Carbonate Sediments Research Articles

Building a bimodal landscape: bedrock lithology and bed thickness controls on the morphology of Last Chance Canyon, New Mexico, USA

Abstract. We explore how rock properties and channel morphology vary with rock type in Last Chance Canyon, Guadalupe Mountains, New Mexico, USA. The rocks here are composed of horizontally to near-horizontally interbedded carbonate and sandstone. This study focuses on first- and second-order channel sections, where the streams have a lower channel steepness index (ksn) upstream and transition to higher ksn values downstream. We hypothesize that differences in bed thickness and rock strength influence ksn values, both locally by influencing bulk bedrock strength and also nonlocally through the production of coarse sediment. We collected discontinuity intensity data (the length of bedding planes and fractures per unit area), Schmidt hammer rebound measurements, and measured the largest boulder at every 12.2 m elevation contour to test this hypothesis. Bedrock and boulder mineralogy were determined using a lab-based carbonate dissolution method. High-resolution orthomosaics and digital surface models (DSMs) were generated from drone and ground-based photogrammetry. The orthomosaics were used to map channel sections with exposed bedrock. The United States Geological Survey (USGS) 10 m digital elevation models (DEMs) were used to measure channel slope and hillslope relief. We find that discontinuity intensity is negatively correlated with Schmidt hammer rebound values in sandstone bedrock. Channel steepness tends to be higher where reaches are primarily incising through more thickly bedded carbonate bedrock and lower where more thinly bedded sandstone is exposed. Bedrock properties also influence channel morphology indirectly, through coarse sediment input from adjacent hillslopes. Thickly bedded rock layers on hillslopes erode to contribute larger colluvial sediment to adjacent channels, and these reaches have higher ksn values. Larger and more competent carbonate sediment armors both the carbonate and the more erodible sandstone and reduces steepness contrasts across rock types. We interpret that in the relatively steep, high-level ksn downstream channel sections, the slope is primarily controlled by the coarse alluvial cover. We further posit that the upstream low-level ksn reaches have a base level that is fixed by the steep downstream reaches, resulting in a stable configuration, where channel slopes have adjusted to lithologic differences and/or sediment armor.

Morphology and Depositional Processes of a Carbonate-Filled Canyon in the Carboniferous KT-II Formation of the Eastern Precaspian Basin, Kazakhstan: Insight from 3D Seismic Data

Canyons in carbonate depositional settings, as important elements of the source-to-sink system, remain poorly studied compared to those in siliciclastic depositional environments. The latest high-resolution three-dimensional seismic data, well logs, and core data at the eastern edge of the Precaspian Basin are used to investigate the geomorphology, infillings, and depositional process of a unique carbonate-filled canyon in the Carboniferous KT-II formation parallel to the carbonate platform, which is distinct from other slope-perpendicular canyons. The canyon has a total length of more than 52.3 km with a nearly N-S orientation and an S-shaped geometry, and the whole canyon can be divided into three segments by two knickpoints. The slope-parallel orientation of the canyon is mainly controlled by the palaeogeomorphology and reverse faults. Due to the collision of the Kazakh and European plates in the early-middle Visean (early Carboniferous), the canyon was formed in a northern tilted, elongated, and restricted palaeotopographic feature between uplifts. The development of reverse faults related to tectonic movement controlled the variations in the width of the canyon and the positions of the knickpoints. Tectonics controlled the orientation and formation of the canyon, while sedimentary processes contributed to its infilling. The well-seismic tie analysis indicated two distinct periods of the canyon fillings, Ss1 and Ss2, which were separated by a second-order sequence boundary. The lower part contained sediments supplied by both sides of the canyon through channels or gullies, and the upper part was dominated by a carbonate platform that prograded from the eastern side of the canyon. The evolution of the canyon can be subdivided into three stages. The increasing stage was mainly characterized by significant upslope erosion through headward retrogressive mass failures in the slope-parallel confined negative relief to form the canyon during the lowstand system tract of Ss1. Subsequently, in the early filling stage, the carbonate factory was productive during the highstand, and massive excess carbonate sediments were transported into the adjacent canyon by channels or gullies on both sides and deposited. The canyon was basically filled, and the morphology became much gentler. During the subsequent late filling stage, the carbonate platform was flooded again during the highstand, and the production rates of the carbonate factory greatly increased. The lateral progradation of carbonate platforms accelerated on the canyon of the early filling stage and further into the inner sag.

