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Miocene carbonate successions found on many isolated oceanic islands throughout south‐east Asia commonly include unconformity capped 'island dolostones' units. On Shidao, located in the Xisha Islands in the South China Sea, well XK‐1 (1268.2 m deep) penetrated a thick succession of carbonates before being terminated in the Jurassic metamorphic basement. The lower part of the succession is composed of the Sanya Formation (Early Miocene) and the overlying Meishan Formation (Middle Miocene), ca 700 m thick, that are formed of limestones and dolostones. An unconformity divides the Sanya Formation into a Lower Member formed largely of limestones, and an Upper Member formed of dolostones (DI‐8). DI‐8 is separated by a major unconformity into lower (DI‐8L) and upper (DI‐8U) units, that both include numerous subaerial unconformities. The overlying Meishan Formation is a limestone succession that includes three unconformity‐capped dolostones intervals (DI‐5, DI‐6 and DI‐7). Strata below the numerous exposure surfaces throughout the Sanya and Meishan formations are highlighted by Fe‐staining, biomoulds, cavities, caves and solution breccia development. The weakly fabric‐retentive dolostones in DI‐5 to DI‐8 are formed of dolomite crystals that have a dirty core encased by a clear rim, with δ18O values of +0.3 to +4.7‰ (average +2.0‰, n = 50), δ13C values of +2.2 to +3.2‰ (average +2.5‰, n = 50), Fe concentrations of 44 to 442 ppm (average 149 ppm, n = 50), Mn concentrations of 5 to 197 ppm (average 27 ppm, n = 50), Sr concentrations of 169 to 257 ppm (average 218 ppm, n = 50) and 87Sr/86Sr values of 0.70847 to 0.70900 (average 0.70879, n = 32). In all cases, dolomitization post‐dated the subaerial exposure. The 87Sr/86Sr values suggest that the dolostones in DI‐8L formed during the late Early Miocene, whereas the dolostones in DI‐8U, DI‐7, DI‐6 and DI‐5 probably formed simultaneously during the Late Miocene even though these units are stratigraphically hundreds of metres apart and were located well below sea‐level. Dolomitization was preferentially focused in those strata with high porosities and permeabilities that allowed easy circulation of the seawater through the rock. This model suggests that dolomitization was not restricted to strata that were at or close to sea‐level. Although dolomitization may have been related to the new oceanographic circulation regime established during that time, the exact properties of the seawater that promoted dolomitization are open to debate. This model of dolomitization may be applicable to many of the 'island dolostones' that are found in the oceans throughout the world. [ABSTRACT FROM AUTHOR]

The formation of ordered dolomite under Earth's surface conditions remains a challenge, although many researchers have now shown that microbes can facilitate the precipitation of this common mineral. This study provides the first evidence of dolomite biomineralization by the green alga Spirogyra (Zygnematales, Chlorophyta), a globally dispersed lacustrine genus. Microbial mats formed by Spirogyra occur in the hyperalkaline and ephemeral lake Caballo Alba, located in Central Spain and characterized by water solutions with high Mg/Ca ratios. Microscopic, geochemical, mineralogical and cytochemical studies have shown that accumulations of carbonates occur within different cells and locations of the alga. Biominerals mostly consist of single crystals of magnesian calcite and dolomite with variable proportions of Ca2+ and Mg2+. High‐resolution transmission electron microscopy and selected area electron diffraction patterns of the carbonates confirm the pervasive presence of super‐lattice reflections typical of ordered dolomite in compositions with greater than 32 mol% MgCO3. The curved surfaces of some carbonate crystals and their attachment to the organic surfaces represent imprints of a confined growth in compartments that act as a template for crystal nucleation and shape the crystals. Dyes reveal chemical differences in these compartments which explain the polymineral biomineralization and suggest that the alga do not strictly control the precipitation process. Zygnematales algae related with Spirogyra date back possibly to the Early Palaeozoic, thus suggesting that dolomite biomineralization could have been a common process through the Phanerozoic. This discovery provides new insight into the biotic formation of orderly dolomite and aids in the reconstructions of past lacustrine environments of dolomite deposition. [ABSTRACT FROM AUTHOR]

