Silty Sediments Research Articles

Microplastics in coastal urban sediments: Discrepancies in concentration and character revealed by different approaches to sample processing

Intertidal, silty sediment samples have been collected from three coastal locations with different uses and anthropogenic signatures in the vicinity of Plymouth, southwest England, and analysed for microplastics (MPs) by two independent means. Firstly, MPs were counted and characterised directly on unprocessed dried sediment under a stereo microscope, and secondly MPs were isolated from sediment by flotation in ZnCl2 solution and filtration before analysis. Direct counting resulted in average (± one standard deviation) numbers of MPs per g of dry sediment of 0.77 ± 0.16 at a marina-harbour, 0.58 ± 0.30 under a busy road bridge and 0.79 ± 0.43 adjacent to country parkland. After flotation and filtration, concentrations were reduced to 0.24 ± 0.11, 0.18 ± 0.06 and 0.48 ± 0.38 MP g−1, respectively. Observations were attributed to hetero-aggregation of small fibres with settling sediment during flotation, and the presence of MPs (including paints) that were too dense to float or that had aggregated or agglomerated with denser sediment and construction material in situ. The findings have implications for the efficacy of flotation procedures, accurate estimations of MP concentrations in sediment and the representativeness of MPs by type, and inter-site comparisons of MPs that are widely reported in the literature.

Mg and Zn isotopic data and supplementary information for "Recycled carbonate-bearing silicate sediments in the sources of circum-Mediterranean K-rich lavas: Evidence from Mg-Zn isotopic decoupling".

Mg and Zn isotopic data and supplementary information for "Recycled carbonate-bearing silicate sediments in the sources of circum-Mediterranean K-rich lavas: Evidence from Mg-Zn isotopic decoupling".

Comparative geochemistry of Early Carboniferous marine red beds (MRBs) and their significance for deep time paleoceanographic reconstructions

Marine red beds (MRBs), also known as oceanic red beds (ORBs), are reddish colored sediments deposited within the marine realm that have been stained during deposition and/or early diagenesis because of changes in the ocean water chemistry in response to paleoceanographic and paleoclimatological factors. Although common throughout the Phanerozoic, marine red beds have mostly been investigated in Upper Cretaceous successions as CORBs (Cretaceous Oceanic Red Beds) - their occurrence and paleoceanographic context within the Paleozoic has yet to be studied in detail. Here we describe and interpret the high-resolution litho- and chemofacies of Early Carboniferous (Late Viséan) marine red beds from the northern coast of Menorca (Balearic Islands, Spain) that mark a rapid transition from siliceous radiolarian chert deposition to re-worked carbonate debrites followed by typical European Culm facies. The succession was deposited within a strait-like narrow basin that formed part of the Alboran-Kabylian-Peloritan-Calabrian terrane in the Western Paleotethys Ocean prior to the onset of the Variscan Orogeny. The marine red bed geochemistry is described in terms of three geochemical endmembers Al-MRBs (clayey), Ca-MRBs (calcareous) and Si-MRBs (siliceous) by using XRF-derived Al2O3, CaO, and SiO2 concentrations. This study finds that Viséan marine red beds from Menorca are predominantly Al-MRBs with subordinate Ca-MRBs. Furthermore, we compare the bulk rock geochemistry of the Viséan marine red beds of this study with the geochemical data of Paleozoic, Mesozoic and Cenozoic marine red beds available in the existing literature (Cambrian, Silurian, Jurassic, Cretaceous, Paleogene and recent Pacific Ocean red clays) to highlight the significance of marine red bed occurrences in relation to (paleo-)bioproductivity fluctuations, deep-water oxygenation and oceanic redox transitions. This study finds that Phanerozoic marine red beds are dominantly Al-MRBs and Ca-MRBs, whilst Si-MRBs are less common. The Early Carboniferous marine red beds of the present study were formed in a distal, deeper-marine environment under oxic-suboxic, oligotrophic conditions. A lack of any significant Fe2O3T enrichment points towards a more complex diagenetic process for the origin of the red color imparted in Phanerozoic MRBs.

