Modern Sedimentary Environments Research Articles

Constraints on dolomite formation in a Late Palaeozoic saline alkaline lake deposit, Junggar Basin, north‐west China

ABSTRACTAlkaline lakes (pH > 9) are among the few modern sedimentary environments that are hydrochemically favourable for low‐temperature dolomite formation. While Mg‐clays and Mg‐evaporites also form more easily in alkaline environments, few studies have focused on how the kinetically inhibited dolomite wins the competition for Mg2+. Here, a basin‐wide survey of the distribution, paragenesis and stable C, O and Mg isotopes of main Mg‐rich minerals in the Late Palaeozoic saline alkaline lake deposit of the north‐west Junggar Basin, north‐west China, is conducted to study the influence of the formation and diagenesis of eitelite, northupite and Mg‐clays on dolomite formation. Large, isolated dolomite crystals (20 to 70 μm in diameter), show positive δ13C values (ranging from +1 to +7‰) and a restricted distribution in the mudstones of the lake‐transitional zone. These crystals have been interpreted as organogenic dolomite driven by methanogenesis via fermentation of organic substrates. The δ18O values of dolomitic mudstones (from −7.4 to +3.4‰), calcitic mudstones (from −15.1 to −3.3‰) and bedded Na‐carbonate evaporites (from +0.08 to +3.7‰), together with their Mg isotopic compositions, suggest that dolomite was not enriched in the most concentrated environments or during stages with most Mg sources, but in the organic‐rich deposits containing few other authigenic Mg‐rich minerals. Dolomite is at a competitive disadvantage for Mg2+ ions compared to Mg‐evaporite and Mg‐clay minerals due to its slow crystallization rates and the deficiency of micritic calcium carbonate precursors. However, it can nucleate and progressively grow into large crystals (>20 μm) if bacterial methanogenesis could effectively lower porewater pH (<8.5) and induce the dissolution of generated eitelite, northupite or Mg‐clays. These findings suggest that high salinity and/or high alkalinity are not always favourable conditions for dolomite formation and highlight the active role of pH fluctuations in inducing low‐temperature dolomite formation.

Reactive vanadium and iron fluxes in different modern sedimentary environments

This study evaluated the removal of the metals (Me) vanadium and iron associated with the operationally defined HCl-extractable (MeHCl), pyrite (Mepyr), and reactive (Mereac = MeHCl + Mepyr) fractions in the sediments of five contrasting sedimentary environments: (1) the oxygen minimum zone (OMZ; San Lázaro and San Blas basins); (2) the anoxic/anoxic-sulfidic sediments of the California Continental Borderland basins and Todos Santos Bay Canyon; (3) the oxic sediments of the Patton Escarpment, Baja California continental shelf, and deep sediments of the Gulf of Mexico (GoM); (4) the Guerrero Negro Hypersaline System (GNHS) in Baja California Sur, Mexico; and (5) the turbiditic deposits of the GoM. The average VHCl concentration ranged from 8 ± 15 nmol g−1 in turbidites to (7.9 ± 4.3) × 102 nmol g−1 in the OMZ, respectively. The Vpyr concentrations were within the narrow range of 6.6 ± 4.2 nmol g−1 (oxic sediments) to 8.6 ± 5.4 nmol g−1 (turbidites), which indicates that the pyrite fraction is not an important reservoir of reactive V. The relative consistency of the Vpyr concentrations (7.2 ± 5.2 nmol g−1; n = 1098), regardless of the sedimentary environment and redox state of the system, allowed us to calculate a global burial value for oceanic sediments of 4.2 ± 3.0 Gg y–1. The VHCl enrichment observed in OMZ and anoxic/anoxic-sulfidic sediments may be due to its incorporation into acid volatile sulfide. The lowest average values of the degree of V pyritization corresponded to the OMZ (0.98 ± 0.68%) while the highest average values corresponded to turbidites (69 ± 32%), with the latter being due to their extremely low VHCl values. The mass accumulation rate (MAR) calculations for reactive Fe and V show that the sediments of all OMZ regions could annually incorporate (12.1 ± 8.6) × 104 Gg and (6.3 ± 3.8) × 102 Gg of reactive Fe and V, respectively. Although the GNHS exhibited the greatest MAR value of reactive Fe and V [(32 ± 28) × 102 g m−2 ky−1 and 35 ± 35 g m−2 ky−1, respectively], this environment currently does not contribute an important percentage of the MAR at the global level due to its small geographical area. However, in the geological past when its geographical area was much greater, the MAR values of reactive Fe and V may have been more important. Although deep sediments (>1000 m water depth) cover a substantial area, they contribute a relatively small, yet significant proportion of the total vanadium (V) deposited in ocean sediments, ranging from 9.7% to 24.8%.