The biogeochemical cycles of carbon and silicon are tightly coupled in modern marine environments due to the pivotal role of planktonic diatoms. Once diatoms are sedimented, the fate of organic matter and biogenic silica is initially governed by bottom water oxygen levels and bacterial communities involved in remineralization processes. The early diagenesis of biosiliceous sediments may result in drastic changes in composition, sometimes hampering palaeoenvironmental reconstruction. Sedimentary successions deposited in the Mediterranean region during the Messinian salinity crisis (5.97–5.33 Ma) allow to explore the early diagenetic transformation of organic matter and biogenic silica in a restricted basin experiencing a severe palaeoceanographic turnover. Sedimentological and petrographic observations coupled to elemental, mineralogical, inorganic and organic geochemical analyses were carried out on biosiliceous (diatomaceous shales and diatom‐bearing mudstones) and associated clay‐rich and dolomite‐rich sediments (dolomitic mudstones) interbedded with primary gypsum layers in the Piedmont Basin (north‐west Italy). The state of preservation of biogenic silica was governed by different pathways of organic matter remineralization, depending on bottom and pore water redox conditions, which were controlled by the structure of the water column. Aerobic respiration of organic matter in a mixed water column and an oxygenated seafloor promoted biogenic silica preservation. In contrast, bottom water anoxia induced by permanent stratification of the water column favoured the concomitant formation of dolomite and authigenic clays. In particular, organic matter degradation through bacterial sulphate reduction increased pore water alkalinity, promoting the precipitation of dolomite. At the same time, the rise in pH promoted the dissolution of biogenic silica which, reacting with pore water cations, ultimately caused the formation of authigenic clays. This study suggests that the apparent annihilation of Mediterranean marine biota during the Messinian salinity crisis partially reflects an early diagenetic bias produced by interactions of the carbon, silicon and sulphur biogeochemical cycles in a restricted basin. [ABSTRACT FROM AUTHOR]

While Ediacaran cap dolostones research is a hot topic, detailed sedimentological analysis of barite‐bearing cap dolostones remains scarce. Here, new data are presented from Xiangerwan in northern Three Gorges, South China, to illustrate the sedimentary characteristics, depositional environment and formation mechanism of the barite‐bearing cap dolostone. In ascending order, the succession is divided into: (i) pebbly dolostone; (ii) dolostone with unusual sedimentary structures (for example, sheet cracks and tepee structures); and (iii) barite‐bearing facies. The gravels of the pebbly dolostone record the slow relative sea‐level rise during post‐glacial transgression, with a local terrestrial influence. Sheet cracks and tepee structures represent water escape structures in the supratidal and upper intertidal zone, and gently wavy stromatolites in barite‐bearing facies are deposits typical of the upper intertidal zone. In addition, primary radial length‐slow dolomite cements were originally precipitated at the seafloor. In general, the cap dolostone was formed in a shallow‐water ramp environment. The form and sedimentary characteristics of barites indicate a primary (syngenetic) origin and help to divide the barites into bladed (Type I) and needle‐like (Type II) variants. With the ongoing melting of the Marinoan glacier and water density stratification, the cap dolostone was gradually precipitated, accompanied by exotic gravels input. The stratification persisted, and special sedimentary structures were widely developed. Phosphorus and barium ions were brought up from the deep basin by upwelling, indicated by the flourishing stromatolites and barite fan deposition. Above the sediment–water interface, the Type I barite first grew upward from substrates, then the Type II barite could grow either in the reverse 'V‐shape' cavities between and within the Type I barite clusters, forming rhythm layers with bedded dolomite, or scattered in the laminae of stromatolites or surrounding dolomite. Finally, the formation of barite fans indicates the end of cap dolostone deposition and deeper water sedimentation. [ABSTRACT FROM AUTHOR]

Lacustrine dolomite nucleation commonly occurs in modern and Neogene evaporitic alkaline lakes. As a result, ancient lacustrine microcrystalline dolomite has been conventionally interpreted to be formed in evaporitic environments. This study, however, suggests a non‐evaporitic origin of dolomite precipitated in a volcanic–hydrothermal lake, where hydrothermal and volcanic processes interacted. The dolomite occurs in lacustrine fine‐grained sedimentary rocks in the middle Permian Lucaogou Formation in the Santanghu intracontinental rift basin, north‐west China. Dolostones are composed mainly of nano‐sized to micron‐sized dolomite with a euhedral to subhedral shape and a low degree of cation ordering, and are interlaminated and intercalated with tuffaceous shale. Non‐dolomite minerals, including quartz, alkaline feldspars, smectite and magnesite mix with the dolomite in various proportions. The 87Sr/86Sr ratios (0.704528 to 0.705372, average = 0.705004) and δ26Mg values (−0.89 to −0.24‰, average = −0.55‰) of dolostones are similar to those of mantle rocks, indicating that the precipitates mainly originated from fluids that migrated upward from the mantle and were subject to water–rock reactions at a great depth. The δ18O values (−3.1 to −22.7‰, average = −14.0‰) of the dolostones indicate hydrothermal influence. The trace and rare earth element concentrations suggest a saline, anoxic and volcanic–hydrothermally‐influenced subaqueous environment. In this subaqueous environment of Lucaogou lake, locally high temperatures and a supply of abundant Mg2+ from a deep source induced by volcanic–hydrothermal activity formed favourable chemical conditions for direct precipitation of primary dolomite. This study's findings deepen the understanding of the origin and processes of lacustrine primary dolomite formation and provide an alternative possibility for environmental interpretations of ancient dolostones. [ABSTRACT FROM AUTHOR]