Analysis of the effect of hydrate on water retention curves in gas hydrate-bearing sediments using gas drainage combined with NMR

Water retention curves play a critical role in numerical simulations for predicting fluid production and sediment deformation behaviors in gas hydrate-bearing sediments (GHBSs). This study uses a new testing assembly that combines gas drainage and low-field nuclear magnetic resonance (NMR) tests to determine the water retention curves of artificially synthesized clay silty specimens. The effect of hydrate on the pore size distributions and water retention curves is analyzed via NMR transverse relaxation time curve distributions, and the mechanism of changes in the water retention curve parameters is further discussed. The results show that hydrate formation decreases the proportion of pores with sizes greater than 15 μm and increases the proportion of pores with sizes less than 3.5 μm in clay silty sediments. Hydrate formation increases capillary pressure and prevents available water migration. The presence of hydrate exponentially increases the normalized capillary pressure but exponentially decreases the normalized curve shape factor, yielding narrower curve distributions. The gas entry pressure and curve shape factor exhibit linear correlations with the pore size distribution parameters. The results imply that the changes in the water retention curves are strongly related to the initial pore size distributions. This study offers a deep understanding of capillary effects-related water retention characteristics and their underlying links with the pore size distributions, and demonstrates that low-field NMR has great potential for characterizing water retention curves of GHBSs.

Petrogenesis of the Paleogene potassium-rich volcanic rocks in the western Yangtze Craton, southeastern Tibetan Plateau

Paleogene potassium-rich volcanic rocks (PVRs) are widespread in the southeastern Tibetan Plateau, providing an opportunity to illustrate the architecture of the lithosphere and further constrain the development of the SE Plateau. Their petrogenesis, together with the geodynamic mechanism, remains in dispute. Mg isotopic compositions of the PVRs in western Yangtze Craton, along with new elemental and Sr-Nd-Pb isotopic data, are utilized to elucidate their petrogenesis and reveal the plausible metasomatic and genetic mechanisms. The PVRs give a compositional spectrum from basaltic trachyandesite to trachyte and are characterized by high large ion lithophile elements concentrations, negative anomalies in high field strength elements, and enriched Sr-Nd-Pb isotopes, suggesting an origin of enriched lithospheric mantle. The correlations among varying major oxides and fractional crystallization simulation results jointly reveal fractional crystallization of olivine and clinopyroxene in the magmatic process. The less-evolved PVRs present similar major oxide contents (e.g., Ti, Na, K) to those melts originated from carbonated peridotite + silicate sediment. Apart from enriched Sr-Nd-Pb isotopes, they also show high La/Yb ratios (11.73–58.95), low Ti/Ti* (0.08–0.71), Hf/Sm (mostly <0.70), and varying δ 26 Mg values (−0.13 to −0.36‰). Combined with the Sr-Nd-Mg isotope numerical simulations, it points out that the mantle source of the PVRs was replenished by a hybrid carbonate-silicic sediment, which was plausibly caused by the Neoproterozoic seafloor subduction. These volcanic rocks also present relatively high K 2 O (3.29–7.50 wt%) and K 2 O/Na 2 O ratios (1.00–2.85). Besides, varying elevated Rb/Sr and low Ba/Rb ratios, together with the La/Yb-Yb non-modal batch melting result, suggest that the enriched mantle source recorded by these PVRs is predominantly constituted by a hybrid phlogopite-bearing garnet-spinel lherzolite. Constrained by petrology studies and seismic tomography, the generation of the PVRs is proposed to be closely associated with the delamination of the thickened lithosphere and thereafter asthenosphere upwelling. • Paleogene K-rich volcanic rocks in the western Yangtze Craton are variably evolved. • These volcanic rocks are sourced from phlogopite-bearing garnet-spinel lherzolite. • Recorded mantle source was replenished by a hybrid carbonate-silicic sediment.