SEDucate: Sedimentary Log Exercises for an Active Learning Environment

Active learning in education has been shown to improve a students’ learning experience in higher education by focusing on a student-centered learning environment. However, development of realistic activities that replicate the complexity of real-world examples without being too complex for the novice learner is often challenging. Here we present SEDucate, a GIS plugin to build realistic active-learning exercises in sedimentology course curriculums by creating unique sedimentary logs for a tailored assessment based on modern sedimentary environments. The automated creation of simple sedimentary logs for a variety of different depositional environments can be implemented in a wide range of exercises and assessments as either individual or group-based work. SEDucate allows for the teaching on the characterization of sedimentary logs, interpretation of paleogeographic environments and the lateral variation observed in modern sedimentary environments.

True substrates: The exceptional resolution and unexceptional preservation of deep time snapshots on bedding surfaces

AbstractRock outcrops of the sedimentary–stratigraphic record often reveal bedding planes that can be considered to be true substrates: preserved surfaces that demonstrably existed at the sediment–water or sediment–air interface at the time of deposition. These surfaces have high value as repositories of palaeoenvironmental information, revealing fossilized snapshots of microscale topography from deep time. Some true substrates are notable for their sedimentary, palaeontological and ichnological signatures that provide windows into key intervals of Earth history, but countless others occur routinely throughout the sedimentary–stratigraphic record. They frequently reveal patterns that are strikingly familiar from modern sedimentary environments, such as ripple marks, animal trackways, raindrop impressions or mudcracks: all phenomena that are apparently ephemeral in modern settings, and which form on recognizably human timescales. This paper sets out to explain why these short‐term, transient, small‐scale features are counter‐intuitively abundant within a 3.8 billion year‐long sedimentary–stratigraphic record that is known to be inherently time‐incomplete. True substrates are fundamentally related to a state of stasis in ancient sedimentation systems, and distinguishable from other types of bedding surfaces that formed from a dominance of states of deposition or erosion. Stasis is shown to play a key role in both their formation and preservation, rendering them faithful and valuable archives of palaeoenvironmental and temporal information. Further, the intersection between the time–length scale of their formative processes and outcrop expressions can be used to explain why they are so frequently encountered in outcrop investigations. Explaining true substrates as inevitable and unexceptional by‐products of the accrual of the sedimentary–stratigraphic record should shift perspectives on what can be understood about Earth history from field studies of the sedimentary–stratigraphic record. They should be recognized as providing high‐definition information about the mundane day to day operation of ancient environments, and critically assuage the argument that the incomplete sedimentary–stratigraphic record is unrepresentative of the geological past.

Mechanisms of Pyrite Formation Promoted by Sulfate-Reducing Bacteria in Pure Culture

Pyrite, or iron disulfide, is the most common sulfide mineral at Earth’s surface and is widespread through the geological record. Because sulfides are mainly produced by sulfate-reducing bacteria (SRB) in modern sedimentary environments, microorganisms are assumed to drive the formation of iron sulfides, in particular pyrite. However, the exact role played by microorganisms in pyrite formation remains unclear and, to date, the precipitation of pyrite in microbial cultures has only rarely been achieved. The present work relies on chemical monitoring, electron microscopies, X-ray diffraction and synchrotron-based spectroscopy to evaluate the formation of iron sulfides by sulfate-reducing bacteria Desulfovibrio desulfuricans as a function of the source of iron, either provided as dissolved Fe2+ or as FeIII-phosphate nanoparticles. Dissolved ferrous iron led to the formation of increasingly crystalline mackinawite (FeS) with time, encrusting bacteria cell walls, hence preventing further sulfate reduction upon day 5 and any evolution of iron sulfides into more stable phases, e.g. pyrite. In contrast, ferric phosphate was transformed into a mixture of large flattened crystals of well crystallized vivianite (Fe3(PO4)2 . 8 H2O) and a biofilm-like thin film of poorly crystallized mackinawite. Although being hosted in the iron sulfide biofilm, most cells were not encrusted. Excess-sulfide delivered by the bacteria and oxidants (such as polysulfides) promoted the evolution of mackinawite into greigite (Fe3S4) and the nucleation of pyrite spherules. These spherules were several hundreds nanometer wide and occurred within the extracellular polymeric substance (EPS) of the biofilm after only 1 month. Altogether, the present study demonstrates that the mineral assemblage induced by the metabolic activity of SRB strongly depends on the source of iron, which has strong implications for the interpretation of the presence of pyrite and vivianite in natural environments.