The upper cretaceous carbonate sediments of the Ras Budran Field, Gulf of Suez, Egypt: stacking patterns and depositional controls

The present study interprets the carbonate stacking patterns and their depositional controls in the Upper Cretaceous strata of the Ras Budran Field in the Gulf of Suez, Egypt, on the basis of integrated well logging and a geological report provided. A set of well logs was run for two wells (RB-A2 and RB-B2), comprising gamma ray, resistivity, density, neutron, sonic, and photoelectric factor. Composite logs and geological reports were also used to help deduce the lithological and biological description of the stratigraphic units under investigation. The stacking patterns of the carbonate rocks yielded three types of electro-facies. The first facies covers the lower part of the Brown Limestone (early Campanian) and reflects a cylindrical (aggrading) stacking pattern. This facies indicates keep-up carbonate deposition in a heterogeneous facies accumulated in shallow water. The second facies covers the upper part of the Brown Limestone (late Campanian) and displays a funnel (prograding) stacking pattern. This facies indicates the depositional setting of a shallow shoreline. The third facies covers the Sudr Chalk (Maastrichtian), for which a serrated stacking pattern indicates an aggradational environment. This pattern indicates a marine depositional setting both distal and deep. Therefore, the formation and evolution of the cyclical stacking pattern of the carbonate rocks reflects variations in accommodation potential that determines the environmental changes. The evolution of these deposits is mainly attributed to a combination of eustatic sea-level fluctuations and tectonic episodes within the basin.

Caliche From Archaeological Zone of Guanajuato Mexico and Nearby Deposits: Geochemical and Mechanical Characterizations

ABSTRACT Currently, several archaeological sites are being studied in the Mexican state of Guanajuato. Among these sites is the Cerro de Los Remedios, located in the municipality of Comonfort, in the state of Guanajuato. The archaeological site shows that among the main materials used for construction was a carbonate-based stone called caliche. Caliche is the result of carbonate sedimentation in sandstone; therefore, its physical and geochemical characteristics are specific to each archaeological zone around the world. Therefore, it was necessary to conduct a study of the chemical, physical, and mechanical properties of the material found. Samples were removed from the CZA archaeological site and characterized by infrared spectroscopy, XRD, X-ray fluorescence and SEM. The results showed that more than 93% of the caliche is composed of CaCO3 as a matrix of micrite and calcite granules. In addition, the physical and mechanical properties were determined, including density, porosity, and uniaxial compression stress. With the results obtained from the samples from the archaeological site, it was possible to identify material banks close to the caliche properties that were most like those used at the site, which can be proposed to be used in the conservation work required at the archaeological site.

The response of calcareous organism to temperature from continental shelf sediments under culture experiment—A case of benthic foraminifera

Calcareous foraminifera are significant indicators of past temperatures, but their ecological characteristics and relationship with temperature have not been extensively studied under culture experiment, particularly in continental shelf environments. In this research, we focused on Jiaozhou Bay (9–26 m) as a case study and collected surface sediments. For the first time, we cultured living foraminifera under five different temperature gradients (6, 12, 18, 24, and 30 °C) to investigate the impact of temperature changes on calcareous foraminifera in continental shelf sediments. Each treatment was replicated three times, and the experiment lasted for 80 days. A total of 4162 live foraminifera were collected. As the culture temperature increased, the foraminiferal parameters (abundance, species diversity, and Shannon-Wiener index) initially increased and then decreased, with the exception of the Margalef index. The study revealed that the ecological inflection point in continental shelf foraminifera occurred at or before 18 °C. Furthermore, a linear relationship was established between the Margalef index and temperature, represented by the eq. Y = 0.0038*T + 0.0962. Notably, the study indicated that foraminifera inhabiting the deeper continental shelf exhibited greater sensitivity to temperature variations compared to the intertidal community. This finding suggests that ecosystems located in deeper waters may be more susceptible to potential harm. Based on projections, crossing the temperature inflection point could potentially result in a 20% decrease in the biomass of calcareous foraminifera in shallow waters, as well as an 8.6 million ton decline in carbonate sedimentation in the future. Overall, this research provides novel insights and contributes to a better understanding of the response of calcareous organisms to temperature, as well as aids in paleotemperature reconstruction efforts.

