Ooid Formation Research Articles

Berthierine/Chamosite ooids and associated phases in Devonian deposits of the Parnaíba Basin, Brazil: Genesis and diagenetic evolution

The Parnaíba Basin Devonian deposits comprise ironstones and organic-rich sediments deposited in an epeiric sea margin under greenhouse conditions, enhanced weathering, reduction of oceanic circulation, and black shale deposition. This study aims to contribute to the understanding of the interrelation between depositional and shallow diagenetic processes of the Devonian units of the Parnaíba Basin, specifically regarding the genesis of iron minerals that constitute the ooidal ironstones of the Itaim and Pimenteiras formations. These units comprise bioturbated heterolites, ooidal ironstones, intraclastic conglomerates, and shales, deposited in a deltaic to neritic, storm-dominated platform. This research investigates drill cores from two wells in the southern part of the Parnaiba Basin, focusing on depositional and diagenetic processes. The proposed model suggests deposition of sediments originally rich in organic matter, under low siliciclastic input, and initially anoxic, transgressive conditions. Under these conditions, ferroan precursor clays were deposited, alternating with periods of reworking by storm currents, generating concentric ooids of berthierine. Bioturbation processes resulted in the concentration of previously formed berthierine ooids during intervals of low sedimentation rates. Eodiagenetic microcrystalline siderite formed along the redox boundary between oxic/bioturbated and anoxic zones. Chamosite formed by partial transformation of berthierine, upon incipient mesodiagenetic evolution of the ooidal ironstones. This study contributes to the understanding of depositional and diagenetic conditions in the Devonian section of the Parnaíba Basin, and of similar deposits, offering insights into the formation and development of berthierine ooids under the influence of sea level fluctuations, oxic/anoxic conditions, sedimentation rate, and bioturbation in sedimentary environments, providing key insights into the paleoenvironmental conditions along the northern Gondwana margin during this period.

Heterogeneous marine response during the Toarcian Oceanic Anoxic Event (TOAE): The potential role of storminess

The Toarcian Oceanic Anoxic Event (TOAE; ∼183 Ma) represents an important hyperthermal and deoxygenation event in the Early Jurassic. However, TOAE marine records are spatially heterogeneous with regard to nutrient levels, primary productivity, redox conditions and organic enrichment. This non-uniform response to global hyperwarming is not readily accounted for by local variations in paleogeography, climate, or water depth. Largely overlooked to date is the intensified storm activity that characterized the TOAE, and the role that this may have played in controlling marine responses to that event. A review of TOAE studies from multiple marine environments suggests that storm intensity covaried with paleoceanographic conditions, such as nutrient availability, primary productivity, redox conditions, and organic-rich sedimentation. At mid-paleolatitude sites, relatively weak storm activity during the TOAE induced short-term watermass oxygenation, and marine settings were mainly characterized by enhanced anoxia (even euxinia), water-column stratification, increased primary productivity (fueled by terrestrial runoff and P regeneration in euxinic settings), and organic-rich sedimentation. At low-paleolatitude sites, TOAE storm activity was relatively strong, and contributed to marine environments characterized by oxic to suboxic conditions, reduced water-column stratification, decreased primary productivity (possibly due to limited P regeneration and upwelling), low sedimentary organic content, and locally high oolite abundance. TOAE marine sites at all paleolatitudes exhibit: i) sea-level rise and enhanced continental weathering fluxes linked to an intensified hydrological cycle; ii) reduced dinoflagellate and increased cyanobacterial activity; and iii) low δ15N values (mainly −1‰ to +3‰) linked to enhanced diazotrophic nitrogen fixation. The spatial heterogeneity of the response of TOAE marine systems is difficult to reconcile with scenarios linking increased terrestrial flux to marine eutrophication, primary productivity increase and organic-rich sedimentation. Consequently, we hypothesize that the intensity of storm activity influenced TOAE marine systems, and that this factor can, at least partially, account for heterogeneous patterns of environmental changes at middle versus low paleolatitudes and open versus restricted marine settings. Importantly, increased storm activity can induce pycnocline deepening via vertical water-column mixing, thereby promoting: i) enhanced aerobic degradation of organic matter (low sediment organic matter content) due to a reduced oxygen-minimum zone; ii) less nutrient upwelling from deep waters into the photic zone (nutrient-depleted upper ocean), and iii) blooms of nitrogen-fixing cyanobacteria (low δ15N) and calcification (ooid formation). Thus, the interaction between storminess and pycnocline depth is a potentially important factor affecting marine environmental changes during TOAE. These findings have implications for modern oceans now experiencing climatic warming and intensified tropical storm activity.

