Abstract Microbialites are valuable archives of environmental change, but their interpretation is often complicated by differential preservation and post‐depositional modification. Fossil Late Pleistocene microbialites preserved along former shorelines of palaeo‐Lake Maquinchao (northwestern Patagonia, Argentina) provide an opportunity to investigate the relationships between microbialite fabrics, preservation and erosion in a lacustrine setting. Petrographic observations were combined with cathodoluminescence, μ‐XRF mapping, scanning electron microscopy, stable isotope analyses, X‐ray diffraction and radiocarbon dating to characterise the internal structure and composition of these fossil microbialites. Two dominant microfabrics were identified: (i) porous micritic shrub fabrics containing filament moulds and microbial‐like structures, and (ii) denser laminated spar–micrite fabrics. These fabrics show a systematic spatial distribution within the build‐ups, with shrub fabrics occurring preferentially in interior portions and laminated fabrics concentrated along outer and upper surfaces. Mineralogical analyses indicate that both fabrics are composed predominantly of low‐Mg calcite, whereas radiocarbon ages constrain carbonate formation between approximately 21 and 14 ka BP. Field observations and microfabric analyses indicate that preservation is strongly influenced by porosity distribution. Dense laminated fabrics form resistant outer layers that enhance preservation, whereas porous shrub fabrics are more susceptible to weathering and erosion. The present morphology of the carbonate build‐ups therefore reflects both primary accretionary processes and subsequent differential preservation. These results demonstrate that differential preservation exerts a major control on the morphology and internal architecture of fossil microbialites and highlight the importance of preservation biases when interpreting lacustrine microbial carbonate records.
Abstract Relative sea‐level change, climate variability and sediment supply exert a first‐order control on the architecture of mixed carbonate–siliciclastic shelves, yet their combined expression in fine‐grained successions with low facies diversity remains difficult to resolve. The Tithonian–Berriasian succession of central Tunisia provides a natural laboratory to address three questions: how many third‐order sequences can be distinguished in a clay‐rich mixed system; how reliably sequence boundaries and maximum flooding surfaces can be located using an integrated proxy set; and how these surfaces relate to regional tectonics and long‐term eustatic trends. Five sections along a ~93 km shelf‐to‐basin transect were logged at decimetre scale and analysed using facies and facies‐association analysis, systematic documentation of discontinuity surfaces, magnetic susceptibility measured selectively in marl–clay intervals, bulk CaCO3 content and XRF‐derived elemental ratios that track detrital input and carbonate productivity. These new data are combined with published palynofacies, clay mineralogy and macrofaunal information. The resulting framework resolves five Tithonian and eight Berriasian third‐order sequences, each constrained by consistent shifts in facies stacking, the distribution and character of condensed surfaces, firmgrounds and hardgrounds, systematic changes in susceptibility and Ti‐, Zr‐ and Ca‐based geochemical ratios and associated changes in biotic assemblages. Correlation across the transect reveals an overall regressive trend overprinted by higher‐frequency transgressive–regressive cycles, with local departures linked to differential subsidence along the North–South structural axis. The results show that integrated sedimentological, geophysical and geochemical methods can recover high‐resolution sequence architecture from monotonous mixed successions, provide a reference shelf model for the southern Tethyan margin during the Tithonian–Berriasian and offer a transferable template for inter‐basinal correlation of sea‐level and climate signals in comparable carbonate–siliciclastic systems.
Abstract Stromatoporoid sponge fossils were major diverse reef‐builders in the Palaeozoic Era; their taxonomic identification relies on thin sections examined under transmitted light microscopy, where vertical and transverse skeletal elements reveal diagnostic architectural features that vary with taxa. These elements typically appear darker than the cement‐filled internal spaces, allowing stromatoporoid taxa to be distinguished. However, stromatoporoid architecture is variable so that judgement of identification is not always unequivocal. Therefore, this study investigates the application of artificial intelligence (AI) to automate stromatoporoid identification, introducing a novel approach to streamline and standardise palaeontological taxonomy. For the first time, both vertical and transverse sections have been simultaneously analysed and integrated into an automated framework. High‐resolution images of thin sections from four well‐established Silurian genera, collected from the West Midlands and Shropshire counties, UK, were used to train supervised machine learning models. The images, captured using plane‐polarised transmitted light microscopy on thin sections, were digitally enhanced to increase contrast and eliminate background noise, ensuring that only skeletal features were used to inform the models. Despite variations in fossil preservation, section orientation and image quality, the AI models achieved classification accuracies of up to 96%. This demonstrates that stromatoporoid skeletal architecture is highly amenable to automated analysis, even under suboptimal conditions. The results represent a significant step forward in the application of AI to palaeontology, reducing reliance on manual identification and accelerating the classification process. Ultimately, this approach provides a new quantitative framework for evaluating stromatoporoid skeletal architecture, strengthening the scientific basis of their taxonomic interpretation and opening new avenues for analysing morphological diversity in hypercalcified sponges.