Survived carbonates in orogenic dunites from recycling of subduction-related sediments

The carbon recycling between the Earth's surface and its interior is accomplished through plate subduction and magmatic processes. The orogenic mantle peridotites document multi-stage metasomatism in the subduction zone and play a significant role in revealing the material transportation and migration between the Earth's different spheres. However, how the migration process of carbon-bearing species influenced the subcontinental lithosphere mantle remains a mystery. To address these issues, we conducted detailed petrography, whole-rock and mineral geochemical compositions, and Sr–C–O isotope studies on two types of peridotites (silicate melt pocket-bearing and carbonate melt pocket-bearing dunites) of the Maowu ultramafic massif in the Dabie orogen. The silicate-melt pockets were captured earlier than the carbonate-melt pockets, and were derived from a large proportion of silicate melts during the formation of garnet pyroxenites. The subsequent carbonate metasomatism was mainly divided into two stages: the stage 1 reflects the reaction between dolomite melts and orthopyroxenes to generate clinopyroxenes with high CaO content and Mg# value (> 92); the stage 2 is the injection of Ca-rich fluids into dunites, forming dolomite veins and volatile-rich assemblages of tremolite, barite and monazite. The clinopyroxenes display high 87Sr/86Sr (0.7079–0.7097) and (La/Yb)N ratios, low Ti/Eu ratios, and different enrichment in Th, U, Sr and light rare earth elements (LREEs), indicating a carbonate metasomatic origin. The equilibrium melts of them are similar to sedimentary limestones with positive Sr anomaly and depleted high field strength elements (HFSEs). The Sr-O isotope mixing simulation shows a mixing of 80–90% carbonate sediments with the upper mantle peridotite. The C-O isotopes of the carbonate minerals have similar variation ranges (δ13CV-PDB: 19.7–15.6 ‰, δ18OV-SMOW: 18.1–28.1 ‰) within the range of sedimentary organic carbon. The 87Sr/86Sr ratios of limestone reservoir (0.708–0.709) suggest the addition of sedimentary carbonates into the mantle wedge at ∼370–500 Ma. Therefore, we propose that the sedimentary carbonates and organic carbon migrated into the deep mantle-wedge through the subducted Tethyan-Ocean slab in the Early Paleozoic, which experienced complex metasomatism and formed as carbonate minerals in the Maowu dunites to achieve the carbon recycling between the sedimentary carbon reservoir and the mantle wedge beneath the North China Craton.

Sulfur disproportionating microbial communities in a dynamic, microoxic-sulfidic karst system.

Biogeochemical sulfur cycling in sulfidic karst systems is largely driven by abiotic and biological sulfide oxidation, but the fate of elemental sulfur (S0 ) that accumulates in these systems is not well understood. The Frasassi Cave system (Italy) is intersected by a sulfidic aquifer that mixes with small quantities of oxygen-rich meteoric water, creating Proterozoic-like conditions and supporting a prolific ecosystem driven by sulfur-based chemolithoautotrophy. To better understand the cycling of S0 in this environment, we examined the geochemistry and microbiology of sediments underlying widespread sulfide-oxidizing mats dominated by Beggiatoa. Sediment populations were dominated by uncultivated relatives of sulfur cycling chemolithoautotrophs related to Sulfurovum, Halothiobacillus, Thiofaba, Thiovirga, Thiobacillus, and Desulfocapsa, as well as diverse uncultivated anaerobic heterotrophs affiliated with Bacteroidota, Anaerolineaceae, Lentimicrobiaceae, and Prolixibacteraceae. Desulfocapsa and Sulfurovum populations accounted for 12%-26% of sediment 16S rRNA amplicon sequences and were closely related to isolates which carry out autotrophic S0 disproportionation in pure culture. Gibbs energy (∆Gr ) calculations revealed that S0 disproportionation under in situ conditions is energy yielding. Microsensor profiles through the mat-sediment interface showed that Beggiatoa mats consume dissolved sulfide and oxygen, but a net increase in acidity was only observed in the sediments below. Together, these findings suggest that disproportionation is an important sink for S0 generated by microbial sulfide oxidation in this oxygen-limited system and may contribute to the weathering of carbonate rocks and sediments in sulfur-rich environments.