The where, when, and how of ooid formation: What ooids tell us about ancient seawater chemistry

Ooids are a common constituent of carbonate rock throughout the last 3.2 billion years. The chemical and physical characteristics of ooids record essential details of the carbonate chemistry and hydrodynamic conditions of their depositional environment. As a result, ooids may be a powerful tool for reconstructing Earth's ancient climates and environments. However, lack of constraints on fundamental aspects of ooid formation, such as the role of abrasion, underscores the potential for additional discoveries utilizing ooids as paleoenvironmental proxies. Here, we use three-dimensional models built by serially grinding and imaging ooids to show how ooid shape changes during growth. We find that ooid abrasion plays only a small role in the formation of ooids, even in giant ooids, where the mass should cause abrasion to be most intense. A lack of abrasion during ooid formation suggests a greater sensitivity of ooid distribution and size to the chemistry of depositional environments. This insight enables us to harmonize our understanding of ooid formation with longstanding questions regarding their spatial and temporal distribution, as well as size variations. With a new database of ooid occurrences, we analyze the influence of seawater chemistry, carbonate shelf area, and evolutionary developments on ooid abundance in the Modern, Last Glacial Period, and throughout the Phanerozoic. Finally, we leverage our new understanding of ooid growth to show how ooid size, including giant ooids, can shed light on relative changes in alkalinity throughout the Phanerozoic.

Organic matter influence on ooid formation: New insights into classic examples (Great Salt Lake, USA; Triassic Germanic Basin, Germany)

ABSTRACTOoids are coated grains composed of a tangential or radial cortex growing around a nucleus. They are common in carbonate deposits of almost any geological age and provide insights into environmental conditions. However, abiotic or biotic factors influencing their formation remain unclear. This study aims to advance current understanding of ooid formation with a multi‐analytical approach (for example, field emission scanning electron microscopy, Raman spectroscopy and micro X‐ray fluorescence) to classic examples from Great Salt Lake, USA, and the Lower Triassic Germanic Buntsandstein Basin, Germany. Both of these deposits represent hypersaline shallow‐water environments where ooids are closely associated with microbial mats. Great Salt Lake ooids are dominantly 0.2 to 1.0 mm in size, ellipsoidal to subspherical in shape, composed of aragonite and contain organic matter. Germanic Buntsandstein Basin ooids are mainly ≤4 mm in size, spherical to subspherical in shape, composed of calcite and currently contain little organic matter. Despite the differences, both ooids have the same cortex structures, likely reflecting similar formation processes. Some Great Salt Lake ooids formed around detrital grains while others exhibit micritic particles in their nuclei. In Germanic Basin ooids, detrital nuclei are rare, despite the abundance of siliciclastic particles of various sizes in the host rocks. Germanic Basin deposits also include ‘compound ooids’, i.e. adjacent ooids that coalesced with one another during growth, suggesting static in situ development, which is supported by the lack of detrital grains as nuclei. Germanic Basin ooids also grew into laminated microbial crusts with identical microstructures, further indicating a static formation. Such microbial crusts typically form through mineral precipitation associated with organic matter (for example, extracellular polymeric substances), suggesting a similar formation pathway for ooids. The inferred key‐role of organic matter is further supported by features in radial ooids from the Great Salt Lake, which commonly exhibit, from their nuclei towards their surface, increasing organic matter contents and decreasing calcification.