The higher abundance of dolomite in the Neoproterozoic compared with that of the Phanerozoic has been linked to elevated marine Mg/Ca ratios, and a hypothesized 'dolomite sea' that promoted widespread syn‐sedimentary fibrous dolomite cement precipitation. This article provides new uranium–lead (U–Pb) chronology, clumped isotope thermometry (TΔ47), combined with δ13C and δ18O, and trace element geochemistry measurements for different types of dolomite, including bulk dolomite matrix (BD), fibrous dolomite cements (DC1) and late sparry dolomite cements (DC2 and DC3), in three Neoproterozoic dolostone profiles from China. The Ediacaran U–Pb ages coupled with nearly identical, low TΔ47 temperatures (average at ca 55‐60°C) for BD and DC1 dolomites, imply that these dolomites were either syn‐depositional or early diagenetic. Some DC1 dolomites yield slightly younger (ca 20 to 50 Ma) U–Pb ages than the BD dolomites, suggesting that they were diagenetic products formed after matrix dolomitization. The estimates of δ18Owater values for dolomitizing fluids suggest that both DC1 and BD dolomites were precipitated from highly evaporated seawater. In contrast, DC2 and DC3 dolomites filling in vugs and fractures have much younger U–Pb ages (mostly <525 Ma), higher TΔ47 temperatures (>95°C), and lower Sr and U contents, which is consistent with the paragenetic sequence that they were deeper burial diagenetic products. A synthesized diagenetic model with evaporative dolomitizing water evolved from anoxic, ferruginous to sub‐oxic and/or oxic, non‐ferruginous conditions is responsible for the decrease of Fe and Mn concentrations in global DC1 and BD dolomites throughout Neoproterozoic time. This study suggests that the widely distributed Neoproterozoic fibrous dolomite cements may be a later diagenetic feature, and thus calls into question their use in palaeoclimate reconstruction and as a principal line of evidence for the Neoproterozoic 'dolomite sea' hypothesis. [ABSTRACT FROM AUTHOR]

The Coorong Coastal Plain in the south‐east of South Australia is a series of uplifted and stranded Pleistocene, beach‐dune, aeolianite complexes (Bridgewater Formation) that range in age from Middle Pleistocene to Holocene. Aeolianite ridges are separated by elongated corridors, the sites of interdune sedimentation. The youngest corridors, sites of Holocene lacustrine deposition, are also locations of syndepositional dolomite formation. These lakes, particularly south of the Coorong Lagoon, are world‐famous and extensively studied. The older corridors, by contrast, are poorly understood. This study investigates the numerous corridor sediments and limestones between aeolianites across the coastal plain. There are two formations: (i) the Padthaway Formation, a middle Pleistocene, Marine Isotope Stage 15 to Marine Isotope Stage 2 deposit with three facies; and (ii) the Glanville Formation, a younger late Pleistocene Marine Isotope Stage 5e, unit with one facies containing warm‐water biota that is not present in the area today. These carbonates are interpreted to record deposition in palaeoenvironments ranging from lacustrine to estuarine–lagoonal. There is very little dolomite in the deposits. Aragonite and Mg‐calcite are present and most abundant in corridors younger than 350 ka. There are only seven samples of Padthaway Formation carbonates that contain any dolomite, with only two containing more than 50%. Limestone and dolomite stable isotopes are similar with negative δ13C values (−3.9 to −8.6‰) and variable δ18O numbers (+3.0 to −2.4‰). Such values are quite unlike those for younger synsedimentary dolomites. Mineralogy and geochemistry suggest original low‐magnesium calcite‐rich sediment that has been modified to varying degrees by meteoric diagenesis. Lack of synsedimentary dolomite is ascribed mainly to more humid climatic conditions in the past. The study emphasizes that lacustrine synsedimentary dolomite forms under semi‐arid conditions when lakes, fed by Mg‐rich and Ca‐rich resurging groundwaters and under bacterial mediation, are evaporated to dryness. [ABSTRACT FROM AUTHOR]