Paleokarst coastal caves at Torricelle Hills (Lessini Mountains, Venetian Prealps, Italy)

This paper describes a set of paleokarst caves at Torricelle Hills near Verona (Southern Alps, Italy.) At this locality, erosional surfaces and paleokarst cavities show that sedimentation of late Paleogene neritic limestones was interrupted by subaerial exposure. Karst features developed during a phase of marine regression that started after the early Oligocene and ended in the mid Miocene. These caves were originally completely filled by iron oxides- and hydrated oxides-rich paleosol sediments (ochre) that, for centuries, have been mined for pigments. Mining activity emptied the caves, leaving the voids and related shapes mostly intact; as a result, the original morphologies have been exhumed, making these caves a rare example of explorable paleokarst. These “ochre caves” were mapped in a series of surveys over a few years. The exploration of overall 4.5 km of accessible passages in four caves yielded a wealth of information on speleological features, stratigraphy, paleontology, and paleogeography, and here we exploit this information to infer the genesis of these unusual caves. Their evolution started in phreatic conditions, characterized by very slowly moving or still waters that led to the formation of solution facets. A vadose phase of development ensued, followed by infilling by reworked soil-derived sediment and associated paragenetic modifications. Sediment accumulation ended with the complete fossilization of the caves under epiphreatic conditions. Siliciclastic and carbonate sediments containing littoral fossils indicate that the caves developed in the vicinity of a coast, and that they were subject to marine ingression. Overall, these paleokarst coastal caves seem to be a fossilized example, well preserved and explorable, of the Carbonate Island Karst Model on larger islands. We interpret these caves as conduits that drained the freshwater lens in a spatially limited carbonate peninsula that existed in this part of the Lessini paleocoastline between the Oligocene and the Miocene.

The development and origin of the two-stage silicification of Upper Jurassic limestones from the northern part of the Kraków-Częstochowa Upland (Southern Poland)

The Upper Jurassic carbonates representing the microbial-sponge megafacies in the area of the Kraków-Częstochowa Upland (KCU) were locally silicified. In the reclaimed Lipówki Quarry, in Rudniki near Częstochowa (in the northern part of the Upland), macroscopically different silicification products were observed in blocks of Upper Jurassic limestones, deposited as mining waste. Two varieties were distinguished: (i) chert concretions representing the I silicification stage and (ii) light-brown, silicified limestones infilling the fractures in chert concretions or forming the cortices around the concretions or forming irregular bodies, all representing the II silicification stage. The diagnostic features are the following: (i) macroscopic development, (ii) the presence of moganite exclusively in chert concretions and (iii) significant differences in crystallinity index (CI) values, namely: 0.1–0.7 for chert concretions and 6.0–6.6 for silicified limestones. The formation of chert concretions was initiated as early as in unconsolidated sediment, whereas the II silicification stage followed the chemical compaction of the limestones. The results of geochemical analyses of the products of both silicification stages indicated that the probable source of silica were the low-temperature hydrothermal solutions. Two types of fractures were found in the chert concretions, generated during different tectonic events. The older, open fractures were formed during the extension of the Late Jurassic sedimentary basin, which formerly occupied the territory of the more recent KCU. These fractures were infilled with unconsolidated, fine-detrital carbonate sediment, in which the concretions were embedded and finally silicified in the II silicification stage. The younger, closed fractures, transversal to those filled by the products of II silicification stage, along which small displacements are evident, document the later tectonic deformations presumably related to Cenozoic faulting.

Characteristics of Early Neoproterozoic Stromatolites from Southern Liaoning, North China: Insights into the Formation of Stromatolites

Stromatolites, among the earliest fossils in Earth’s history, are widely distributed on the margins of the North China Precambrian carbonate platform. The formation processes of stromatolites reveal the biomineralization and evolution of early life in the Precambrian. The well-preserved stromatolitic dolostones recorded in the Ganjingzi Formation are developed around Yuanjiagou village, in southern Liaoning Province. The morphology of the Ganjingzi stromatolites manifests in stratiform, columnar, and domal forms. A tripartite lamina structure including light laminae and two types of dark laminae is observed in thin sections. The origins of dark laminae were related to microbial metabolism, while the light laminae were the result of the recrystallization of synsedimentary marine cement. Hardground substrate and carbonate fragments were suitable for microbes to colonize, suggesting that microbes can adapt to various current energy settings. A comparison of the growth environment, morphology, and laminae features between the Ganjingzi stromatolites and modern carbonate stromatolites from Hamelin Pool and Lagoa Vermelha suggest that the Ganjingzi stromatolites may have been formed in a restricted tidal-flat setting with high salinity and evaporation. The role of microbes that form modern stromatolites in inducing precipitation of carbonate or binding sediments, might contribute to the formation of the Ganjingzi stromatolites. The formation process of the Ganjingzi stromatolites indicates that the microbial communities, favorable substrate, and synsedimentary marine cement were the key factors in promoting the development of the Neoproterozoic stromatolites on the northeastern margin of the North China Craton.