Geochemical and Stable Fe Isotopic Analysis of Dissimilatory Microbial Iron Reduction in Chocolate Pots Hot Spring, Yellowstone National Park

Chocolate Pots hot spring (CP) is an Fe-rich, circumneutral-pH geothermal spring in Yellowstone National Park. Relic hydrothermal systems have been identified on Mars, and modern hydrothermal environments such as CP are useful for gaining insight into potential pathways for generation of biosignatures of ancient microbial life on Earth and Mars. Fe isotope fractionation is recognized as a signature of dissimilatory microbial iron oxide reduction (DIR) in both the rock record and modern sedimentary environments. Previous studies in CP have demonstrated the presence of DIR in vent pool deposits and show aqueous-/solid-phase Fe isotope variations along the hot spring flow path that may be linked to this process. In this study, we examined the geochemistry and stable Fe isotopic composition of spring water and sediment core samples collected from the vent pool and along the flow path, with the goal of evaluating whether Fe isotopes can serve as a signature of past or present DIR activity. Bulk sediment Fe redox speciation confirmed that DIR is active within the hot spring vent pool sediments (but not in more distal deposits), and the observed Fe isotope fractionation between Fe(II) and Fe(III) is consistent with previous studies of DIR-driven Fe isotope fractionation. However, modeling of sediment Fe isotope distributions indicates that DIR does not produce a unique Fe isotopic signature of DIR in the vent pool environment. Because of rapid chemical and isotopic communication between the vent pool fluid and sediment, sorption of Fe(II) to Fe(III) oxides would produce an isotopic signature similar to DIR despite DIR-driven generation of large quantities of isotopically light solid-associated Fe(II). The possibility exists, however, for preservation of specific DIR-derived Fe(II) minerals such as siderite (which is present in the vent pool deposits), whose isotopic composition could serve as a long-term signature of DIR in relic hot spring environments.

Nucleation and stabilization of Eocene dolomite in evaporative lacustrine deposits from central Tibetan plateau

AbstractIn past decades, the formation of dolomite at low temperature has been widely studied in both natural systems and cultured experiments, yet the mechanism(s) involved in the nucleation and precipitation of dolomite remains unresolved. Late Eocene dolomitic deposits from core in the upper Niubao Formation (Lunpola Basin, central Tibetan Plateau, China) are selected as a case study to understand the dolomitization process(es) in the geological record. Dolomite formation in Lunpola Basin can be ascribed to a different mechanism forming the large quantities of replacive dolostones in the geological record; and provides a potential fossil analogue for primary dolomite precipitation at low temperature. This analogue consists of an alternation of laminated dolomitic beds, organic‐rich and siliciclastic layers; formed in response to intense evaporation interpreted to take place in a continental shallow lake environment. Mineralogical, textural and stable isotopic evaluations suggest that the dolomite from those dense‐clotted laminated beds is a primary precipitate. At the nanoscale, these dolomitic beds are composed of Ca–Mg carbonate globular nanocrystals (diameter 80 to 100 nm) embedded in an organic matrix and attached to clay flakes. Micro‐infrared spectroscopy analyses have revealed the presence of aliphatic compounds in the organic matrix. Microscopic and elemental compositional studies suggest that clay surfaces may facilitate the nucleation of dolomite at low temperature in the same way as the organic matrix does. The dolomite laminae show values for δ18OVPDB from −3.2 to −1.76‰ and for δ13CVPDB from −2.62 to −3.78‰. Inferred δ18OSMOW values of the lake water reveal typical evaporitic hydrological conditions. These findings provide a potential link to primary dolomite formation in ancient and modern sedimentary environments; and shed new light on the palaeoenvironmental conditions in central Tibet during the Eocene.