Abstract Fluctuations in the abundance of low‐temperature (T) dolomite have occurred throughout geologic time and have been linked to changes in hydrothermal activity, sea level and burial of organic matter suggesting ties to the global biogeochemical cycle of carbon. However, low‐T dolomite remains an enigmatic mineral due to poorly understood controls on its formation. Recent studies have suggested that both biotic and abiotic nucleation surfaces, such as extracellular polymeric substances (EPS) and clays, play an important role in formation. Despite both abiotic and biotic factors having been demonstrated to trigger the formation of low‐T dolomite, comparative studies are lacking. Moreover, the exact mechanisms occurring in the presence of EPS, clays and silica need to be clarified. Thus, this study focused on isolating and characterising EPS from hypersaline sabkhas, a known locale for primary dolomite, as well as clay minerals, and using these materials as reactive surfaces to induce precipitation. Within this study, carbonaceous precipitates formed and were subsequently subjected to high‐resolution analysis including Raman spectroscopy and Near Edge X‐ray Absorption Fine Structure (NEXAFS) to characterise their structure and interaction with specific nucleation sites. The data collected during these experiments has shown that carbonates preferentially nucleate on EPS and most likely on carboxylic functional groups. Additionally, the data show that EPS and clays by themselves may be inadequate to facilitate precipitation. Other factors such as solution chemistry, dissolved Si concentration, as well as organic material and the associated functional groups play a critical role in low‐T dolomite formation. It was also observed that too much organic material could play an inhibitory role. The confirmation of preferential nucleation of carbonates on carboxylic groups lends credence to the currently proposed organomineralisation model of low‐T dolomite precipitation. Furthermore, the data highlight the critical importance of environmental dynamics and their interpretation in the global biogeochemical cycle of carbon.
Abstract The Eocene–Oligocene boundary was a period of major global climatic and tectonic change, yet its expression in the Zagros Foreland Basin remains controversial due to complex facies relationships and the overprint of the Pyrenean orogenic phase. This study aims to resolve this stratigraphic enigma across three key sections using an integrated multi‐proxy approach combining biostratigraphy, geochemistry and mineralogy from 670 thin sections and 30 geochemical samples. The multi‐proxy approach provides independent, cross‐validated lines of evidence for interpreting depositional environments and sequence boundaries, moving beyond correlation based on lithostratigraphy alone. Results delineate two distinct boundary expressions. In the interior Fars, an erosional unconformity separates the Lutetian Orbitolites complanatus‐Coskinolina roberti‐Rhapydionina liburnica zone of the Jahrum Formation from the Rupelian Nummulites vascus‐N. fichteli zone of the Asmari Formation. In coastal Fars, a continuous transition is marked by the Late Eocene Turborotalia cerroazulensis‐Hantkenina zone of the Pabdeh Formation grading into the Rupelian Asmari. Geochemical data validate these models: the Pabdeh Formation shows high CaO and Sr. with low Al2O3, indicating a pelagic setting, while the Jahrum Formation has higher terrigenous proxies. SEM‐EDS confirms syngenetic dolomitisation in the interior platform and pyrite framboids in the basinal Pabdeh, indicating anoxia. A Monte Carlo sensitivity analysis confirms 97.3% confidence for the unconformity and 89.4% for the conformable interpretation. This work quantitatively demonstrates that a paleobathymetric gradient was the dominant control on sequence architecture, effectively neutralising the Pyrenean tectonic signal in deeper basinal settings. This framework provides a predictive model for stratigraphic correlation in foreland basins globally.
Abstract Submarine fan deposits are important archives of Earth's history and serve as the record of turbidity current events that transfer large amounts of terrestrial sediment and carbon into the ocean. The geomorphology, depositional processes and facies architecture change significantly along the submarine‐fan depositional profile, particularly at the channel–lobe transition zone (CLTZ). However, few studies have documented detailed facies architecture and associated sedimentary structures in the CLTZ, which are important for paleoenvironmental interpretations and morphometric information essential to reconstructing sediment transport dynamics and reservoir connectivity in ancient successions. Excellent coastal‐cliff outcrops from ancient CLTZ deposits of the Upper Cretaceous Point Loma Formation in San Diego, California constrain the facies architecture of the CLTZ and the three‐dimensional geometry of supercritical‐flow bedforms. We update previous work on sand‐rich and mud‐rich lobe complexes at Sunset Cliffs and present the first description of the uppermost sand‐rich CLTZ complex that contains antidune deposits arranged in predominantly metre‐thick tabular packages. Paleocurrent data suggest a compensatory evolution between elements and complexes that was probably driven by updip avulsions. The data generated by this study help to characterise the detailed facies architecture and bedform geometry for CLTZ deposits, which enables a better understanding of the sediment transport dynamics of the channel–lobe transition zone. These data, when combined with other ancient and modern analogues, can be used to predict reservoir connectivity and heterogeneity in CLTZ deposits that form hydrocarbon reservoirs and potential carbon storage sites.