Sediment accumulation rates and carbonate fluxes of deep-sea sediments in the southern Gulf of Mexico

Here we present the mass and carbonate annual fluxes, collected by two sediment traps at 1000 m depth, located on the western and southern deep-water region of the Gulf of Mexico (GM), and compare them with the total mass and carbonate accumulation rates (CAR) from 46 sediment cores retrieved from continental slopes and the abyssal plain of the southern Gulf of Mexico (sGM). CaCO3 fluxes in the sediment traps located close to 1000 m water depth, range between 15.1 and 22.4 g m−2 yr−1, these values fall on the high-end range of the mean fluxes reported for the global ocean (9–22 g m−2 yr−1). The carbonate fluxes from the water column are remarkably close to the CAR in the lower slope and abyssal sediments of the southern GM, implying that dissolution processes of biogenic carbonate in the sediments below 1000 m are negligible in the sGM. The spatial patterns of CARs in the sediments are mainly linked with surface productivity of calcareous organisms and export to depth over the continental slope and abyssal plain regions, with mean values of 15.7 ± 5.7 g m−2 yr−1 (0.15 ± 0.06 mol m−2 yr−1). However, there are some large areas over the abyssal plain to the north of the Campeche escarpment showing enhanced CAR values most likely as a result from two processes, lateral input from the Yucatan carbonate shelf region and massive collapse events originating at the Campeche escarpment and deposited to the north of the Yucatan Platform. These processes can account for an additional increase in carbonate mass accumulation by approximately 8 g m−2 yr−1 in this area, showing a decreasing gradient towards the N of the escarpment. Lateral transport by turbidity currents of carbonate platform sediments along submarine canyons and probably episodic gravity flows further aid to explain the localized higher carbonate mass accumulation (> 20 g m−2 yr−1) observed at the foot of the continental slopes to the west and south of the southern GM. We estimate total carbonate accumulation estimates for the entire Gulf of Mexico, from the upper slope to the abyssal plain (1000–3750 m), considering the northern deep-water region reported by Giering et al., 2018, of 0.09 to 0.12 Tmol/yr accounts for 0.8 to 1.8% of all carbonate sedimentation in the ocean below 1000 m. Taking into considerations its small areal extent of <0.3% of the global ocean the GM carbonate accumulation accounts for a factor of 3 to 6 its areal importance, implying that changes in the production and export of carbonate in the GM due to surface ocean warming and acidification processes in this inland sea will have a sensible effect in the global carbon cycle.

Constraining the onset and environmental setting of metazoan biomineralization: The Ediacaran Nama Group of the Tsaus Mountains, Namibia

The advent of animal (metazoan) biomineralization marks a fundamental transition in Earth's history, facilitating the diversification of novel body plans as well as the biological control of carbonate sediment production, resulting in a permanent restructuring of the global carbon cycle. The oldest known skeletal metazoans, the tubular ‘cloudinids’ e.g., Cloudina, appeared during the terminal Ediacaran of the Nama Group, Namibia, but precisely when, where, and why metazoans first acquired the ability to biomineralize and their timing relative to the regional δ13Ccarb profile is undocumented. Assessing possible environmental triggers for this key event therefore requires accurate constraint of the age, paleoenvironmental setting and the geochemical context of the earliest cloudinids.Here we present new stratigraphic, sedimentological, and geochemical (δ13Ccarb and δ18Ocarb) data from the oldest strata (Dabis Formation) of the Nama Group from the Witputs Sub-basin, exposed in the Tsaus Mountains of the Tsau Khaeb National Park (formerly Sperrgebiet). This constrains the oldest confirmed occurrence of Cloudina to limestones of the lower Kliphoek Member, ca. 551–550 Ma, that were deposited laterally-equivalent to the more proximal fossiliferous Kliphoek quartzite. These limestones have dominantly negative δ13Ccarb values that immediately precede recovery from the basal Nama negative δ13Ccarb excursion (BANE), and were deposited in shallow waters after a transition from semi-restricted, evaporitive-dolomitic to open marine carbonate settings. Regional correlation shows that Cloudina first appeared during an interval of dominantly low oxygen and unstable, regional marine redox conditions. We conclude that data do not support a long-term shift towards more stable, oxygenated conditions as a driver for the first appearance of skeletonization in metazoans, but do suggest that open marine carbonate settings with high carbonate supersaturation were required to support the calcifying Cloudina. In the Nama Group, the appearance of Cloudina also notably coincided with a major sea level lowstand, which we hypothesise may have enabled oxygenation of these more distal settings. Such an expansion of shallow marine oxygen may therefore have provided an evolutionary driver for innovations that were not metabolically costly, such as the onset of Cloudina biomineralization.