Radiocarbon Chronology/Growth Rates of Ooids from Great Salt Lake, Utah

Ooids (calcium carbonate coated grains) are common in carbonate environments throughout geologic time, but the mechanism by which they form remains unclear. In particular, the rate of ooid growth remains elusive in all but a few modern marine environments. In order to investigate the rate of ooid growth in a non-marine setting, we used 14C to date ooids from Great Salt Lake, Utah, a well-known site of aragonitic ooids. Bulk ooids obtained from the northern shore of Antelope Island and the northeast shore of Great Salt Lake near Spiral Jetty were sieved into different size fractions and produced mean ages ranging between 2728±15 and 4373±20 14C yr BP. Larger ooids were older than smaller ooids, implying that larger ooids grew in the environment for a longer duration, with the caveat that bulk age dating integrates the growth history of an ooid. To better resolve growth history, ooids from the coarse fraction were sequentially dissolved, and 14C ages were obtained for each dissolution step to create a time series of ooid growth. The results of the sequential dating indicate that the coarse Great Salt Lake ooid growth began between 5800-6600 ± 60 14C yr BP while their outer cortices are nearly modern. Sequentially dated ooids from the South Arm of Great Salt Lake at Antelope Island record a nearly linear growth history (~ 10-15 µm/kyr), whereas ooids from Spiral Jetty record somewhat faster growth between ~6000 and 4000 years ago (0.03 – 0.06 µm/yr) followed by a 10x slower growth history for the remainder of their lifespan (0.003 – 0.008 µm/yr). The lifespan of Great Salt Lake aragonitic ooids is two to six times longer than those from modern marine environments, and thus provides a unique end member for understanding the mechanisms behind ooid formation. The ooid age range indicates that geochemical parameters measured from bulk ooid dissolution integrates over ~6000 years and thus does not represent a geochemical snapshot in time, as some previous studies have suggested.

Microborings reveal alternating agitation, resting and sleeping stages of modern marine ooids

ABSTRACTOoids are abundant carbonate grains throughout much of Earth's history, but their formation is not well understood. Here, an in‐depth study of microbial bioerosion features of Holocene ooids from the Schooner Cays ooid shoals (Great Bahama Bank, Eleuthera, Bahamas) and the Shalil al Ud ooid shoals in the Arabian/Persian Gulf (Abu Dhabi, United Arab Emirates) is presented. No obvious differences were found in ooid size distribution, cortex layer thickness, the composition of nuclei or euendolithic community when comparing ooids from both locations. Microendolithic borings are present in most studied ooid surfaces, but the intensity of (micro‐)bioerosion varies significantly. Applying an epoxy vacuum cast‐embedding technique allowed the identification of ichnotaxa and their inferred producers (various genera of diatoms, cyanobacteria, coccolithophores and unspecified bacteria). Euendolithic taxa have specific low‐light tolerances and light optima. This implies that information about the relative bathymetry (seafloor versus burial within an ooid shoal) and ecology for ooid cortex formation can be obtained via the presence or absence of their respective ichnotaxa. The history of a statistically significant number of ooid cortices can be translated into dune dynamics and the temporal variations thereof by allocating the inferred index producer to a defined burial or light penetration zone. In this context, ooid formation can be divided into four stages: (i) an agitation stage in the water column, characterized by the colonization of grains by photoautotrophs; (ii) a resting stage, characterized by temporary burial of the ooid, leading to immobilization and a shift towards heterotrophs; (iii) a sleeping stage, characterized by prolonged burial and colonization by organotrophs; and (iv) a reactivation stage, characterized by a resurfacing of the ooid and a subsequent shift towards photoautotrophs. The sleeping stage is presumably a stage of ooid degradation where bioerosion, mainly by heterotrophic fungi and bacteria is particularly active.

A tale of two Arabian Cenozoic ooids: Factors controlling the differential growth of ooids across the Arabian Plate