Evaluation of single-sided nuclear magnetic resonance technology for usage in geosciences

Because of its mobility and ability to investigate exposed surfaces, single-sided (SiS) nuclear magnetic resonance (NMR) technology enables new application fields in geosciences. To test and assess its corresponding potential, we compare longitudinal (T 1) and transverse (T 2) data measured by SiS NMR with those of conventional geoscientific laboratory NMR. We use reference sandstone samples covering a broad range of pore sizes. Our study demonstrates that the lower signal-to-noise ratio of SiS NMR data generally tends to slightly overestimated widths of relaxation time distributions and consequently pore size distributions. While SiS and conventional NMR produce very similar T 1 relaxation data, unbiased SiS NMR results for T 2 measurements can only be expected for fine material, i.e. clayey or silty sediments and soils with main relaxation times below . This limit is given by the diffusion relaxation rate due to the gradient in the primary magnetic field associated with the SiS NMR. Above that limit, i.e. for coarse material, the relaxation data is strongly attenuated. If considering the diffusion relaxation time of 0.2 s in the numerical data inversion process, the information content is blurred over a range larger than that of conventional NMR. However, our results show that principle range and magnitudes of the relaxation time distributions are reconstructed to some extent. Regarding these findings, SiS NMR can be helpful to solve geoscientific issues, e.g. to assess the hydro-mechanical properties of the walls of underground facilities or to provide local soil moisture data sets for calibrating indirect remote techniques on the regional scale. The greatest opportunity provided by the SiS NMR technology is the acquisition of profile relaxation data for rocks with significant bedding structures at the μm scale. With this unique feature, SiS NMR can support the understanding and modeling of hydraulic and diffusional anisotropy behavior of sedimentary rocks.

The Depositional Mechanism of Hydrothermal Chert Nodules in a Lacustrine Environment: A Case Study in the Middle Permian Lucaogou Formation, Junggar Basin, Northwest China

Although chert deposits are limited in geological distribution, their geological and geochemical characteristics can provide important information to reconstruct paleoenvironmental and diagenetic processes. For the Permian period, cherts are utilized to trace global silicon cycles and hydrothermal activities in relation to the Permian Chert Event. In Northwest China, Permian chert nodules have recently been discovered in both the southeastern and northwestern margins of the Junggar Basin. We conducted an analysis of the mineralogy, petrology and geochemistry of chert nodules of the Lucaogou Formation in the southeastern margin of the Junggar Basin to identify silicon sources and determine the precipitation mechanism of chert nodules. As evidenced by petrology, the chert nodules were mainly composed of crypto-microcrystalline silica (94.33% on average), with development of a soft-sediment deformation structure, indicating the synsedimentary deposition of silicon. Proven by trace elements, high Eu/Eu* ratios (average 2.14), low total rare earth element content (average 6.03 ppm), low LaN/YbN ratios (average 0.17) and low Y/Ho ratios (average 25.25) in chert nodules supports the hydrothermal source of silicon. The wide distribution of authigenic metal-bearing minerals and the significant positive Eu anomalies observed suggest that the chert depositions in the Lucaogou Formation intermittently received high-temperature (&gt;250 °C) hydrothermal fluids, likely associated with the initiation of the Bogda Rift in the middle Permian. Following rapid cooling down and differential compaction, siliceous sediments dehydrated and deformed, finally forming chert nodules.

