposition. Much of the previous work on this formation has been done using drill cores, with development limiting surface exposure. While this allows us to build a full stratigraphic sequence of deposition, it has the potential to miss smaller‐scale variability and sedimentary structures. It is also difficult to understand the three‐dimensional geometries, architecture and relationships between the depositional elements from drill core alone. An outcrop in North Bergen, NJ that was recently exposed during construction provided an opportunity to study the primary and secondary sedimentary structures of a previously hidden part of the Stockton Formation. These sediments and structures were investigated in the field and laboratory, with geochemical data assisting in the interpretation of their origin and history. Despite its small size, this outcrop provides strong evidence for varying rates of basin subsidence, with rocks deposited during more rapid subsidence also showing secondary structures associated with major earthquakes. Rapid subsidence during fluvial deposition resulted in the preservation of more fine‐grained, lower‐porosity sediments. Recently the Newark Basin has been identified as having characteristics that are favourable for CO2 storage, however, a detailed understanding of sedimentary lithologies is essential to assessing that potential. If these periods of rapid subsidence have similar lithological consequences basin‐wide, this could potentially impact the capacity for carbon sequestration, by reducing porosity and permeability in many areas, and potentially creating barriers to flow.

Chapter 6. Albertine Rift Marginal Lacustrine Environments

Marginal lacustrine depositional environments in the Albertine Rift are highly sensitive to short-term, climatically controlled lake level fluctuations. As such, they act as one of the prime recorders of paleo-shoreline transgression or regression in rift-fill stratigraphy. In both Lakes Edward and Albert, modern day seasonal winds blow parallel with the long-axis of the rift basin, developing wave dominated shorelines at the end of a lake, but sheltered rift flanks display coastal features associated with longshore drift. The different types of shoreline around Albertine Rift lakes today produce characteristic geomorphological features and associated lithofacies, with key sedimentary structures and fauna/flora that can include hippopotamus, elephant, crocodile and fish remains. These can be identified in Pleistocene – Holocene onshore sedimentary successions around the Lake Edward and Lake Albert rift basins, providing a record of glacial – interglacial climatic cyclicity in the Equatorial Tropics of East Africa. Northern hemisphere interglacials result in a wet phase and lake level rise in the Albertine Rift, with marginal lacustrine shorelines back-stepping landward. Whereas, high latitude glacials cause corresponding periods of aridification and rift lake level fall, with paleo-shorelines prograding out towards the centre of the basin.

Inner architecture of fan delta front deposits: An outcrop example from Xiguayuan formation, Luanping Basin, northeast China

The fan delta front deposit is a notable reservoir type in the basin margin and typically exhibits high heterogeneity in reservoir architecture. With the deepening of exploration and development of oil and gas fields, the study of lower-order depositional architectures is relatively weak, and their implication on remaining oil distribution and enrichment are poorly understood. Field outcrops of the fan delta front deposit exhibit first-hand data of lithofacies, association, and depositional architectures, effectively providing insights for underground reservoir research to guide the development of oil fields, such as high water–cut oil fields in the late development period. Herein, we considered the fan delta front outcrops of the Sangyuan section as an object to reveal the depositional architectures located in the Luanping Basin, northeast China. Using meticulous artificial fieldwork and unmanned aerial vehicle observation, we focused on the 4th-order (single sandbody) and 3rd-order (composite laminaset groups) architectural units. Based on field observations, quantitative statistics, and model matching and guidance, architectural models of the fan delta front in the Sangyuan section were established in response to the high-frequency lake-level. In addition, genetic types of remaining oil controlled by architectural features in Wenchang B oil field during the high water–cut stage were further discussed, and suitable strategies and measures are proposed to produce different types of remaining oil. Results indicate that there are twelve basic types of lithofacies and three types of a single sandbody developed in the Sangyuan section, and evidently, the dimensions and spatial stacking patterns of 4th-order architectural units are closely related to high-frequency lake-level fluctuation, that is, with the rise of lake-level, the thickness and width of a single sandbody gradually decrease, whereas the width–thickness ratio gradually increases. Meanwhile, the spatial stacking patterns of sandbody gradually evolution from the downcut type to the superposition or butted type and finally to the isolated type. A single sandbody is divided into one or more composite laminate groups by the interfaces of 3rd-order architectural elements, which are identified according to the flow regime and sedimentary structures. Different orders of architectural units have different controlling effects over remaining oil distribution and enrichment, and the corresponding technical measures are proposed to enhance the remaining oil recovery. The above mentioned analyses can provide a valuable reference for oil and gas production in the late stage of an oil field with a similar depositional environment.