Ooids are a critical yet enigmatic carbonate grains in the geological record. Previous studies have focused their efforts on studying the mode of ooid formation throughout the Phanerozoic via experimental, numerical simulations, and field studies. To date, however, there is little discussion around factors governing the texture, mineralogy, structure, size, and geometry of Cenozoic ooids, particularly in Saudi Arabia. The present article aims to provide insights on key factors controlling the differential growth of Cenozoic ooids in Saudi Arabia by comparing two contrasting carbonate environments and tectonic settings. Here, we studied the Holocene (0.4–2 kya) ooids developed in the tidally dominated carbonate ramp of the Arabian Gulf under a passive margin setting and the Miocene (15–21 mya) ooids from the Red Sea that were developed in a tectonically active, wave-dominated fault-bounded carbonate platform. The Holocene ooids were characterized by aragonitic, heavily micritized and small (max:550 μm) ooids with bioclast-dominated nuclei. In contrast, the Miocene ooids were replaced by dolomite, larger in size (max:820 μm) and significantly less micritized with variable nuclei compositions, such as rock fragments and bioclasts. The variation in the intensity of bioerosion and micritization suggest the Miocene ooids experienced lesser resting stage time than the Holocene ooids developed under the tide regime due to active wave actions. In addition, the Miocene ooids were developed in a fault-bounded margin allowing a higher carbonate saturation state to concentrate in that zone while the Holocene ooids were developed under a widespread distribution of carbonate saturation state in a vast carbonate ramp setting. These findings indicate that original platform geometry and depositional system (tide-vs wave-dominated) are substantial in dictating the duration of ooid resting stage and distribution of carbonate saturation state across the platforms. All these combined, allows the Miocene ooids to grow larger and reaching its equilibrium size faster. This study highlights the importance of coupling platform architecture and other key environmental parameters (wave actions, sediment supply, and tectonic activities) in understanding the formation of Cenozoic ooids.

Tracking depositional and geochemical variations in the Cambrian North China Platform: Insights from sedimentology, geochemistry, and C-O isotopic records

The Cambrian carbonate sequence in North China is of significant global and regional scientific importance due to its continuous and thick exposure and the data on the major environmental changes and biological events of the early Phanerozoic, which can be gleaned from it. While the well-exposed Cambrian outcrops have been utilized to unravel the processes governing these events, few studies have focused on the outcrops' spatio-temporal depositional patterns and geochemical variations. Therefore, our study address, these issues by integrating fieldwork with petrographic and geochemical analyses. In the North China Platform (NCP), the Cambrian carbonate sequence is predominantly composed of six third-order depositional sequences (~370–400 m thick) that can be divided into nine microfacies, representing three primary microfacies associations, namely tidal flat, lagoon, and shoal. In contrast to the standard carbonate sequence stratigraphy model and many other examples of Cambrian platforms, the deposition of oolitic grainstones in the NCP are overlain by the deep-water calcareous mudstone and shale during a period of slow fall of sea-levels. Such a particular style shows a similar trend to the Schlager model of a type III sequence boundary that formed between a highstand systems tract (HST) and a transgressive system tract (TST). Scanning electron microscope (SEM) observations show an abundance of microbial fossils, a nanosphere, and remnants of extracellular polymeric substances (EPS) inside the dark micrite of normal regressive deposits (oolitic grainstone), suggesting the significant role of microbial activity in the formation of ooids. The δ18O isotopic values range from −4.15 to −11.75‰ VPDB, portraying pervasive diagenetic alterations with a variable degree of diagenesis at different localities, and both marine- and burial-related diagenesis. In contrast, the δ13C VPDB values have a very narrow range, from −2 to +2‰ VPDB, suggesting little to no influence of meteoric diagenesis. At the same time, the slight depletion of δ13C isotope values may reflect the role of microbial processes during the genesis of ooids. This study highlights the importance of microbial activity and marine-related diagenetic overprints during the formation of carbonate facies in the Cambrian Period and provides evidence that carbon isotopic excursions observed during different major environmental and biological events in the NCP are related to primary signals and not altered by intensive meteoric diagenesis.

Microbial Activity and Neomorphism Influence the Composition and Microfabric of Ooids From Great Salt Lake, UT