Interaction of Gd2Si2O7 with CMAS melts for environmental barrier coatings

Environmental barrier coatings (EBCs) prevent the oxidation of ceramic matrix composites (CMC), which are used as components in gas turbines. However, EBCs deteriorate more rapidly in real environments, molten silicate deposits accelerate the deterioration of EBCs. In this study, high-temperature behavior sintered Gd2Si2O7 with calcia-magnesia-alumina-silica (CMAS) melt at 1400 °C for 0.5, 2, 12, 48, and 100 h was investigated. HT-XRD results showed that at 1300 °C, CMAS and Gd2Si2O7 chemically reacted to form Ca2Gd8(SiO4)6O2 (apatite). The reaction layer became thicker as the heat-treatment time increased, and the thickness of the reaction layer has increased following a parabolic curve. With the extension of the reaction time from 0.5 to 100 h, the thickness of the reaction layer increased from approximately 98 to 315 µm. It was confirmed that Ca2Gd8(SiO4)6O2 grew vertically on the Gd2Si2O7 surface. Vertical and horizontal cracks were found after reacting at 1400 °C for 100 h, but no interfacial delamination occurred in this study. In addition, the effects of CaO:SiO2 molar ratios, monosilicates (RE2SiO5) and disilicates (RE2Si2O7), heat-treatment time, and cation size were determined and compared with the results of previous studies (Gd2SiO5, Yb2SiO5, and Er2Si2O7).

Review of the type material for two Palaeozoic ostracod species from southeast Australia

The ‘Illaenus’ band of the Costerfield Siltstone in the Costerfield-Heathcote area contains pockets of Silurian (lower Wenlock) deposits that contain a rich invertebrate fauna. Several assemblages from the ‘Illaenus’ band have been described and include ostracod species referable to 13 genera. These specimens were housed at the Australian Government Bureau of Mineral Resources in Canberra, which was damaged by fire in 1953. Of the documented ostracod specimens, only three remain, but are in a degraded state. We reinterpret this historically important material as representing the types of Ctenobolbina proxima and Kayatia prima. Tamara T.A. Camilleri [tamara.camilleri@deakin.edu.au] and Mark T. Warne [mark.warne@deakin.edu.au], School of Life and Environmental Sciences and Centre for Integrative Ecology, Melbourne Campus, Deakin University, Geelong, Victoria 3220, Australia, and Museums Victoria, GPO Box 666, Melbourne, Victoria 3001, Australia. Elizabeth A. Weldon [l.weldon@deakin.edu.au], School of Life and Environmental Sciences and Centre for Integrative Ecology, Melbourne Campus, Deakin University, Geelong, Victoria 3220, Australia.

Silicate Scaling Formation: Impact of pH in High-Temperature Reservoir and Its Characterization Study

Silicate scaling tends to form and be aggravated during high pH Alkaline Surfactant Polymer (ASP) floods and this silicate scale deposition affects oil production. Hence, it is important to examine the conditions that lead to silicate scale forming. The severity of the silicate scaling reaction, the type and morphology of silica/silicate scale formed in an experimental ASP flood were studied for pH values 5, 8.5, and 11, whilst the temperature was kept constant at 90 ℃. In addition, the impact of calcium ion was studied and spectroscopic analyses were used to identify the extent of scaling reaction, morphology type and the functional group present in the precipitates. This was performed using imagery of the generated precipitates. It was observed that the silica/silicate scale is most severe at the highest pH and Ca:Mg molar ratios examined. Magnesium hydroxide and calcium hydroxide were observed to precipitate along with the silica and Mg-silicate/Ca-silicate scale at pH 11. The presence of calcium ions altered the morphology of the precipitates formed from amorphous to microcrystalline/crystalline. In conclusion, pH affects the type, morphology, and severity of the silica/silicate scale produced in the studied scaling system. The comprehensive and conclusive data showing how pH affects the silicate scaling reaction reported here are vital in providing the foundation to further investigate the management and prevention of this silicate scaling. Copyright © 2022 by Authors, Published by BCREC Group. This is an open access article under the CC BY-SA License (https://creativecommons.org/licenses/by-sa/4.0)

Nitrogen Incorporation in Potassic and Micro- and Meso-Porous Minerals: Potential Biogeochemical Records and Targets for Mars Sampling.