Abstract The processes governing dolomite [CaMg(CO3)2] formation remain among the most debated topics in sedimentary geology. Although primary dolomite can precipitate at low temperatures in certain modern environments, its scarcity today contrasts sharply with its abundance in ancient rocks—a discrepancy known as the ‘dolomite problem’. Dolomite typically forms through two pathways: (1) primary precipitation during early diagenesis, often influenced by microbial activity and organic matter and (2) secondary replacement of preexisting carbonates during burial at higher temperatures. In this study, we investigate Mg isotope fractionation in a modern sabkha in southern Qatar to evaluate its potential as a tracer of dolomite formation processes. We analysed δ26Mg and δ44Ca in surface‐ and pore waters, authigenic clays and organic‐ and leached dolomite‐containing fractions. Ca isotopes reveal an ~1‰ fractionation between pore water–organic matter and dolomite, consistent with a two‐step, biologically mediated formation pathway. Contrary, only minor 26Mg enrichment in the organic fraction relative to pore water suggests that Mg isotopes alone provide limited evidence for such microbial mediation. Dolomite δ26Mg values (~−2.15‰) align with predictions for temperature‐dependent inorganic precipitation. Overall, the results indicate that microbial activity probably influences dolomite formation indirectly by altering local water chemistry rather than having a distinct Mg isotopic fractionation. These findings refine the application of Mg isotopes as proxies for dolomite genesis and offer new insights into carbonate diagenesis in saline environments.
Abstract The Permian–Triassic transition is characterised by major environmental changes and the largest known mass extinction event in the Phanerozoic. However, successions with a relatively complete sedimentological and palaeontological record across the Permian–Triassic are limited to a few well‐known sections. The Antalya and Aladağ Nappes in south‐western Türkiye provide extensive outcrops of the Permian–Triassic transition. Some key investigations have revealed the importance and potential for these successions to improve our understanding of the events across the Permian–Triassic. Here, we incorporate 3D open‐access virtual outcrop models with a virtual field guide to introduce the Permian–Triassic transition in south‐western Türkiye, to improve the accessibility, reproducibility and sustainability of fieldwork findings. The fossiliferous Upper Permian to Lower Triassic successions in the studied locations reach over a kilometre thickness. Due to the contemporaneous opening of the Neotethys Ocean, the exposures from the Antalya Nappe (Çürük Dağ in Kemer; Öznurtepe in Gazipaşa; and Demirtaş in Alanya) are considered to be deposited on a southward facing carbonate platform in the Neotethys Ocean, whilst successions from the Aladağ Nappe (Taşkent in Konya) are considered to be deposited on the northern side of the carbonate platform towards the Palaeotethys Ocean. In all sections, the Changhsingian (uppermost Permian) is represented by highly fossiliferous platform carbonates. The Changhsingian successions terminate with a thin oolitic grainstone (‘transitional oolites’), which is identified as the Permian–Triassic mass extinction interval and is characterised by a negative carbon isotope excursion. The transitional oolites are overlain by microbialite‐dominated carbonates, and then oolite‐dominated carbonates deposited in the Griesbachian. This carbonate‐rich deposition was replaced with a mixed carbonate‐siliciclastic succession later in the Early Triassic (Dienerian–Spathian), when marine ecosystems slowly recovered. These environmental and biotic changes are similar to the known record from the tropical palaeolatitudes in the western to eastern Palaeotethys.
Abstract The Permian succession of the Paraná Basin records the progressive disconnection from the Panthalassic Ocean that bathed the southwestern Gondwana Supercontinent from the Ordovician to the early Permian. The development of the Gondwanides Belt in the southwestern part of the continent acted as an orographic barrier, restricting marine connections and trapping marine waters in a megalake. In this study, we present a detailed analysis of the transition from marine to continental environments in the Serra Alta and Corumbataí formations, documenting high‐resolution stratigraphic sequences with significant hydrological and salinity changes. Our data illustrate how sedimentation and accommodation dynamics shaped the evolution of this continental‐scale lake system, which was greatly influenced by meteorological phenomena, including storms and seiches. The megalake experienced transitions between overfilled, brackish to freshwater balanced‐fill and saline underfilled stages, associated with distinct fourth‐order transgressive and regressive stratigraphic sequences. These changes in water balance and salinity fostered the development of a unique, endemic bivalve‐dominated fauna derived from marine ancestors, highlighting the basin's response to changing environmental conditions. Notably, this study identifies tectonic events and climate shifts as primary allogenic forces controlling deposition. Meanwhile, local sediment dynamics and episodic events such as storms and seiches originated key autogenic changes in the resulting stacking patterns. Such meteorological phenomena generated an intriguing heterolithic pattern in the fine‐grained lake deposits, which would otherwise be confused with astronomical tides. Our results provide insights into the understanding of sedimentological processes in large lacustrine systems, with implications for paleoclimatic and paleoenvironmental reconstructions in megalakes from the geological record.