The sediment along the shorelines of Great Salt Lake (GSL), Utah is dominated by ooids, concentrically-coated carbonate sand grains. Two characteristics differentiate GSL ooids from typical modern marine ooids: well-developed radial aragonite microfabrics and the ubiquitous occurrence of a Mg-silicate phase. The radial microfabrics have formed the basis of conceptual models applied to understand the formation of radial fabrics in ancient ooids, but the formation of the Mg-silicates, and the relationship between Mg-silicates and radial aragonite crystals have received little attention. The occurrence of Mg-silicates in GSL ooids is surprising because GSL lake water pH is ~8.3, too low for Mg-silicate precipitation (requires pH>8.7). We use transmitted light microscopy, element mapping via wavelength-dispersive x-ray spectroscopy with electron microprobe, scanning electron microscopy, and synchrotron x-ray fluorescence (XRF) mapping and sulfur K-edge absorption spectroscopy to explore the spatial relationships between the mineral phases in GSL ooids. We observe large euhedral aragonite crystals penetrating Mg-silicate zones and cutting across laminar cortices, suggesting that the characteristic radial aragonitic fabrics in GSL ooids, traditionally interpreted as a primary structure, are enhanced, or in some cases entirely created via neomorphism. XRF maps reveal that Mg-silicate zones co-occur with elemental sulfur (S0), which we interpret as a metabolic intermediate of microbial sulfur cycling. This co-occurrence supports our hypothesis that microbial sulfur cycling plays a key role in the formation of GSL ooids by locally shifting pH beyond the threshold for Mg-silicate precipitation. This compositional fingerprint could serve as a biosignature in ancient lacustrine strata where Mg-silicates co-occur with carbonate minerals.

Tidal dynamics drive ooid formation in the Capricorn Channel since the Last Glacial Maximum

Relative sea-level changes can dramatically alter coastal geomorphology and coastlines, which, in turn, can fundamentally alter tidal regimes. The Great Barrier Reef (GBR) has undergone around 120 m of relative sea level (RSL) rise since the Last Glacial Maximum, ∼20,000 years ago (ka). Ooid grains (sand sized carbonate sediment) that formed in shallow water (>5 m depth) and under prolonged hydrodynamics forcing are now found at depth ranging between 100 and 120 m under the present day GBR Gulf of Capricorn. The apparent inconsistency between preferential conditions for their formation and the actual environment where they are found at present-day could be used to infer past regional hydrodynamic conditions. Here, we focus on the regional changes in the GBR tidal dynamics over the last 16.8 ka to show that sea-level rise on the GBR has caused significant changes on tidal patterns and dynamics. To do so, we used the first multi-scale palaeo-tidal finite element coastal tidal model of the GBR over five time slices (present day, ∼10 ka, ∼12 ka, ∼15 ka and 16.8 ka), representing the position of RSL at 0 m, 20 m, 45 m, 75 m and 96 m below present. We show that favourable conditions for ooid formation only existed for a short period of time between 16.8 and 11 ka. At that time, the Gulf of Capricorn was a wide shallow shelf with strong currents constantly agitating grains, providing rapid burial, exposure and re-burial cycles. We show that these conditions only existed for a short period of time and hence explain the presence of ooid grain formation in the GBR at that time. We propose ooids formed within the Capricorn Channel at a time of lower RSL than expected and then underwent sub-tidal transportation to their final deposition place via tidal currents, explaining the inconsistency with their age and the depth at which they were found.

Holocene Lacustrine Abiotic Aragonitic Ooids from the Western Qaidam Basin, Qinghai-Tibetan Plateau

Carbonate ooids are a significant component of shallow water carbonate deposits in the present and geologic past, yet their origin and formation mechanism have been the subject of continuing debate. This study focuses on the well-preserved Holocene aragonitic ooids collected from the west Qaidam Basin, Qinghai-Tibetan Plateau (QTP). The mineralogical and chemical compositions, and stable (δ13C and δ18O), and radiocarbon isotopes of the ooids were analyzed to investigate their formation and develop a depositional model. The ooids formed approximately 5377±61 cal BP, and their cortices were composed of microcrystalline aragonite, with most nuclei being quartz grains. Stable carbon and oxygen isotopes indicate that authigenic aragonite precipitation is driven by evaporation and associated degassing of CO2 under turbulence conditions in a shallow alkaline lakes. Furthermore, eletron microscopy showed no presence of microfossils in ooid cortices or other evidence of microbial activity. Therefore, we propose that aragonite precipitation during ooid formation is most likely induced abiotically by increasing alkalinity due to evapoconcentration of lake waters based on an absence of an efficient carbonate-inducing metabolic pathway. New observations and detailed analyses of aragonitic ooid samples in the Qaidam Basin provide an improved understanding of the origin and formation processes of carbonate ooid in modern environment and the geologic past.