We measured the N concentrations and isotopic compositions of 44 samples of terrestrial potassic and micro- and meso-porous minerals and a small number of whole-rocks to determine the extent to which N is incorporated and stored during weathering and low-temperature hydrothermal alteration in Mars surface/near-surface environments. The selection of these minerals and other materials was partly guided by the study of altered volcanic glass from Antarctica and Iceland, in which the incorporation of N as NH4+ in phyllosilicates is indicated by correlated concentrations of N and the LILEs (i.e., K, Ba, Rb, Cs), with scatter likely related to the presence of exchanged, occluded/trapped, or encapsulated organic/inorganic N occurring within structural cavities (e.g., in zeolites). The phyllosilicates, zeolites, and sulfates analyzed in this study contain between 0 and 99,120 ppm N and have δ15Nair values of -34‰ to +65‰. Most of these minerals, and the few siliceous hydrothermal deposits that were analyzed, have δ15N consistent with the incorporation of biologically processed N during low-temperature hydrothermal or weathering processes. Secondary ion mass spectrometry on altered hyaloclastites demonstrates the residency of N in smectites and zeolites, and silica. We suggest that geological materials known on Earth to incorporate and store N and known to be abundant at, or near, the surface of Mars should be considered targets for upcoming Mars sample return with the intent to identify any signs of ancient or modern life.

An experimental investigation into the evolving instability of a subaqueous mild silty slope under progressive waves

The silty sediments that are widely distributed in estuaries and deltas are susceptible to slope instability due to ocean storms. Flume experiments were carried out in this study to investigate the characteristics and failure process of a subaqueous mild silty slope subjected to ocean waves. The results show that the wave steepness varies accordingly to wave propagation over the mild silty slope due to shoaling effect. Where the wave steepness is up to the threshold value of 0.04, liquefaction may occur in silty slopes due to the substantial amount of residual pore water pressure development. In this process, the maximum wave pressure appears near the wave breaking point where it may be increased by about 40% compared with that at bottom of the slope; this means that liquefaction may primarily first occur in regions near the wave breaking point and become the predominant trigger for the overall slope instability. The maximum liquefaction and sliding depth of silty seabed may reach 0.1–0.15 times the wavelength for wave steepness of 0.04–0.10. Unlike the failures of a loose sandy slope that are characterized by a slumping stage along with a sudden rise in pore water pressure, wave induced failure of the mild silty slope progresses in three distinct stages, i.e., (i) initial erosion and scouring, (ii) coupled development of liquefaction and scour, and (iii) oscillating and sliding of sediments.

Sedimentary geochemistry of deepwater slope deposits in southern Lake Tanganyika (East Africa): Effects of upwelling and minor lake level oscillations

AbstractLake Tanganyika ranks among the most valuable modern analogs for understanding depositional processes of carbonaceous sediments in ancient tropical rifts. Prior research on Lake Tanganyika has emphasized the importance of bottom-water anoxia, depositional processes (hemipelagic settling versus gravity flows), and large-scale (100s of meters) lake level change on the quality of sedimentary organic matter content. Here, facies analysis and numerous organic geochemical tools (elemental, carbon isotope, and programmed pyrolysis) were applied to a radiocarbon-dated core from southern Lake Tanganyika to investigate the accumulation of carbonaceous sediments in a deepwater slope environment influenced by high-frequency climatic fluctuations accompanied by only minor (10s of meters) lake level changes. Considerable variability in lithofacies and geochemistry characterizes the ∼ 1030-year-long core record, chiefly driven by climate-mediated changes to the lake's upwelling system. Laminated diatom oozes and sapropels with mean total organic carbon (TOC) concentrations and hydrogen indices of 6.9 wt.% and 385 mg hydrocarbon/g TOC, respectively, characterize sediments deposited during periods of strong upwelling and variable water levels. Silty sediments deposited via gravity-flow processes were likewise rich in organic matter, likely due to preservation-enhancing bottom-water anoxia. Dilution by reworked tephra was the chief constraint on organic enrichment at the study site. Data from this study reveal that oscillations in atmospheric and limnological processes in the absence of major shoreline movements can result in geochemically diverse deepwater slope sediments, which have implications for improving depositional models of petroliferous continental rift basins.