Utilization of chemostratigraphy to the subregional scale correlation of Permo–Carboniferous sediments, eastern Saudi Arabia

The following chemostratigraphy study was completed on the Permo–Carboniferous Unayzah Group in Eastern Saudi Arabia. The aim of the study was to determine whether the technique could be used to identify formation and member boundaries on a subregional scale, with the area extending for c. 550 km in a NW–SE direction and 350 km NE–SW. A total of 30 375 core and cuttings samples were analysed from 225 wells using ICP (Inductively Coupled Plasma) and XRF (X-ray Fluorescence) instruments, with data acquired for 42–53 elements for each sample in the range Na-U in the periodic table. Of these, details of 14 wells are discussed in the present work. Based on variations in Zr/Ti, Zr/U, Zr/Y, Y/U, U/Th, Gd/Zr, Nb/Yb and Nb/Th it is possible to identify a hierarchical order of one zone, four subzones and five divisions, these chemozones being correlative on a subregional scale. By comparing geochemical, biostratigraphic, sedimentological and wireline log data in selected reference wells, it was possible to relate chemozones to particular members of the Unayzah Group. The differences in geochemistry between these chemozones/members is largely explained by changes in provenance, though depositional environment may have exerted at least some influence.

Dolphin-shaped island: Exploring the natural resources and radiological hazards of Wadi El Gemal Island

The objective of this study is to assess the natural resources and radiological risks of Wadi El Gemal Island by examining its topography, mineralogy, geochemistry, and radioactive distributions. This island, which is situated at the outlet of Wadi El Gemal in Egypt's southeastern Desert, has a unique shape resembling a dolphin based on Landsat imagery. It's a part of the Wadi El Gemal-Hamata Protectorate and is notable for its diverse environmental, geological, economic, and archeological features, including recent reefs, sandy deposits, Quaternary carbonate sediments, and mangroves. The main natural resources on the island are fauna, mangrove forests, and flora. Samples collected from the island were analyzed using a NaI detector to measure the concentrations of radionuclides such as 238U, 232Th, 226Ra, and 40K, which were found to be within acceptable levels according to UNSCAR guidelines. The radionuclide 238U, 232Th, 226Ra, and 40K activity concentrations of the collected samples were 32.55 ± 9, 12.63 ± 4, 12.49 ± 4, and 325 ± 34 Bq/kg, respectively. Regarding radiological hazard indices, the values of absorbed gamma dose rate (36.06 ± 5.42 nGy/h), radium equivalent activity (73.88 ± 14.4 Bq/kg), annual effective dose indoor (0.18 ± 0.03 mSv/y) and outdoor (0.04 ± 0.01 mSv/y), internal (0.29 ± 0.05) and external (0.2 ± 0.03) indices, and excess lifetime cancer index (0.15 ± 0.05 × 10−3).This is suggest that there is no significant risk associated with these sediments.

Substantial incorporation of isotopically heavy reduced U species into marine carbonate sediments

The uranium (U) isotope ratio (238U/235U, reported as δ238U) in marine carbonates is an important proxy for reconstructing changes in past oceanic redox conditions, based on the essential assumption that U in carbonates originates from dissolved U(VI) in seawater. However, diagenetic processes may induce the addition of isotopically heavy U(IV) into the carbonates, complicating the application of the U isotope proxy. So far, the valence states of trace amounts of U and its relation with U isotope composition in marine carbonate sediments have not been directly studied. In this study, we have calibrated an ion-exchange chromatographic method to separate U(IV) from U(VI) in geological carbonate samples (e.g., modern and fossil corals, stalagmites, cold seep carbonates) and quantified their contents by mass spectrometry. Our study confirms that modern coral carbonates are faithful archives of U(VI) sourced from seawater. Surprisingly, drill core samples from a modern coral carbonate platform, which bear no sign of alteration inferred from conventional diagenetic proxies (e.g., Mn/Sr), show significant positive correlation between U(IV) fraction and bulk carbonate δ238U, suggesting U(IV) is present in the marine carbonates and drives δ238U toward higher values than that of seawater. We therefore suggest that coupled valence and isotope analyses of U in marine carbonates could provide critical constraints toward reliable reconstruction of marine redox evolution in the Earth’s history.

