Depositional Record最新文献

The Dohat Faishakh sabkha in Qatar was among the first modern environments studied to understand low-temperature dolomite formation in association with gypsum and other evaporites. Since the 1960s, research conducted in this sabkha has significantly influenced geological models that remain widely used today, helping in the interpretation of sedimentary sequences that dominated certain periods of Earth's history. Here, we present results of an investigation of the dolomite occurring in this sabkha using techniques more advanced than those available during the initial pioneering studies. By integrating our new results with previously published data, we establish an ‘identity card’ for this sabkha dolomite and the environment it forms. The dolomite exhibits a rhombohedral morphology, contains 50.8 mol% Mg, and has an ordering degree of 0.25 (poorly ordered). Isotopic values are approximately: δ¹³C = 5.0‰, δ¹⁸O = 4.1‰ and δ²⁶Mg = −2.6‰ to −1.5‰ and Δ₄₇ = 0.611‰. Annual temperature data indicate an average of 32.2°C in the subsurface intervals with the highest dolomite content. The associated pore water has an Mg/Ca ratio of 156, a salinity roughly nine times that of sea water and a pH of 6.9. Sediment total organic carbon is ~2%. Microbial diversity in the dolomite-bearing layers is dominated by Euryarchaeota—an extremophilic, opportunistic and metabolically versatile archaeal phylum. Together, these data provide a reference for identifying sabkha-type dolomites in the geological record, calibrating paleoclimatic proxies and interpreting biomarker signals that may be recorded in ancient dolomites.

The Norian Dolomia Principale in the Southern Alps, northern Italy, is composed of fully dolomitised carbonate platforms, locally interrupted by intraplatform basins that are only partially or not affected by dolomitisation. We studied the intraplatform basinal limestone of the Calcare di Zorzino as well as the dolomitised deep-water Monte Zenone bioherm, which—with an area of ~8 km²—is considered to be one of the largest bioconstructions in the Southern Alps, is isolated within the basinal deposits and is detached from the Dolomia Principale platform. We combine field and petrographic observations with thin section microscopy, X-ray diffraction, cathodoluminescence and both conventional and clumped stable-isotope analyses (δ¹³C, δ¹⁸O, ∆₄₇) of several adjacent carbonate lithologies ranging from non- to fully dolomitised. Reordering models indicate that early diagenetic ∆₄₇ signatures are preserved in bioherm dolomite, whereas Calcare di Zorzino micrite may have experienced both recrystallisation and thermal resetting during the thermal history of the Norian carbonate sequence. Resulting clumped-isotope derived temperatures (T(∆₄₇)) and calculated paleofluid compositions (δ¹⁸O_Fluid) of well-preserved micrite, matrix and cement dolomite document that—in contrast to the Dolomia Principale platform—growth of the Monte Zenone bioherm on a tilted and drowned platform block was controlled by the syn-tectonic and fault-controlled ascent of fluids associated to the Late Triassic–Early Jurassic rifting phase. These deep and warm fluids fostered bioherm growth, with early diagenesis providing the hard substrate for colonisation of microbial communities as well as serpulids and molluscs, and the eventual dolomitisation of the entire bioherm body.

It is important to understand the long-term climate variability for better insight into the climate change scenario. The monsoon-fed alluvial rivers of western India provide potential archives for palaeohydrological change. Here we attempt to reconstruct the palaeohydrological variability from the terrace sediments along the Orsang River, a tributary of the Narmada River during the last 1500 years using multiple proxies. The results reveal three distinct depositional phases: I. >1358–1003 cal yr. BP, II. 1003–600 cal yr. BP and III. 600–191 cal yr. BP in the Orsang Basin. The depositional phases I and III correlate with significant global cold and arid climatic phases i.e. DACP and LIA respectively whereas phase II represents the MWP. The high discharge conditions in the Narmada River during the MWP resulted in back flooding and slackwater sedimentation in the Orsang River especially ~704 cal yr. BP as suggested by the Sr-Nd isotopic ratios and high AOM. A significant influx of terrestrial organic matter related to high erosion and runoff due to intense precipitation events prior to 1358, ~1003 and ~601 cal yr. BP in the Orsang River (tributary) marks the climatic transitions. The terrace deposits in the Orsang tributary have recorded the high-magnitude flood events in the Narmada River during wet periods (MWP), and in the tributaries during dry periods, (DACP and LIA). The study demonstrates that tributary terrace sequences are valuable archives for palaeoflood inferences and the use of multiple proxies helps in demarcating local versus regional hydrological events. The inferences underscore the sensitivity of monsoon-dominated fluvial systems to climatic excursions. The regional heterogeneity highlights that climatic shifts during the Late Holocene were not globally uniform. While periods of a stronger Indian Summer Monsoon matched those in Southeast Asia, they contrasted with trends in the Eastern Mediterranean and the Middle East.