Latest Pleistocene and Holocene Floodplain Evolution in Central Europe—Insights from the Upper Unstrut Catchment (NW-Thuringia/Germany)

The upper Unstrut River is located in Germany at the modern Central European climate boundary of Cfb and Dfb climate. The river drains a loess landscape, which has experienced important environmental changes throughout the last 12,000 years. To evaluate the impacts of these changes on floodplain evolution, a multi-proxy research program, consisting of 2D electrical resistivity tomography profiling (ERT), vibracoring, and sedimentological investigations, 14C and OSL dating were applied. From base to top the investigations the following fluvial deposits were revealed: (1) gravels embedded in a fine-grained sediment matrix (interpreted as fluvial bedload deposits); (2) silty sediment with pedogenic features (interpreted as overbank floodplain deposits); (3) peat and tufa deposits (interpreted as wetland deposits) intercalated by pedogenetically influenced silty sediments (interpreted as overbank deposits); (4) humic silty sediment with some pedogenic features (interpreted as overbank floodplain deposits); and (5) silty sediments (interpreted as overbank deposits). Radiocarbon and luminescence dates yielded the following periods for sediment formation: (1) Younger Dryas to Preboreal period (around 11.6 cal ka BP); (2) Preboreal to early Atlantic period (approx. 11.6 to 7.0 cal ka BP); (3) early Atlantic to late Subboreal period (approx. 7.3 to 3.4 cal ka BP); (4) late Subboreal to early Subatlantic period (2.9 to 2.3 cal ka BP); and (5) late Subatlantic period (approx. 1.0 to 0.6 cal ka BP). The results suggest that floodplain development during the latest Pleistocene and early Holocene (approx. 11.6 to 7.0 cal ka BP) was considerably controlled by climatic conditions and short-term climate variabilities, which caused gravel deposition and overbank sedimentation. Afterwards floodplain conditions varied between rather stable (peat and tufa development, initial soil formation) and active periods (deposition of overbank fines). In this context, active periods with increased sediment input prevailed from approx. 5.1 to 3.4 cal ka BP, 2.9 to 2.3 cal ka, and 1.0 to 0.6 cal ka BP, temporally corresponding well with increased land-use phases of the past. In conclusion this study demonstrates that the investigated Unstrut catchment has reacted very sensitively to natural and human-induced changes during the latest Pleistocene and Holocene. Consequently, this high vulnerability to external changes should be considered in future river predictions or river management.

Kinetic study of fulvic acid on methane hydrate formation and decomposition in clay-rich silty sediments

Humic substances and salts are widely distributed in marine sediments, a phenomenon that undoubtedly increases the complexity of recovering methane from hydrate. In this work, kinetic experiments were carried out to determine the role of fulvic acid (FA) on methane hydrate formation and decomposition in marine sediments. We used the mixed clay-rich silty with pore water, containing 3.0 wt% NaCl, to simulate the sediments, with experimental pressure (12.7–14.6 MPa) and temperature (8.6–16.2 °C) conditions adopted from the hydrate reservoir of Shenhu area, South China Sea (SHCS). Results revealed that FA has favorable/unfavorable effects on methane hydrate formation in the sediments, in a concentration-dependent manner. FA solutions, at concentrations of 6.0 and 10.0 wt%, inhibited the formation of methane hydrate, because the high-water holding capacity itself could reduce water activity. Conversely, 2.0 wt% FA solution accelerated methane hydrate formation by arranging surrounding water molecules with H-bonds. Interestingly, it was found that the addition of FA could significantly weaken NaCl's inhibitory effect on methane hydrate formation, while water conversion ratios increased ∼2/3-fold relative to NaCl without FA solution containing systems. These results suggested that presence of FA in marine sediments promoted hydrate reformation during reservoir production. Besides, we found that there is a synergistic effect between FA and NaCl in the process of methane decomposition.