Effect of chemical environment on copper tailings reinforced by microbially induced carbonate precipitation

The accumulation of large amounts of tailings produced in the beneficiation process of ores is one of the main challenges for mining industry related to their retention in tailings ponds and the potential hazard of tailings dams’ failure. To enhance stability of tailings dams, microbially induced carbonate precipitation (MICP) has emerged as a novel technique for tailings treatment. However, the intricate chemical environment within tailings ponds affects the growth of microorganisms and can adversely impact the efficiency and effectiveness of MICP. This study investigates for the first time the factors that influence the growth of Sporosarcina pasteurii in a copper tailings pond characterized by weak acidity and the presence of copper ions. The adverse effects of the chemical environment in copper tailings ponds on MICP are investigated through mechanical tests and microscopic analyses on four groups of MICP-reinforced copper tailings. The results revealed that copper ions inhibited the growth of bacteria, while pH had an insignificant effect on bacteria. Hence, copper ions are identified as the main factor that weakens the reinforcing effect of MICP on copper tailings. Even though the weakly acidic environment has been shown to have a minor impact on the performance of MICP-reinforced copper tailings, maintaining the acid resistance of the reinforced tailings is crucial for ensuring long-term reinforcement effectiveness. It was also found that increasing the concentration of the bacterial solution and applying it in multiple injections, while controlling the pH of the cementation solution, could enhance the survival rate and activity of bacteria and improve the carbonate sedimentation environment. This would strengthen the reinforcing effect of MICP. The results demonstrated that MICP has promising potential for tailings treatment, but it is essential to address the adverse effects of the chemical environment in tailings ponds. The findings from this study provide valuable reference data for subsequent field experiments and engineering practices and facilitate the application of biomineralization in tailings ponds.

Characterizing the water–rock interactions and groundwater flow processes in the Paleozoic to Cenozoic strata aquifer systems with contrasting mineralogy at basin scale: Qingyi river, east China

This study aims to identify water–rock interaction and groundwater flow processes in aquifer systems with contrasting mineralogy at the basin scale. Investigations have been carried out using major ion geochemistry and environmental isotopes (δ2H and δ18O of water, δ13C values, 87Sr/86Sr ratios, and 14C activities) within aquifers with contrasting mineralogy in the Qingyi river basin (QRB, east China) composed of marine carbonate and continental granitic sediments with successive strata during Paleozoic, Mesozoic and Cenozoic. Analysis of 95 rain, surface and groundwater, and 12 solid rock samples found that dissolution of limestone, silicate minerals, and evapotranspiration dominated the hydrogeochemical process in the lower, upper, and middle basin, respectively. The various major ion and 87Sr/86Sr ratios of groundwater in each aquifer suggest that local lithology minerals dissolution controls the water–rock equilibria in the groundwater system. The 87Sr/86Sr ratios are correlated with both major ion geochemistry and δ13C values, constraining the process of varying proportions of silicate and carbonate minerals dissolution. Percolating water from soils carrying dissolved CO2 plays an important role in controlling the hydrogeochemical processes in the groundwater system throughout the QRB. Inter-aquifer flow has been proved by the well correlated relations between 87Sr/86Sr ratios and δ13C values or/and Sr/Na and Ca/Na ratios, as well as the irregular groundwater apparent 14C ages (1.78 to 8.52 kyr) with respect to position in the basin. Admixture ratios of 9–50% are derived from the groundwater in limestone aquifer. The rapid decrease in carbonate groundwater mixing ratio is accompanied by an increase in 87Sr/86Sr ratios, indicating that groundwater is derived Sr from silicate weathering and abundant carbon from the soils. Regional inter-aquifer flow occurs in the buried carbonate aquifers based on isotopic evidence, which could result in karstic groundwater leakage in the Wuhu cityside and should be given special attention.