Origin and depositional characteristics of supported conglomerates

Abstract The origin and depositional characteristics of supported conglomerates in the Mahu Sag, Junggar Basin, Xinjiang, China, are examined. Based on the terminological comparison, modern sedimentary survey and core description, the initial connotation and similarities and differences in definition between supported conglomerates and other similar concepts are discussed, the modern sedimentary environment in which supported conglomerates develop is analyzed, and the sedimentological characteristics of supported conglomerates formed in different depositional environments revealed by the cores of Mahu conglomerate oil field in the Junggar Basin are described. The supported conglomerate is similar in texture to grain supported conglomerate and openwork conglomerate but has differences from them, so it is suggested to keep the term “supported conglomerate”, but the formation mechanism of supported conglomerate needs to be re-examined. Through field survey of modern sediments in Baiyanghe alluvial fan, Huangyangquan alluvial fan, and Wulungu Lake in Xinjiang, it is found that supported gravels not only formed by flooding events but also by sieving, avalanching, fluvial sorting as well as wind and wave reworking in the depositional environments such as inter-mountain creek, colluvium fan, gravel channel on gobi and the fan surface, lake beach, delta front, subaerial debris flow and subwater grain-flow etc. Supported gravels could form supported conglomerate after being buried. Supported conglomerates of seven different origins have been recognized in the cores of the Triassic and Permian stratum of Mahu Depression, Junggar Basin, namely, supported conglomerates in gravel channel deposits, in wind reworked channel deposits, in gravel beach bar deposits, in wave reworked delta front deposits, in mouth bar deposits and in debris flow deposits respectively. The study shows the supported conglomerates may be formed by a single depositional event or by multi-events during the post-depositional sedimentary reworking and even in diagenesis stage. Through flume experiment, numerical simulation, empirical model and modern sediment survey, infiltration process of gravelly channel can be reconstructed and the primary pore structure of supported gravel can be estimated. Statistics on physical properties of various types of reservoirs in Triassic Baikouquan Formation of Mahu oilfield show that granule conglomerate and pebbly conglomerate have higher porosity and permeability, while the cobble and coarse pebble conglomerate have lower permeability, which indicates that the supported gravels are easy to be reworked by post depositional filtration and diagenesis, and thus decrease in porosity and permeability.

Evolution model of a modern delta fed by a seasonal river in Daihai Lake, North China: determined from ground-penetrating radar and trenches

While deltas fed by seasonal rivers are common in modern sedimentary environments, their characteristics remain unclear as compared to those fed by perennial rivers. This study identifies a small delta discharged by a seasonal stream flowing into Daihai Lake, in northern China, which is driven by ephemeral and high-energy flood events. Detailed 3D facies architecture was analyzed using ground-penetrating radar (GPR) and sedimentary logs from outcrop and trenches. Four types of radar surfaces, including truncations of underlying inclined strata, weak reflections, and depositional surface of downlap and onlap, were identified. Six radar facies (high-angle oblique-tangential, low-angle subparallel, gently plane parallel, plane-parallel, chaotic, and continuous strong reflection) were identified based on distinctive reflections, including amplitude, continuity, dip, and termination patterns. Five depositional units (Unit A to E) were documented from proximal to distal delta. Seasonal discharge signatures include significant grain-size decrease over short distance, abundant Froude supercritical flow sedimentary structures, poorly developed barforms, and small-scale scour and fill structures. Records of lake-level and sediment budget were evaluated over the past 60 years. In highstand stage (1960–1980), amalgamated channel (Units A and B), and delta front (Unit C) were deposited. In slope stage (1980–1996), the lower deposits (Units A, B, C) were eroded by Unit D with a distinct truncation surface. In lowstand stage, most eroded sediments bypassed the incised channel and accumulated in the distal part, in which a new depositional unit was formed (Unit E). The model demonstrates that deltas fed by seasonal rivers tend to accumulate large amounts of sediments carried by high magnitude floods within short periods.