Microbial mats writing the fossil record of true substrates in clastic rock successions

True substrates are bedding surfaces that represent the original environmental surfaces that existed at the time of burial. In the clastic depositional regime, there are abiotic and biotic causes that may contribute to the formation of such true substrates. This paper focuses on how sediment-stabilizing microbial mats may increase the preservation potential of true substrates. Baffling and trapping enrich fine-grained particles and heavy minerals in mat textures inducing a layer of heterogeneity in the otherwise homogenous deposits. Together with in situ mineral precipitation during the life time of the microbial mat and post-depositional diagenesis, baffling and trapping contributes to processes leading to bedding plane preservation. Microbial mats interact with the hydrological system of a coastal environment causing typical microbially induced sedimentary structures (MISS). The morphologies of MISS can indicate climate seasonality and zone of the respective true substrate. The interaction of animals and plants with microbial mats allow reconstructing events that took place during the exposure of the original environmental surface. Finally, true substrates displaying MISS are well-defined indicators for life exploration of Earth's oldest sedimentary rock successions and equivalent lithologies on other planets.

Evaporites and red beds of a syn-rift Atlantic series: evidence of continental depositional environment (Berrechid sub-basin, Morocco)

ABSTRACT The Berrechid sub-basin of Morocco consists of Late Triassic–Early Jurassic evaporites and red beds that are part of the syn-rift Atlantic series. Although some of the Berrechid sub-basin has been extensively studied, the depositional environment of the Upper Salt-Mudstone Formation has long been debated, with some early studies proposing a marine origin. This study documents the sedimentological, mineralogical, and geochemical characteristics of core materials from two boreholes and mine outcrops covering the entire series of the Upper Salt-Mudstone Formation. Our analysis of the lithology, mineralogy, sedimentary structures, sedimentary textures, and geochemical data suggests deposition in a continental setting. In addition, detailed observations of sedimentary structures and textures indicate additional features of periods of flooding, evapoconcentration, and desiccation. The continental setting in a dry climate is also supported by the occurrence of Classopollis pollen. This study provides new insights of the paleoenvironmental conditions in the Berrechid sub-basin, and significantly advances our understanding of the paleoenvironment and climate of Pangea during the Late Triassic–Early Jurassic period.

The Occurrence Characteristics and Enrichment Patterns of Shale Oil in Different Lithofacies: A Case Study of the Fengcheng Formation in the Mahu Sag.

The supply of shale oil is of significant importance to the sustainability of energy resources. However, due to the diversity of shale lithofacies, the occurrence characteristics and enrichment patterns of free oil and adsorbed oil remain unclear. Based on this, taking the shale oil reservoir of the Fengcheng Formation in the Mahu Sag as the research object, a series of experiments were conducted, including X-ray diffraction analysis (XRD), total organic carbon (TOC) content analysis, high-pressure mercury injection (HPMI), and field emission scanning electron microscopy (FE-SEM). In addition, multitemperature pyrolysis was used to preliminarily determine the contents of free oil and adsorbed oil in different lithofacies, and a laser scanning confocal microscope (LSCM) was employed to visually characterize the occurrence characteristics of shale oil at the lamina scale. Principal component analysis (PCA) was combined to assess the effect of pore structure parameters on the contents of free oil and adsorbed oil in different lithofacies. The results showed that the samples could be categorized into three lithofacies: continuous laminated felsic shale, discontinuous laminated felsic shale, and agglomerated mixed shale. Based on sedimentary structural characteristics, the shale oil in the continuous laminated felsic shale indicated obviously laminated distribution, while for the shale oil in the discontinuous laminated felsic shale, the laminated distribution of shale oil was weakly distributed, exhibiting a district distribution along the fracture. The histogram counts of the two lithofacies adsorbed oil fluorescence were higher than those of the free oil. The agglomerated mixed shale oil was widely distributed and exhibited a platy pattern, and the histogram counts of the free oil fluorescence were higher than those of the adsorbed oil. Analysis of the pore principal component parameter F (including neighborcount, rugosity, aspect ratio, and symmetry) revealed that the pores in the continuous laminated felsic shale and the discontinuous laminated felsic shale could provide more effective adsorption sites, resulting in higher contents of adsorbed oil. The pore type of the agglomerated mixed shale was quite different from the other two lithofacies, with higher free oil content compared with adsorbed oil. Combined with the sedimentary structure and the occurrence characteristics of shale oil, the shale oil enrichment patterns were finally categorized into lamina-pore controlled type, lamina-pore-fracture controlled type, and fracture-pore controlled type, respectively. Overall, this study aims to facilitate efficient exploration and production of shale oil and gas resources.