Atypical ooid diversity in the Upper Cretaceous Yacoraite Formation, Argentina

AbstractThis study reports on ooid diversity from different lithotypes of the Yacoraite Formation (Salta Group basin) in the Central Andes of north‐west Argentina. The ooids display a variety of internal and external morphologies that may be deployed as proxies for seawater chemistry and hydrodynamic processes. A short review of nomenclature problems is first discussed, followed by presentation of a two‐fold quantitative and qualitative methodology. Our proposed classification addresses internal and external ooid characters in order to understand growth in response to various environmental processes at an individual particle level. This classification allows discrimination between a variety of morphologies, but also evaluation of the complexity of the processes involved in ooid formation as seen in the fossil record. This study evaluates whether the ooids present within the Yacoraite Formation share similarities with ooids formed in marine versus marine lagoon and/or lacustrine environments. A possible lacustrine interpretation finds its origin in the diverse assemblage of ooid morphotypes present, which exceeds the variations described for marine ooids. However, growth paths and occurrence of various compound morphologies point to intense marine recycling, suggesting little accommodation. Together with other sedimentological characteristics, for example, bi‐directional cross‐bedding, tidal shoals, hummocky cross‐stratification and exposure surfaces, these features suggest that marine processes had an impact on the sediments. Therefore, the ooid assemblage of the Cretaceous Yacoraite Formation was most likely formed in a shallow coastal lagoon in the framework of an epicontinental sea that at times experienced marine flooding events. A detailed evaluation of processes involved in oolite formation is needed in order to improve the stratigraphic and palaeoenvironmental understanding of ooid formation through time. This includes examining alternating constructive and destructive stages, early binding and cementation, reworking, recycling and averaging processes.

Quantitative evaluation of the roles of ocean chemistry and climate on ooid size across the Phanerozoic: Global versus local controls

AbstractOoid size in natural systems can be predicted based on the expected equilibrium between precipitation and abrasion rates. Therefore, ooid size distributions may enable quantitative inferences about past ocean and climate conditions from sedimentology that complement insights from geochemistry. A few previous attempts have been made to compile information on ooid sizes across the Phanerozoic, but no systematic ooid size dataset is available and equilibrium models of ooid growth have not been explored comprehensively. In this study, convolutional neural network‐based segmentation was used to automate ooid size measurement and provide a systematic sampling of ooid sizes spanning the Phanerozoic. This data set is coupled with Monte Carlo simulations to quantitatively explore the interplay between three testable physicochemical parameters (long‐term average carbonate saturation state, bed shear velocity and intermittency of transport) in determining equilibrium ooid size. The numerical model shows that a typical‐sized ooid (≤1000 µm) can grow under a wide range of parameter combinations whereas giant ooids (>2000 µm) can only form under a more restricted set of circumstances. Furthermore, the model predicts that the formation of giant ooids is common when conditions favourable for rapid CaCO3 precipitation are achieved. This finding is consistent with the naturally rare occurrences of giant ooids, which developed under exceptional environmental conditions and are primarily associated with intervals of bimineralic seas and elevated sea‐surface temperature (greenhouse or transitional climate). Whereas global controls could help explain secular trends in ooid size, significant spatial variability of ooid size in several time intervals further demonstrates the importance of local environmental parameters in the growth of ooids. Together, spatial and temporal variation in ooid size distributions can complement geochemical proxies in quantitatively reconstructing ancient tropical marine environments.

Distribution and Controlling Growth Factors of Ooids in Qinghai Lake, Northern Tibet Plateau, China

Ooids are coated carbonate grains, which exist in shallow water marine and lacustrine environments. There is an ongoing debate about whether the origin of ooids is inorganic or organic. Qinghai Lake is the largest inland lake in China, and ooids are seen on the lake shore. This paper focuses on whether environmental energy has an impact on the growth and size of ooids. Through hydrochemical analysis, thin section observation, and scanning electron microscope, the carbonate coats of beach sands from Qinghai Lake were studied. The research shows that the carbonate-coated grain content from the different shores of the lake present variations. The hydrodynamics and particularly the waves seem to control the distribution of carbonate coats in the lake shore, not the hydrochemical condition. In addition, the integrity and thickness of carbonate coats from the shores with a strong hydrodynamic force are high and thick, respectively. The carbonate coats are often observed on medium-grained sands, and the maximum carbonate-coated grain occurred under the strongest waves, indicating that ooids can be produced only when hydrodynamic force and particle size are well matched. Bacteria or extracellular polymeric substances are not observed within the ooid cortices by scanning electron microscopy. So, bacteria may not be a major factor in the formation and growth of ooids, but hydrodynamic forces appear to play a great role in carbonate grain coat distribution, integrity, thickness, and ooid grain size.