Carbonate and siliceous rock horizons at the boundary of Eocene and Oligocene deposits in the Ukrainian Carpathians as geotourism sites

&#x0D; &#x0D; &#x0D; The outcrops of carbonate and siliceous rock horizons at the boundary of Eocene and Oligocene deposits in the Ukrainian Carpathians represent potential sites for geotourism. They are located in the northern part of the Skyba zone, in settlements (Boryslav, Verkhne Synyovydne, Maydan, Skole) that can be reached by various vehicles. The sites are of interest not only due to the possibility to be shown for tourists in situbut also due to the possibility to tell them about sedimentological history of the Carpathians, the whole Tethys basin and theWorld Ocean being the reason fortheir formation. The article considers global and regional events at the Eocene-Oligocene boundary, exemplified by deposits of the Sheshory horizon and lower siliceous horizon of the Lower Menilite Sub-Formation as well as sedimentologicaland lithogenetic aspects of chertsorigin.In sections of the Carpathian flysch the clay-calcareous and siliceous- clay rocks of the Sheshory (Eocene) and Rybnyk (Oligocene) horizons are overlaid by a pack of carbon-bearing cherts(mainly phthanites) of the Menilite Formationlower siliceous horizon. This was fixed as the «Eocene final event» (climatic cooling, mass extinction of some groups of marine organisms, including certain types of warm-water foraminifera), and had a global significance. At that time at the regional scale separation of the Paratethys Sea from the Tethys Ocean as well as sharp change in biogenic sedimentation (from carbonate to siliceous) in the Carpathian sedimentary basin occurred.At the Eocene and Oligocene boundary and then during the Oligocene the World Ocean demonstrated a series of events that ultimately led to the transformation of a global system of ocean waters circulation from the Cretaceous-Eocene, marked with uniformly warm climate throughout the planet, to contemporary with more contrast climate and distinct climatic zones.The change of warm-water plankton organisms for those who areaccustomed to live in cold seas can be illustrated by the results of study of calcareous and siliceous rocks of the Eocene-Oligocene deposits in the Ukrainian Carpathians.The Sheshory horizon marls are composed of cryptocrystalline clay- carbonate matter with numerous remains of warm-water forms of planctonic foraminifera from a group of globigerina (Globigerinida) sized up to 0.1 mm. The number and size of these organisms significantly reduced in the rocks of Rybnyk horizon. No Their traces are notfound in siliceous formations.Various researchers have justified biogenic, volcanogenic, or chemogenic origin of these cherts. Authors of the article believe that these rocks are the product of sedimentation of biogenic siliceous deposits and their post-sedimentary transformation.&#x0D; &#x0D; &#x0D;

The effect of overconsolidation on the mechanical behaviours of hydrate-bearing silty sediments

CO2 sequestration via gas hydrate method shows great potential on promoting global carbon cycling and reducing greenhouse effect through injecting CO2 to hydrate reservoirs. However, the overconsolidation state induced by hydrate formation or interconversion of CO2 and CH4 hydrate would affect the stability of silty reservoirs after CO2 sequestration, but currently, few studies have been presented on this topic. To this end, a sequence of overconsolidation and drained shear tests on CO2 hydrate-bearing silty sediments (HBSSs) were carried out in this study to simulate and elucidate the effect of overconsolidation on the mechanical behaviours of CO2 HBSSs. The results reveal that all HBSSs with different hydrate saturations exhibit strain hardening and shear contraction behaviours with or without overconsolidation, while an increase in the overconsolidation ratio (OCR) enhances the deviator stress and alleviates the contraction effect. In addition, a strengthening effect on the stiffness, strength and resistance of deformation of HBSSs is induced by overconsolidation, while this effect becomes more prominent as the hydrate saturation increases. HBSSs with higher OCRs and hydrate saturations generally have lower stress-dilatancy ratios. Moreover, the shear paths of HBSSs with higher OCRs deviate farther from the normal consolidation line of hydrate-free silty sediments. This study is expected to provide theoretical guidance for the CO2 sequestration via gas hydrate in silty reservoirs.