Origin and process of carbon isotopic reversal of tight sandstone gas in the Shixi gas field, eastern margin of Ordos Basin

Stable carbon isotopic compositions of natural gas have long been employed to determine its origin, accumulation, and migration. A reversal of the natural gas carbon isotope sequence, i.e. δ13C1 > δ13C2 > δ13C3, was discovered in the tight sandstone gas (TSG) of the Shixi gas field at the eastern margin of the Ordos Basin. This paper presents a study to determine the causes of carbon isotopic reversal hence to better understand the gas accumulation in these tight sandstone reservoirs. The molecular composition and carbon isotopes on 81 wellhead gas samples were measured from the TSG reservoirs of the Pennsylvanian in the Shixi gas field. Results indicate that the molecular compositions of the Shixi gas field comprise methane ranging from 88.9 wt% to 98.9 wt%, ethane ranging from 0.49 wt% to 1.12 wt%, and minor propane with content <0.01 wt%. The non-hydrocarbons are mostly CO2 which is <1.2 wt% and the rare helium ranges from 0.02 wt% to 0.23 wt% with an average of 0.09 wt%. The CO2 was probably derived from the decomposition of carbonate sediments under a high temperature owing to the varied δ13CCO2. Hydrocarbon is considered predominantly from coal-derived humic source rocks and minor oil-type gas. The isotopes of CO2, helium and vitrinite reflectance of the source rocks, i.e. coal and carbonaceous mudstone, indicate that the study area is lacking conditions for the formation of biogenic gas. The carbon isotopic reversal with δ13C1>δ13C2>δ13C3 in the Shixi gas field was interpreted as a result of secondary cracking under a local high temperature >200 °C.

Redox changes and mercury signature during the Lower Alum Shale Event (mid-Tournaisian, Mississippian) in the Rhenish Massif: Implications for oxygenation history and volcanism in southern Laurussian shelf and Palaeotethys Ocean

The mid-Tournaisian Event, also called the Lower Alum Shale Event (LASE), was a global anoxic event that occurred ca. 355 Ma ago. This event is connected to drastic facies changes from pelagic carbonate sedimentation to widespread black organic-rich siliceous shales and radiolarites in many parts of the world. For the first time, high-resolution inorganic geochemistry and framboidal pyrite analyses were applied to decipher depositional condition changes during the mid-Tournaisian anoxic event in the Rhenish Massif, then situated on southern shelf of the Laurussia continent. The lower and middle parts of the LASE interval in the study area were deposited under anoxic conditions at the bottom and water column, while the upper part was deposited under restricted but better‑oxygenated conditions. Additionally, a minor Hg anomaly was found during the LASE deposition in the Rhenish Massif, confirming volcanic overprint during the event. The scenario of palaeoenvironmental changes obtained in the Rhenish Massif is compared to other areas where the LASE was recorded and studied, providing a wider, panregional picture of changes in marine settings during that important Mississippian event. The results obtained in the present study show that the conditions during the LASE in pelagic settings were not uniform, with variable bottom-water oxygenation and different proximity and influence of volcanic activities. The numerous regional magmatic centers are considered as potentially responsible for the drastic depositional changes at a local scale and the bioproductivity increase at the global scale. However, their total contribution caused the climatic turnover responsible for the Gondwana deglaciation and resultant global transgression leading to development of anoxia in many parts of the world during this mid-Tournaisian anoxic event.

Sedimentological and Mineralogical Fingerprint of the Beach Sediments, Western Mediterranean Coast, Egypt

The composition and texture of beach sediments along continental coastlines vary considerably. Beach sands made completely of carbonate sediments are prevalent along Egypt's Western Mediterranean Coast. Forty-five sediment samples were collected from various locations (El-Dekhela, Kilo-21, Abu Talat, and Sidi Krir) along the Western Mediterranean Coast of Egypt. The sedimentological and mineralogical attributes of the collected sediments were investigated using collaborative techniques. The beach sediments in the coastal region between El-Dekhela and Sidi Krir consist mainly of carbonate sands that are mineralogically composed of dolomite, calcite, and aragonite with smaller amounts of quartz, sylvite, anhydrite, gypsum, and trace amounts of microcline. They contain ooids with bioclasts and very few heavy minerals, except for notable amounts of these minerals in the El-Dekhela region. These heavy minerals primarily consist of opaque minerals, amphiboles, and pyroxenes, with smaller amounts of epidote, zircon, tourmaline, rutile, garnet, kyanite, monazite, sphene, staurolite, and biotite. The combination of heavy minerals in these deposits indicates a wide variety of igneous, metamorphic, and sedimentary rock types as potential source rocks. Heavy mineral concentrations decrease westward from the Nile River's mouth near Rosetta to Egypt's Western Mediterranean Coast. Most of the studied ooids are white colored and usually have a pearly luster; these characteristics indicate that these sediments were formed in agitated water. The current study reveals that the ooids in the study area originated from a variety of sources, including autochthonous and allochthonous ooids that eroded and broke, implying that they may have originated from coastal plain carbonate ridges.