Giant ooids of microbial origin from the Zhangxia Formation (Cambrian Miaolingian Series) in North China

The role of microorganisms in the formation of giant ooids is one of the areas of long-term controversy in ooidal research, but it has not been confirmed conclusively. Abundant giant ooids developed in the Zhangxia Formation of the Cambrian Miaolingian Series in North China. Giant ooids in the study area were examined by using Polarized Light Microscopy and Field Emission Environmental Scanning Electron Microscopy. The nuclei of the ooids consist of micritic pellets or radial ooids with diameters less than 2 mm and are formed in a weak-agitating seawater environment. Their cortices are concentric, and are characterized by the alternations of the dark laminae of micritic calcite or Girvanella filaments and light laminae of microsparry calcite. In the environments of inter-bank sea with the alternating development of medium and low energy and chiefly weak-agitating conditions, giant ooids were formed under the joint action of Girvanella filamentous growth, biologically-induced calcification and/or biologically-influenced calcification and inorganic calcium carbonate precipitation. The microfossils of Girvanella are distributed in inner and outer cortices of giant ooids, especially dense in the latter. This distinctly indicates that microbes play a significant role in the formation of giant ooids, and also provides a vital example for discussing the microbial origin of giant ooids.

Post-depositional modification of carbonate ooids by sulfate-reducing bacteria: Evidence from the Lower–Middle Jurassic, Tethyan Himalayas of southern Tibet

The role of microbial vs. abiotic processes in ooid formation has been a controversial topic for over a century. Recent sedimentological, geochemical and biomarker data indicate that microbes make an important contribution to the construction, destruction and modification of ooids. Here, we present a detailed petrographic and chemical investigation of well-preserved red oolites from the Lower–Middle Jurassic Nieniexiongla Formation in the Tethyan Himalaya of southern Tibet. Petrographic studies and electron probe microanalysis indicate that the red coloration is due to the presence of hematite minerals. The hematites show euhedral and amorphous spheroidal morphology and residual elemental sulfur was detected within minerals. The δ34S values in the leachate (δ34SNaCl) and in carbonate associated sulfate (δ34SCAS) of red oolites are lower than gray oolites and micritic limestones. Moreover, δ34SNaCl and δ34SCAS values are lowest in samples with the highest content of hematitic ooids. Together, these observations suggest that the hematites are transformed from pyrite. The Fe2+ (and possibly SO42−) provided by pore-water combined with degradation of syn-depositional organic matter within ooids produced anoxic microenvironments that facilitated local pyrite precipitation. Some spheroidal pyrites were subsequently transformed to euhedral crystals through continuous growth of the constituent microcrystals. This study demonstrates that organomineralization mediated by sulfate-reducing bacteria can produce pyrite grains within carbonate ooids. The results emphasize the important role that microbial communities play in the deposition and modifications of ooids.

Ooid distribution and fabric in the Miaolingian of Xiaweidian Section, Beijing (North China Platform)