The degradation of exopolymeric substances (EPS) by heterotrophic bacteria, concomitant release of calcium ions and precipitation of carbonates were studied in a temperate mountain lake, Lac d'Ilay, Jura France. Phytoplankton blooms in this lake produced large amounts of exopolymeric substances (EPS; 1.8–3.0 mg L⁻¹), probably inhibiting CaCO₃ precipitation by binding Ca²⁺ as shown by the saturation index of calcite and aragonite remaining well below 1. EPS settled to the sediments, where additional polymeric substances were produced by the benthic community. The total amount of EPS decreased downcore from ca 50 μg/g dry sediment near the surface to ca 1.5 μg/g dry sediment at the bottom (120 cm depth). A decrease in acidity, protein and sugar content, and calcium-binding capacity of EPS with depth coincided with active calcite precipitation. Aerobic and anaerobic EPS-degrading heterotrophic enrichments were obtained from the top, middle and bottom of the core. Doubling times of aerobic cultures from the top were six times shorter than those of cultures obtained from the bottom of the core, but anaerobic growth rates were similar across all enrichments. Aerobic turnover rates of organic compounds decreased by a factor of 4–5 from top to bottom; anaerobic rates were similar at all depths, except for the turnover of polymers, which was negligible at the surface compared to rates at the middle and bottom. All enrichments released calcium when grown on EPS. Growth on calcium-saturated EPS in anaerobic cultures obtained from the bottom of the core was the slowest, but still released 26% of the Ca in 20 days. This release during EPS degradation explained an increase in free calcium ions with depth reported in a previous study and may account for a large fraction of the carbonate mud. This suggests that sediments should be considered as an important source of biogenic carbonates.

Pennsylvanian time was characterised by widespread transgressive depositional systems that spanned non-marine to fully marine environments across the North American midcontinent. This study presents new palynological and Rock-Eval pyrolysis data from 113 samples from five cores in the northern Forest City Basin within a depositional-environment framework previously published which integrated a wide range of physical, chemical and biogenic attributes. A total of 153 palynomorph taxa were identified and we interpret depositional environments within a Palaeozoic hydrosere framework. Swamp forest assemblages are dominated by arborescent lycopods (~40%), sub-arborescent lycopods (~15%) and ferns (~25%), with gymnosperms and sphenopsids each composing <10% of the population. Kerogen analyses reveal abundant charcoal, and the ratio of elongated to detrital charcoal decreases upwards from the Kilbourn Formation to the Swede Hollow Formation, reflecting both spatial and temporal changes in wildfire occurrence and charcoal transport processes. Rock-Eval pyrolysis of 30 samples confirms dominantly terrestrial organic matter with Type III and Type IV kerogen. The prevalence of Type IV kerogen and abundant charcoal fragments point to frequent palaeo-wildfires. Stratigraphically, the Kilbourn Formation represents the wettest interval, whereas the overlying Kalo and Floris formations record increased representation of fluvial floodplain and upland assemblages, indicating drier climatic intervals. The Swede Hollow Formation marks a partial return to wetter conditions, coinciding with renewed marine influence recorded by the Oakley Shale. Collectively, these results reveal that the Early–Middle Pennsylvanian landscape of the Forest City Basin was a complex mosaic of swamp forest, floodplain and fluvial upland environments influenced by climatic fluctuations, base-level changes and periodic wildfire activity. The integration of palynological and geochemical data provides the first detailed reconstruction of ecosystem gradients and wildfire patterns across this midcontinent basin, highlighting its role as a key sediment transfer zone and ecological link between continental and marginal marine systems during Pennsylvanian time.

During recent sampling of the alkaline and hypersaline Lake Nuoertu in the Badain Jaran Desert, Inner Mongolia, PR China, various living microbial mat morphologies, along with associated lithified microbialites and stromatolitic tufa were discovered. Our fieldwork confirmed preliminary findings from the late 1990s that linked carbonate formations to subterranean freshwater discharge into the alkaline lakes of the Badain Jaran desert. This field report provides an overview of the microbialite morphologies, including phyto-microbialites, columnar and domical-linked as well as hemispherical roll-up structures, their redox stratification and water quality measurements at several lake sites and one groundwater spring. Environmental DNA data of differently pigmented mat layers reveal a well-defined vertical microbial zonation with cyanobacteria and aerobic heterotrophs dominating the top layer and deeper anaerobic layers characterised by anoxygenic purple sulphur bacteria and sulphate reducing bacteria, as well as sulphur bacteria and fermenters in strictly anoxic zones. This work serves as a preliminary report, highlighting the newly documented alkaline environment that hosts a key living microbialite community in China.