Magnesite formation in playa environments near Atlin, British Columbia, Canada

The hydromagnesite-magnesite playas near Atlin, British Columba, Canada are unique Mg-carbonate depositional environments that have formed at Earth’s surface since the end of the last deglaciation. This study elucidates the mechanisms, pathways, and rates of magnesite (MgCO3) formation in these near-surface environments, which are challenging to study in short-duration laboratory experiments because magnesite precipitation is extremely slow at low temperature. The Atlin playas, having formed over millennia, contain abundant magnesite as well as a suite of other Mg- and Ca-carbonate minerals. Mineralogical and textural evidence demonstrate that hydromagnesite [Mg5(CO3)4(OH)2·4H2O] forms at least in part through transformation of more hydrated phases, e.g., lansfordite (MgCO3·5H2O). Deposition of these hydrated Mg-carbonate minerals is limited by the evaporative flux, and thus, is effectively transport-controlled at the scale of the playas. Magnesite is a spatially distinct phase from hydromagnesite and its crystal morphology varies with depth indicating variable crystal growth mechanisms and precipitation rates. Particle size distributions and mineral abundance data indicate that magnesite formation is nucleation-limited. Furthermore, mineralogical data as well as stable and radiogenic isotope data support magnesite formation starting after the majority of hydromagnesite had been deposited likely resulting from long induction times and slow precipitation rates. Hydrated Mg-carbonate minerals precipitate relatively rapidly and control pore water chemistry while magnesite remains highly supersaturated, and thus, is reaction-controlled. This difference in controlling regime allows for magnesite abundance to increase over time without the loss of hydromagnesite such as through its transformation, which the data also does not support. We estimate rates of magnesite formation (nucleation + crystal growth) in the range of 10-17 to 10-16 mol/cm2/s over approximately 8000 years. This study helps to elucidate the geochemical conditions needed to form Mg-carbonate minerals in ancient and modern sedimentary environments and provides insights into facilitating long-term storage of anthropogenic CO2 within Mg-carbonate minerals.

Facies, sedimentary environments and sequence stratigraphy of Cretaceous deposits in the northwest of Azerbaijan, Iran

The Cretaceous deposits in Morakan, located in the northeast of Khoy, a town in western Azerbaijan province, Iran, are primarily made up of carbonate rocks. These deposits, which are of 717 meters thick, lie on the clastic facies of Jurassic and are covered with beneath the pelagic beds of the Upper Cretaceous. To investigate the facies, sedimentary environment and sequential stratigraphy of these successions, a stratigraphic section was made in the south of Morakan village. Carbonate facies in this section have deposited in open marine, bar, lagoon, and tidal flat facies belts. The study of these facies and comparing them with old and modern sedimentary environments reveals that these succes-sions have deposited in a carbonated platform of rimmed shelf type. The study of the vertical succes-sion of microfacies shows four main sequences in the form of system tracts of TST and HST, which are made up of shallowing and deepening parasequences. The first lower boundary of the sequences is the type SB1 unconformity, and other identified sequences are the type SB2 unconformity.

Carbonates’ precipitation by halophilic bacteria as a potential biosignature for the search for life on Mars

The mechanisms of Mg-carbonate precipitation in the Earth's modern sedimentary environments has not yet been completely elucidated. However, it is known that the microbial activity is significant to facilitate or induce mineral formation. The organic EPS matrix secreted by microorganisms provide an ideal physicochemical environment to the mineral nucleation. Sediments and water samples were collected at Lagoa Vermelha, Araruama (RJ), Brazil. This lagoon is characterized of being a hypersaline lake, where we can find a range of microorganisms known as halophilic and halotolerant extremophiles that have adaptation strategies to compensate for the harmful effects of high salinity, such as EPS biosynthesis, which is one of the most common protection mechanisms in bacteria, helping to maintain the integrity of their cells’membrane. In the present project, the bacterial capability of bioprecipitation using bacterial isolates from a hypersaline lagoon was investigated by combining microbiological, microscopic and geochemical analytical techniques. The isolates were evaluated for their ability to produce bioprecipitates using this multi-technique approach that includes Scanning Electron Microscopy (SEM) with EDS, XRD and Raman spectroscopy. It was possible to characterize the carbonates formed by the bacteria isolated. Finally, these carbonates could represent a potential target for astrobiological studies of potential potential biosignatures for the search for life beyond Earth.

Heavy Metal Pollution Characteristics in the Modern Sedimentary Environment of Northern Jiaozhou Bay, China.