Full-Body Impressions: A category of trace fossils unique to exceptionally preserved bedding planes and indicators of true substrates

Certain biogenic sedimentary structures can be considered to be reliable indicators of true substrates. Full-body impressions, or FBIs, are trace fossils that are shallow, highly-detailed impressions of the ventral axial anatomy of low body mass animals made while living . Their mode of preservation makes FBIs distinct from undertracks, body moulds, or passive tool marks. FBIs are made at the air- or water-substrate interface during sedimentary stasis. Associated trace fossils, like those that exhibit excavations with preserved waste mounds and very near-surface burrow roofs, located on the same bedding plane, can support the interpretation of true substrate preservation. Integration of the biogenic and sedimentological data from both Carboniferous and Pleistocene sites with well-preserved bedding-plane trace fossil assemblages has confirmed the conditions under which true substrates are preserved. These include periods (as brief as days to weeks) of sedimentary stasis, followed by gentle deposition without erosion, and without the need to invoke biofilms or biomats as consolidants. The potential for these taphonomic conditions exist in both alluvial and lacustrine environments, as well as intertidal-estuarine valleys, as described by other published examples of FBI preservation. Future FBI discoveries will prove useful in identifying true substrates across a range of time periods and environments.

Study on Depositional Elements and Sandstone Body Development Characteristics Under High-Resolution Sequence Framework of the Fuyu Oil Layer in Bayanchagan Area

Summary With the development of oil and gas exploration, the tight oil in Daqing Oil Field has attracted attention. In recent years, the Fuyu oil layer in the northern Songliao Basin has become a focal point for exploration; however, the study of depositional elements in this area remains insufficient. Based on several analyses of sandstone thickness, depositional environment, heavy minerals, rock color, sedimentary structures, and logging curve characteristics, as well as stratigraphic division and comparison in the Bayanchagan area’s Fuyu oil layer, it has been revealed that the clastic source directions are northwest and west. The area features river-dominated, shallow water lacustrine delta deposits. These deposits are further divided into two subenvironments: delta plain and delta front, encompassing eight depositional elements. The main depositional elements are distributary channels, crevasse splays, floodplains and overbank sand lenses, subaqueous distributary channels, river mouthbars, interdistributary bays, and frontal splays. The whole oil layer in the study area is in the background of a transgressive deposition, during which lake level oscillates and several water regressions occur, thus establishing the sedimentary evolution characteristics of the oil layer in the study area. The Fuyu I (FI) sandstone thickness and oil layer thickness in the study area are the largest, with the widest distribution range, while the Fuyu III (FIII) is the least developed; the Distributary channels are the most effective oil and gas depositional elements. Vertically, the FI and Fuyu II (FII) oil layers are the most developed, and gradually deteriorate downwards.

Characterizing Sub‐Seafloor Seismic Structure of the Alaska Peninsula Along the Alaska‐Aleutian Subduction Zone