Abstract Early marine diagenesis can alter the δ13C values of carbonate sediments in the marine environment, hindering interpretations of changes in global carbon cycling through geological time. In this study, the influence of sediment accumulation rate on the localisation and intensity of diagenetic alteration in the marine burial environment is documented. New measurements of δ18O and δ13C values of bulk carbonate sediments as well as siliciclastic and total organic carbon content were conducted on Holocene–Pleistocene aged sediments collected by Ocean Drilling Program Leg 133 on a proximal–distal transect on the slope adjacent to the Great Barrier Reef. Significantly lower bulk carbonate δ13C values and total organic carbon content were found to occur during periods of reduced sediment accumulation, but sediments in these intervals lack obvious sedimentological evidence of alteration, like the development of hardgrounds. New findings suggest that changes in sediment accumulation rate may explain large, asynchronous changes towards lower stable carbon isotope values of marine carbonates deposited in oxygenated slope settings in the geological record. Based on these results, periods of reduced sediment accumulation rate are proposed to enhance diagenetic reactions by providing continued access to oxidants. Alteration during periods of low sediment accumulation rate is likely to be exacerbated in settings containing mixed siliciclastic‐carbonate sediments. Finally, while this new observation indicates that subtle early marine burial diagenesis can obscure records of the global carbon cycle preserved in periplatform sediments, new insights may prove useful in constraining the dynamic nature of sedimentation patterns in slope environments through Earth history.
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: delta C-13 = 5.0 parts per thousand, delta O-18 = 4.1 parts per thousand and delta Mg-26 = -2.6 parts per thousand to -1.5 parts per thousand and Delta(47) = 0.611 parts per thousand. Annual temperature data indicate an average of 32.2 degrees 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 similar to 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 similar to 8 km(2)-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 (delta C-13, delta O-18, triangle(47)) of several adjacent carbonate lithologies ranging from non- to fully dolomitised. Reordering models indicate that early diagenetic triangle(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(triangle(47))) and calculated paleofluid compositions (delta O-18(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 similar to 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, similar to 1003 and similar to 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 mu g/g dry sediment near the surface to ca 1.5 mu g/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 (similar to 40%), sub-arborescent lycopods (similar to 15%) and ferns (similar to 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.
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 similar to 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.
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.
This study investigates the formation mechanisms of anthropogenic tufa deposits resulting from the chemical breakdown of legacy paper mill sludge (PMS) at the former Dalmore Paper Mill site in Auchendinny, Scotland. Tufa, a form of calcium carbonate (CaCO 3 ), typically forms in natural freshwater environments; however, this research explores its precipitation through interactions between alkaline industrial waste (PMS) and atmospheric CO 2 . Field and laboratory analyses were carried out to assess the geochemical characteristics of the stream water, as well as the mineralogical and isotopic composition of the associated tufa deposits. X‐ray diffraction (XRD) analysis revealed that PMS is composed predominantly of calcite, while tufa samples also contain minor amounts of quartz and kaolinite. Stream water measurements indicated alkaline conditions (pH 8.27–8.98) and elevated calcium concentrations, with calcite saturation indices suggesting conditions favourable for carbonate precipitation. Stable isotope analysis of tufa deposits (δ 13 C −24.62‰ to −13.74‰; δ 18 O −17.50‰ to −7.66‰) revealed a dominant contribution from atmospheric CO 2 , confirming a precipitation mechanism driven by CO 2 ingassing and hydroxylation reactions. The results support a model where rainwater infiltrates PMS heaps, leaching calcium into stream waters. As these calcium‐rich waters mix with atmospheric CO 2 , supersaturation occurs, leading to rapid calcite precipitation. The resulting tufa deposits exhibit laminated structures and high porosity, indicating episodic deposition under variable geochemical conditions. This study provides the first detailed evidence of tufa formation from PMS and suggests that such industrial waste materials can act as carbon sinks, capturing and mineralising atmospheric CO 2 . These findings expand the understanding of anthropogenic carbonate systems and highlight the potential of PMS as a low‐cost material for environmental remediation and carbon sequestration, supporting sustainable waste management strategies and contributing to climate mitigation goals.