The particle size distribution characteristics of sediments and the concentrations of heavy metals in Jiaozhou Bay were investigated in this study. The average concentrations of Cu, Pb, Zn, Cr, Cd, Hg and As were 36.88, 29.60, 82.08, 77.48, 0.083, 0.048 and 11.00mg/kg, respectively. The heavy metal concentrations were highest in the eastern sediments, followed by those at the top of the bay, and the lowest concentrations were observed in the central region. Overall, a decreasing trend from the center of the bay to the periphery was observed. Additionally, the distribution of heavy metals in sediments was not completely controlled by sediment particle size. The degree of heavy metal contamination was evaluated using the geoaccumulation index and Hakanson's method. The results revealed that the level of heavy metal pollution in the sediments was relatively low and that the main pollution elements were Cu and Hg. In addition, the sediments are associated with various levels of potential ecological risk due to the high pollution levels of Hg and Cd.

Distribution of biogenic silica in seafloor sediments on the East China Sea inner shelf: Seasonal variations and typhoon impact

The content and distribution of biogenic silica (BSi) in sediments, as effective proxies for marine paleo-productivity and sedimentary environment evolution studies, has significant seasonal and event-based differences. Based on measurements of BSi content in the seafloor sediments acquired during three surveys in winter and before and after typhoons in summer in the Fujian-Zhejiang coastal mud deposition center (MDC-ZFC), respectively, the seasonal and event-based (typhoon process) differences of BSi in seafloor sediments are discussed by combining with data of the sediments grain sizes and the abundances of dominant diatom species. Conceptual models of the modern sedimentary environment (including BSi) are summarized. The study results show that the BSi in the seafloor sediments in the study region is relatively low, generally less than 1%. Generally, the BSi in the winter seafloor sediments is lower than that under normal summer sea conditions and the BSi in the sediments after the summer typhoons is higher than that under normal summer sea conditions. The differences in the origin of biogenic siliceous materials (seasonal growth of siliceous organisms) and the marine hydrodynamic environment (such as seafloor sediment resuspension and elutriation caused by strong marine hydrodynamic processes) are the major factors for different relative BSi content. The seasonal and event-based variations of BSi have important significance in paleo-oceanography and event sedimentology studies.

Wave-Created Mud Suspensions: A Theoretical Study

We studied wave-created high-density mud suspensions (fluid mud) using a one-dimensional water column (1DV) model that includes k-ε turbulence closure at a high vertical resolution with a vertical grid spacing of 1 mm. The k-ε turbulence model includes two sediment-related dissipation terms associated with vertical density stratification and viscous drag of flows around sediment particles. To this end, the calibrated model reproduces the key characteristics (maximum concentration and thickness) of fluid mud layers created in laboratory experiments over a large range of wave velocities from 10 to 55 cm/s. The findings demonstrate that the equilibrium near-bed mud concentration, Cb, is solely determined from the balance between erosion and deposition fluxes, whereas the thickness of the fluid mud layer is mainly controlled by sediment-induced density stratification, which dissipates turbulence and hence eliminates turbulent sediment diffusivity at the top of the fluid mud layer, the lutocline. Our model stands in contrast to those that suggest that upward sediment diffusion is close to zero at the interface between the fluid mud layer and the overlying fluid. Instead, our model suggests that the upward diffusive flux of fluid mud flows peak at the lutocline and is compensated for enhanced settling fluxes just above it. Our model findings also support the existence of the gelling-ignition process, which is critical for the development of fluid mud beds in modern sedimentary environments.

The last interglacial (MIS 5e) sea level highstand from a tectonically stable far-field setting, Yorke Peninsula, southern Australia