This study is an attempt to describe the distribution and fabric of the Cambrian ooids from the Xuzhuang, Zhangxia and Gushan formations at the Xiaweidian section. The oolitic banks occupy the upper parts of the 3rd order depositional sequences recognized for these formations. Petrographical techniques were applied to describe the sedimentary features of ooid grains. Different characteristics of ooids including size distribution, composition, morphology and the internal and external cortical architecture were taken into consideration. Radial-concentric, micritic, superficial, composite, pseudo-oids, neomorphosed and geopetal ooids are properly studied under the microscope. Different fabrics of ooids have been linked to their different depositional settings, and a variety of sub-environments has been established. The oolitic grain banks are composed mainly of calcite, with noteworthy presence of aragonite and dolomite. The two-fold role of microorganisms during and after the formation of ooids can be recognized under the microscope. The mechanism of ooids fabric modification has been elaborated in detail. Firstly, the dark laminae in several ooids most probably show the remains of filamentous cyanobacteria taking part in the construction of ooids. Secondly, they destroy the cortex through boring, which is then subsequently filled by aragonite. In order to apprehend the sedimentological features of the Miaolingian strata in the Xiaweidian section, this research highlights the distribution of oolites and their resultant fabric in response to relative sea-level variations. The Miaolingian ooids in the Xiaweidian section provide a good reference example of the depositional pattern of oolitic grain banks.

Cambrian ooids, their genesis and relationship to sea-level rise and fall: A case study of the Qingshuihe section, Inner Mongolia, China

ABSTRACT: TheCambrian strata at the northwestern margin of the North China Platform in InnerMongolia hold thick oolitic-grain bank deposits.Generally, the strata are dominated by calcareous mudstone of shelf facies in the lower part, micritic limestone consisting of deep to middle ramp facies in the middle part, and oolitic limestone encompassing shallow ramp to grain bank facies in the upper part of each formation. The shelf and deep ramp facies are the result of relative sea-level rise, while oolitic limestones developed in response to relative sea-level fall. Microscopically, the studied ooids are represented by radial crystal structures and concentric laminations with or without cores, single crystal or neomorphosed ooids, and highly bored ooids. The size andmorphology of the ooids indicate a two-fold mechanical influence of microbes; constructive in the Miaolingian and destructive in the Furongian ooids. Based on these observations, it can be inferred that microbes (predominantly composed of filamentous fossils of cyanobacteria) excreted extracellular polymeric substances (EPS) to develop multiple bacterial biofilms microbial mats. The subsequent decay of the EPS through sulfate reducing bacteria most likely caused precipitation around these ooids. The depositional style of ooids occupying the upper parts of the formations, and their possible genesis from microbes provide clue for regional correlation, as well as affirm biological control in the formation of ooids.

Implications of giant ooids for the carbonate chemistry of Early Triassic seawater

Abstract Lower Triassic limestones contain giant ooids (>2 mm) along with other precipitated carbonate textures more typical of Precambrian strata. These features appear to have resulted from changes in seawater chemistry associated with the end-Permian mass extinction, but quantifying the carbonate chemistry of Early Triassic seawater has remained challenging. To constrain seawater carbonate saturation state, dissolved inorganic carbon, alkalinity, and pH, we applied a physicochemical model of ooid formation constrained by new size data on Lower Triassic ooids from south China, finding that the Triassic giant ooids require a higher carbonate saturation state than typifies modern sites of ooid formation. Model calculations indicate that Early Triassic oceans were at least seven times supersaturated with respect to aragonite and calcite. When combined with independent constraints on atmospheric pCO2 and oceanic [Ca2+], these findings require that Early Triassic oceans had more than twice the modern levels of dissolved inorganic carbon and alkalinity and a pH near 7.6. Such conditions may have played a role in inhibiting the recovery of skeletal animals and algae during Early Triassic time.

Microbially Mediated Organomineralization in Paleozoic Carbonate Ooids

This article reports the first results of studies on microbially mediated organomineralization in carbonate ooids from the Paleozoic sections of the Timan–Northern Ural region, which were formed in different environments and had different original mineral composition (calcite, Mg-calcite, and dolomite). Using a scanning electron microscope, mineralized microbial biofilms preserved in various forms and the modified morphology of primary grains under the effect of organic acids were identified. The interrelation between microbes and organomineralization has been established, which was observed in the form of conserved nuclei of the amorphous phase of calcium carbonate on the surfaces of mineralized biofilms, including EPS relics in the composition of ooid crusts. Carbon and oxygen isotope data showed the difference in Paleozoic ooid formation from saline lagoons to open shallow sea. The Raman spectra revealed the state of structure order of carbonaceous materials (CM) ranging from amorphous to weakly ordered carbon in ooids, which enables the data on the isotope composition of ooid carbonates to be considered corresponding to their primary composition and the environmental conditions under which they were formed.