Cambrian–Silurian sediments in the southeastern Qilian Orogen, NE Tibetan Plateau: Constraints on crustal and tectonic evolution of microcontinents in the northern Proto-Tethys Ocean

The Qilian Orogen preserves abundant Early Paleozoic records, revealing the tectonic evolution of the northern Proto-Tethys Ocean on the periphery of the northern margin of eastern Gondwana. We present detrital zircon U–Pb and Hf isotopes, and whole-rock element and Nd isotopic geochemical data on the Early Paleozoic sedimentary rocks in the southeastern Qilian Orogen. Detrital zircon grains from these rocks are clustered into seven distinct age groups: 495–426 Ma, 710–501 Ma, 1043–726 Ma, 1347–1054 Ma, 2009–1609 Ma, 2874–2274 Ma, and >3100 Ma. Detrital zircon and geochemical results suggest that their provenances are Early Paleozoic subduction–accretion related rocks, Neoproterozoic basements of the Qilian Block, and recycled sediments from Precambrian units. The whole-rock εNd(t) values of the sedimentary samples range from –13.0 to 2.5, and the εHf(t) values of Neoproterozoic–Early Paleozoic detrital zircon grains range from –28.6 to 15. Hf and Nd isotopes indicate marked growth and reworking of the continental crust in the early Neoproterozoic and Ordovician–Early Silurian. As revealed by age spectra of detrital zircon, we infer that the Qilian Block was tectonically related to blocks on the northern margin of eastern Gondwana during the Early Paleozoic, such as the North Qinling, Indochina and South China blocks. We also suggested that the Qilian Block experienced double-sided subduction of the North and South Qilian oceans, and developed sedimentation in the inter-arc and back-arc basins during the Ordovician to earliest Silurian. Finally, sedimentation occurred in an Early Silurian retro-arc foreland basin and Middle Silurian foreland basin during the syn-collisional stage.

Numerical analysis on gas production from silty hydrate reservoirs in the South China sea by depressurizing: The effect of permeability reduction caused by pore compression

Natural Gas Hydrate (NGH) is widely discovered in silty sediments with a high compressibility in the South China Sea. The permeability reduction in these reservoirs during depressurization is inevitable. Here, we innovatively take the effect of permeability reduction into account to improve the accuracy of production forecast and suggest the optimal production pressures for two different typical reservoirs (i.e., Class I: Site W19 and Class III: Site SH2) in this area. In this study, two typical reservoir-scale production models of NGH reservoir are built, and the effects of bottomhole pressure and pore compressibility and permeability correlation coefficients on gas and water extraction are quantitatively evaluated. The simulation results indicate that for silty NGH reservoirs, the effect of permeability reduction on gas and water recovery during depressurization cannot be overlooked. Pore compressibility and permeability correlation coefficients will significantly affect production, and with an increase in both parameters, the cumulative gas and water recovery will decrease. However, the total gas production will firstly increase and then decrease with a reduction in bottomhole pressure. In addition, the optimal production pressures of sites W19 and SH2 are 1.5 MPa and 5.5 MPa respectively. These findings can give some valuable suggestions for future field tests in this region.

The effect of siliceous sponge deposition on Permian paleocommunity structure

AbstractA dramatic shift from carbonate-rich to chert-rich marine strata occurred during the Permian and is frequently attributed to the increased activity of siliceous sponges and their biosiliceous sedimentation. The first-order ecologic consequences of this transition, if any, remain opaque. We analyze fossil occurrence data from the Phosphoria Basin (western North America) to test whether the presence of siliceous sponges, which are correlated with basin-wide chert strata, influenced the recruitment of benthic fauna. Using published lithologic descriptions, we categorized fossil collections by formation, facies, and lithology and used these data to code detrended correspondence analysis and nonmetric multidimensional scaling ordinations. We also analyzed the clustering of taxa into faunal units termed biofacies.Results from these analyses indicate that fossil collections occurring in chert and carbonate are closely associated in faunal composition and community structure. These collections preferentially occur in the inner- to mid-ramp facies, in agreement with previous studies. Although largely similar in composition, collections of chert and carbonate lithology exhibit differences in the frequency and abundance of accessory brachiopod taxa (e.g., Composita and Hustedia), possibly a result of greater biosiliceous sedimentary input.