Yorke Peninsula in southern Australia is an important region for reconstructing relative sea-level histories due to its location on the eastern margin of the tectonically stable Gawler Craton and in one of the world's geographically most remote far-field locations from the Pleistocene ice sheets. Richly fossiliferous, skeletal carbonate sands of the last interglacial (125 ka) Glanville Formation crop out in the coastal cliffs along large sectors of southern Yorke Peninsula. Sedimentary facies include deepening-upward intertidal to shallow subtidal facies, relict storm beach facies and cobble and boulder beach deposits in more exposed, higher energy locations. During deposition of the Glanville Formation, southern Yorke Peninsula had a different coastal geography with two prominent marine corridors extending across the southern-most portion of the entire peninsula. In a 3 km long coastal cliff section in southern Hardwicke Bay, the Glanville Formation crops out as an upward-deepening intertidal-subtidal succession capped by supratidal and subaerially-exposed sediments with pervasive calcrete development. The sedimentary succession passes upwards from a basal unit of intertidal sand flat facies with abundant gastropods (Batillaria diemenensis) near the upper bounding (disconformity) surface, upwards into richly fossiliferous shelly sands (coquina) representing a shallow subtidal facies formed by sediment aggradation in response to a relative sea-level rise. The subtidal facies is dominated by the bivalve molluscs Katelysia sp. and Amesodesma angusta, signifying a water deepening event. The subtidal facies is in turn overlain by pedogenically modified skeletal carbonate sands with pervasive calcrete development signifying a relative sea level fall at the end of the Last Interglacial Maximum. The upper-bounding surface of the shallow-water subtidal facies ranges from 2.4 to 3.0 m Australian Height Datum (AHD) and by analogy with modern sedimentary environments suggests a maximum palaeo-sea level of 4.8 ± 1.0 m during the Last Interglacial Maximum. Uranium-series ages of 127.3 ± 2.1 to 115.0 ± 5.4 ka on specimens of the solitary coral Plesiastrea versipora from the subtidal facies confirm that the succession was deposited during the Last Interglacial Maximum, and are consistent with the independent stratigraphical evidence that the highstand event was represented by a single phase of relative sea-level rise. Correlation with other occurrences of the Glanville Formation in southern Australia has also been confirmed by aminostratigraphy.

Modern sedimentary facies, depositional environments, and major controlling processes on an arid siliciclastic coast, Al qahmah, SE Red Sea, Saudi Arabia

The facies and environments along the arid siliciclastic coast of Red Sea in Al Qahmah, Saudi Arabia are studied to establish a depositional model for interpretation of ancient rocks deposited in rift settings. Field and petrographic studies of 151 sediment samples in an area of 20 km2 define seven main facies types: beach, washover fan, tidal channel, dune, sabkha, delta, and wadi (seasonal stream). The wadi and delta facies are composed of poorly to moderately well-sorted, gravelly, medium-to-fine sands. Delta-front sands are redistributed by southward longshore currents to form a beach. Beach facies is composed of well-to-moderately sorted fine sands with minor gravels, which contain high concentrations of magnetite, ilmenite, garnet, pyroxene, amphibole, epidote, titanite, and apatite grains, indicating strong winnowing. Crabs and other burrowers destroy primary sedimentary structures and mix sediments in foreshore and backshore of the beaches. Wind and storm surge rework foreshore and backshore sediments to form washover fans. Sabkha facies occurs extensively in supratidal depressions behind beach, are flooded by rainstorms and spring tide, and capped by a 5-cm-thick crust composed of interlaminated halite, quartz, albite, minor gypsum and biotite, and rarely calcium carbonate. Halite occurs as thin sheets and gypsum as nodules with a chicken-wire structure. Clastic fraction in sabkha sediments ranges from coarse silt to coarse sand with moderate sorting, and is transported by currents and wind. Tidal inlets and tidal creeks assume abandoned wadis and are filled by muddy sand. Sand dunes and sand sheets are 1–7 m high and widely distributed due to variable wind directions. Fine-grained dune sands are moderately well sorted, whereas sheet sands are coarser and poorly sorted due to vegetation baffling. Most eolian sands are sourced from beach deposits. This suite of complex riverine, wave, tidal, wind, chemical, and biological processes form the facies mosaic along the arid Al Qahmah coast, which is strongly affected by climate-driven evaporation and wind action.

Asymmetrical, inversely graded, upstream-migrating cyclic steps in marine settings: Late Miocene-early Pliocene Fish Creek-Vallecito Basin, southern California