AbstractA shallow sub‐seafloor seismic model that includes well‐determined seismic velocities and clarifies sediment‐crust discontinuities is needed to characterize the physical properties of marine sediments and the oceanic crust and to serve as a reference for deeper seismic modeling endeavors. This study estimates the seismic structure of marine sediments and the shallow oceanic crust of the Alaska‐Aleutian subduction zone at the Alaska Peninsula, using data from the Alaska Amphibious Community Seismic Experiment (AACSE). We measure seafloor compliance and Ps converted wave delays from AACSE ocean‐bottom seismometers (OBS) and seafloor pressure data and interpret these measurements using a joint Bayesian Monte Carlo inversion to produce a sub‐seafloor S‐wave velocity model beneath each available OBS station. The sediment thickness across the array varies considerably, ranging from about 50 m to 2.80 km, with the thickest sediment located in the continental slope. Lithological composition plays an important role in shaping the seismic properties of seafloor sediment. Deep‐sea deposits on the incoming plate, which contain biogenic materials, tend to have reduced S‐wave velocities, contrasting with the clay‐rich sediments in the shallow continental shelf and continental slope. A difference in S‐wave velocities is observed for upper oceanic crust formed at fast‐rate (Shumagin) and intermediate‐rate (Semidi) spreading centers. The reduced S‐wave velocities in the Semidi crust may be caused by increased faulting and possible lithological variations, related to a previous period of intermediate‐rate spreading.

Bioturbators as ecosystem engineers in space and time

AbstractBiogenic sedimentary structures offer a unique perspective for understanding the role of the biosphere in the interaction with other Earth subsystems and the building up of our planet. The record of their ancient equivalents provides a wealth of information for reconstructing the role of bioturbators as ecosystem engineers using multiple ichnological proxies and methods. In this study, we present an overview of how bioturbation has worked across a broad range of spatial and temporal scales from the perspective of ecosystem engineering. Comprehensive and systematic datasets allow analyses at regional and global spatial extents, and especially over long temporal scales where sampling intensity and rock record biases can be considered. Our results underscore the significance of the Cambrian Explosion in the establishment of modern‐style shallow marine ecosystems and of the Ordovician Radiation for their deep marine counterparts, as well as the continuous ecosystem impact of bioturbation during Palaeozoic terrestrialization. Comparable datasets for the rest of the Phanerozoic have not yet been compiled. However, preliminary information indicates that colonization of ultra‐deep tiers, the rise to prominence of regenerators, increased burrowing efficiency, and increased compartmentalization of the endobenthic ecospace, were products of the Mesozoic Marine Revolution. The trace‐fossil record offers hard data to evaluate bioturbation as a driving force in ecosystem re‐structuring and as a key factor in geobiological cycles. Models assessing these fundamental issues should be rooted empirically at different scales, from both autoecological and synecological to macroecological, making the best possible use of the rich and rapidly developing ichnological toolbox.

Different nitrogen conditions regulating extracellular polymeric substances and erosion resistance of sewer sediment: Mechanism of microbial metabolism and molecular response

Nitrogen biotransformation plays a vital role in the metabolism of microbial communities in sewers. Extracellular polymeric substances (EPS) secreted by microbial communities can form gel-like sewer sediments, causing clogging of the sewer. However, knowledge on the effects of varying nitrogen conditions on the erosion resistance of sewer sediments and EPS produced by sewer microorganisms is limited. In this study, two typical organic/inorganic nitrogen ratios of sewage were reproduced in simulated sewer reactors, i.e., 3/7 (R1 group) and 7/3 (R2 group). Higher organic nitrogen (ON) concentrations were found to increase the critical erosion shear stress by 26.43%; this was ascribed to increased particle diameter, weakened electrostatic repulsion of sediments and stimulated EPS secretion in the R2 group. The protein and polysaccharide contents of the R2 group were 48.84% and 34.25% higher than those of the R1 group, respectively, which was supported by increased gene abundances for aromatic amino acid synthesis, general secretory pathways of protein, and synthesis of precursors and polysaccharides. Tightly-bound EPS in R2 group exhibited increased contents of hydrophobic protein secondary structures and intermolecular hydrogen bonds, thereby promoting the formation of gel-like sediment structures with enhanced erosion resistance. However, the microbial diversity and the abundance of key genes involved in EPS generation and secretion (e.g., tyrB, yajC, secB, gumF, and gumH) obviously decreased in the R1 group. Moreover, high ON concentrations increased microbial diversity and enhanced microbial glycolysis, tricarboxylic acid cycle and ammonium assimilation. This study reveals the formation mechanisms of EPS in sewer sediments under different nitrogen conditions and their effects on sediment erosion resistance, which contributed to improved sewer system operation and sewer sediment control.