Microbially induced sedimentary structures (MISS) derive from the interaction between sediments and unicellular microorganisms and are of interest to the search for the earliest signs of life in ancient rocks, on Earth and on Mars. Here, we describe a type of MISS in the form of a polygonal structure characterised by a domed rim. Despite earlier suggestions of a biological origin, several aspects regarding their formation mechanism, the criteria to distinguish these structures from abiotic mud cracks, and their preservation potential in the geological record remain insufficiently constrained. To identify and define the distinctive features characterising this MISS, we have investigated the morphology and formation mechanism of polygonal microbial mats in two modern sabkha locations in Qatar. The sedimentological, mineralogical and geochemical analyses suggest that the studied polygons result from microbe-mineral interactions causing the stabilisation of detrital minerals (e.g. quartz, feldspar) and precipitation of micrite comprised of calcite and high-Mg calcite. The polygonal morphology is mainly the result of two co-occurring mechanisms: shrinkage, as a result of desiccation and microbial growth. A ∼1.5 cm-sized domed-rim has been identified as the key morphological feature that is exclusively present in and that allows for the recognition of, the polygons that form in association with a growing microbial mat. In cross-section, a domed-rim is comprised of millimetre-thick laminae showing angular relationships (e.g. overgrowth of older lamina sets) that, we argue, cannot be formed in the absence of growing biomass. Finally, through the study of a radiocarbon-dated ancient sabkha outcrop, it is shown that domed-rim microbial polygons can survive degradation and diagenesis, producing a mineral fossil structure that can potentially be preserved for billions of years.

Anhydrite is a common type of cement occurring in the tight (low permeability) sandstone reservoirs in continental lake basins. The precipitation of anhydrite cement is governed by fluid activity during diagenesis and its precipitation controls the pore evolution in tight sandstones. It is important to understand the formation mechanism of anhydrite cement in order to forecast the quality of tight sandstone reservoirs ahead of drilling. Jurassic Shishugou Group lacustrine sandstones of the Fukang Sag of Junggar Basin, NW China, are important low-permeability oil reservoirs. In the Shishugou Group sandstones, abundant anhydrite cement occurring mainly along a deep fault zone is associated with significant oil and gas shows. The electronic probe and laser ablation multi-collector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS) were applied to investigate the formation time, precipitation conditions, material sources and precipitation process of anhydrite cement and provide new insights for fluid activity, diagenetic system and the formation of pores in the tight sandstone. Anhydrite cement is mainly formed by the dissolution and reprecipitation of early diagenetic calcite cements, feldspars and volcanic rock fragments at the mesodiagenetic stage. The dissolution of calcites and detrital grains (feldspars and volcanic rock fragments) provided sufficient Ca²⁺ for both pore-filling and grain-replacive anhydrite. Organic acids can provide abundant SO₄²⁻ from the Permian petroleum source rock and transport it to the Shishugou Group sandstone through the deep-penetrating faults. The sufficient SO₄²⁻ in an acidic environment is favourable for the precipitation of anhydrite at the mesodiagenetic stage. The precipitating process of the anhydrite cement in the Shishugou Group sandstone involved obvious mass transfer and dissolution in an open diagenetic system due to the hydrocarbon charging through deep faults.

This study examines the climatic controls on dolomite precipitation through a multiproxy investigation of a carbonate-rich sediment core from Salinas Lake, a hypersaline playa in Alicante, south-eastern Iberia. The ~120,000 year record captures depositional cycles and palaeoenvironmental changes driven by late Pleistocene to Holocene climate variability. Integrated analyses of sedimentology, lithology, geochemistry (elemental concentrations, total organic carbon, stable carbon and oxygen isotopes), scanning electron microscopy, microbial community characterisation and palynology reconstruct lake hydrology and its influence on carbonate mineralogy. The sediment succession is marked by alternating calcite- and dolomite-rich intervals, with dolomite crystals displaying morphological evolution from spherical to rhombohedral forms with depth. Stable isotope signatures (δ¹³C: −6.5‰ to −2.4‰ VPDB; δ¹⁸O: −2.3‰ to +4.9‰ VPDB), alongside microbial structures such as extracellular polymeric substances (EPS) and internal crystal voids, suggest a biologically mediated precipitation mechanism. These mineralogical shifts closely correspond to rapid hydrological changes driven by Dansgaard–Oeschger climate oscillations, with dolomite formation favoured under arid, evaporative conditions that concentrate Mg and Ca ions and promote microbial mat development. Halophilic microbial communities, capable of catalysing carbonate precipitation, probably enhance dolomite nucleation and growth through EPS production and geochemical modulation. This work underscores the complex interplay between climate, hydrology, microbial activity and sedimentary mineral formation, providing new insights into the longstanding ‘dolomite problem’ within sedimentary environments.

Sediment is an important facet of sand cay reefs as it is responsible for reef accretion and island formation, with shifts in the proportions of sediment producers being proxies for ecological shifts. However, manual sediment analyses require experts to identify thousands of sand grains by hand before beginning data analysis. To accelerate the process, we developed the Sediment Analysis Neural-network Data-engine (SAND-e) to estimate the proportions of sediment producers, based on segmentation and classification of carbonate sand grains from microscope camera imagery. Sediment from Darvel Bay was used for training due to the variability of sand cay reefs available in that area. SAND-e segmented 1686 images into 32 883 grains within 3.5 h. The grains were then fed through SAND-e's classifier ensemble, containing four classifiers that voted to classify the grains into one of five classes (calcareous algae, coral, foraminifera, molluscs and ‘other’) in 1 hour. Both SAND-e and 11 humans annotated grains from the same dataset to ensure that SAND-e's accuracy was within the already accepted error rate deriving from multiple human annotators.