Cyclic steps are ubiquitous in modern sedimentary environments, yet their recognition remains sparse in the rock record. Here, we interpret three sets of undulating backsets (1 to 3) recognized in the late Miocene-early Pliocene Latrania Formation in the Anza-Borrego Desert, the Fish Creek-Vallecito Basin, southern California, USA as the first cm- to dm-scale outcrop record of cyclic steps, based on asymmetrical cross-sections, upstream migration, and inversely graded laminae. Upstream migration and asymmetrical cross-sections of backsets and concomitant backset laminae are attributed to supercritical-to-subcritical flow transitions through weak hydraulic jumps, which are composed of: (i) thin (tens of centimetres) and slower (reported as flow velocities (Ū) of 0.45 to 1.45ms−1, with mean value of Ū=0.89ms−1) subcritical (represented by internal Froude numbers (Fr) of 0.67 to 0.99, with mean value of Fr=0.84) turbidity currents on the stoss sides, and (ii) thin (tens of centimetres) and faster (reported as Ū of 0.99 to 4.03ms−1, with mean value of Ū=2.24ms−1) supercritical (represented by Fr of 1.84 to 3.07, with mean value of Fr=2.42) turbidity flows on the lee sides. The inversely graded laminae in the troughs of backsets are 2 to 5cm thick, and consist of two discrete divisions: (i) 1 to 2cm thick, lower finer-grained divisions made up of parallel laminated siltstones, overlain by very fine- to fine-grained sandstones, and (ii) 2 to 3cm thick, upper divisions composed of medium- to coarse-grained sandstones, with sporadic occurrence of subrounded pebbles. These inversely graded laminae are related to stratified, collisional and/or frictional traction carpets under conditions of high fall-out rates. Due to the poor preservation potential of cyclic steps, the rock record of cyclic steps is generally considered to be rare. The present outcrop-based study presents a detailed analysis of sedimentary facies, growth patterns, and flow dynamics of marine cyclic steps, thereby contributing to better understand architectural styles and lithological properties of cyclic steps in the geological record.

Impact of rapid urbanization on the modern sedimentary environments in a semienclosed embayment: Tong’an Bay, southeast China coast

ABSTRACTTong’an Bay (TAB) is a semienclosed embayment in the north of Xiamen Island. The sea area of TAB was mainly used for aquaculture in enclosed areas before 2003. Since 2003, the districts surrounding TAB have been undergoing a rapid urbanization process, and the scale of land used for urban construction in these districts has been continuously increasing. Under the backdrop of rapid urbanization, the coastline of TAB has largely been transformed into an artificial coastline. Due to the large scale of land reclamation and, dredging activities and the opening of the Gaoqi-Jimei (GJ) Seawall, there have been relatively significant changes in the coastal and submarine geomorphology in central northern, western, and southern TAB and Wuyuan Bay as well as a significant increase in the area where artificial submarine geomorphologies are distributed; the number of surface sediment types in the TAB has increased, and sediments in the sea area surrounding engineering projects have been coarsening; the sediment transport pattern has not significantly changed at the mouth of TAB and in central northern TAB, but the suspended sediment transport direction near the GJ Seawall has changed to a certain extent.

Microbial acceleration of aerobic pyrite oxidation at circumneutral pH.

Pyrite (FeS2 ) is the most abundant sulfide mineral on Earth and represents a significant reservoir of reduced iron and sulfur both today and in the geologic past. In modern environments, oxidative transformations of pyrite and other metal sulfides play a key role in terrestrial element partitioning with broad impacts to contaminant mobility and the formation of acid mine drainage systems. Although the role of aerobic micro-organisms in pyrite oxidation under acidic-pH conditions is well known, to date there is very little known about the capacity for aerobic micro-organisms to oxidize pyrite at circumneutral pH. Here, we describe two enrichment cultures, obtained from pyrite-bearing subsurface sediments, that were capable of sustained cell growth linked to pyrite oxidation and sulfate generation at neutral pH. The cultures were dominated by two Rhizobiales species (Bradyrhizobium sp. and Mesorhizobium sp.) and a Ralstonia species. Shotgun metagenomic sequencing and genome reconstruction indicated the presence of Fe and S oxidation pathways in these organisms, and the presence of a complete Calvin-Benson-Bassham CO2 fixation system in the Bradyrhizobium sp. Oxidation of pyrite resulted in thin (30-50nm) coatings of amorphous Fe(III) oxide on the pyrite surface, with no other secondary Fe or S phases detected by electron microscopy or X-ray absorption spectroscopy. Rates of microbial pyrite oxidation were approximately one order of magnitude higher than abiotic rates. These results demonstrate the ability of aerobic microbial activity to accelerate pyrite oxidation and expand the potential contribution of micro-organisms to continental sulfide mineral weathering around the time of the Great Oxidation Event to include neutral-pH environments. In addition, our findings have direct implications for the geochemistry of modern sedimentary environments, including stimulation of the early stages of acid mine drainage formation and mobilization of pyrite-associated metals.