The sedimentary record of the Pahrump Group in Death Valley comprises well‐exposed successions of mixed carbonate and siliciclastic deposits. Despite the abundance of studies focussing on the depositional dynamics of mixed carbonate – siliciclastic deposition in the Phanerozoic, the record of similar Proterozoic examples is comparatively sparse. Using high‐resolution stratigraphy and microfacies analyses, this study investigates the Tonian Horse Thief Springs Formation within the Pahrump Group of Death Valley, California, in order to propose first‐order constraints on the interplay between carbonate and siliciclastic deposition in the early Neoproterozoic. The mixed successions are unlike many previously studied examples interpreted as being caused by glacio‐eustatic sea‐level changes, thus arguing for a more relevant tectonic influence on siliciclastic deposition. This interpretation is supported by detailed microfacies analyses of siliciclastic‐rich dolostones, which show abundant soft‐sediment deformation features suggesting sudden pulses of sandstone deposition onto a shallow marine carbonate shelf. The Tonian carbonate factory recovered quickly after being smothered by siliciclastics, particularly due to abundant stromatolite growth. A new relationship between siliciclastic input, carbonate fabric diversity, and carbonate preservation is established based on microfacies analyses, putting forward that siliciclastic deposition had a significant impact on the formation and preservation of later‐stage diagenetic dolomite cements, as well as on stromatolite morphology and carbonate fabric diversity within the microbialites. This study shows how repeated siliciclastic incursions had a significant impact on the Proterozoic carbonate factory of the Horse Thief Springs Formation, as well as on diagenetic modification and preservation of shallow marine carbonates, and establishes previously unexplored relationships between carbonate and siliciclastic strata in the Proterozoic. [ABSTRACT FROM AUTHOR]

The Coorong Lakes, South Australia, are one of the models for unravelling the 'Dolomite Problem'. Critically, today only a few modern environments remain where large quantities of very high magnesium calcite (VHMC; Ca0.5Mg0.5CO3; also described as protodolomite or disordered dolomite) and magnesite (MgCO3) precipitate. Previously conducted laboratory studies demonstrate that carbonate minerals can precipitate via classical and non‐classical crystallization pathways. This study uses the preserved crystal sizes, morphologies and microstructures of Ca–Mg carbonates in the Coorong Lakes (Milne Lake, Pellet Lake and North Stromatolite Lake) to evaluate which crystallization pathway most likely occurred. In the fine‐grained sediments of these lakes, very high magnesium calcite and magnesite occur as aggregate particles of nanocrystals (<100 nm). Rietveld refinements using X‐ray diffraction data give modelled Lvol–IB crystallite size values of <120 nm for all carbonates. Transmission electron microscopy shows that, within VHMC and magnesite particles, nanocrystals have an almost identical orientation of their crystal lattice fringes. This is morphologically similar to Ca–Mg carbonates formed via an amorphous carbonate precursor in non‐classical crystallization laboratory experiments. Precipitation of carbonate minerals via an amorphous‐to‐crystalline pathway requires the water to be supersaturated relative to both crystalline and amorphous phases. In the Coorong Lakes, surface water likely only becomes supersaturated relative to amorphous carbonate phases in the late summer after extensive evaporation. Observations suggest that VHMC and dolomite do not directly precipitate from bulk modern seawater, despite oversaturation relative to the crystalline phases, because seawater is undersaturated with respect to amorphous calcium magnesium carbonate, thus limiting the precipitation through a non‐classical crystallization pathway. [ABSTRACT FROM AUTHOR]

This work evaluates an exceptionally complex natural laboratory, the Middle Triassic Latemar isolated platform in the northern Italian Dolomite Mountains and explores spatial and temporal gradients in processes and products related to contact metamorphism, dolomitization and dedolomitization of marine limestones. The relation between petrographic change and re‐equilibration of geochemical proxy data is evaluated from the perspective of carbonate‐archive research. Hydrothermal dolomitization of the limestone units is triggered by dykes and associated hydrothermal fluids radiating from the nearby Predazzo Intrusion. Detailed petrography, fluid inclusion analysis, δ13C and δ18O data and 87Sr/86Sr isotope ratios shed light on the extreme textural and geochemical complexity. Metamorphic and diagenetic patterns include: (i) peak‐metamorphic and retrograde‐metamorphic phases including three dolomite marbles, two dedolomite marbles, brucite, magnesium silicates and late‐stage meteoric/vadose cement at the contact aureole; (ii) four spatially defined episodes of dolomitization, authigenic quartz, low magnesium calcite and late‐stage meteoric cement at the Latemar isolated platform; and (iii) kilometre‐scale gradients in δ13C values from the contact aureole towards the platform interior. Results shown here are relevant for two reasons: first, the spatial analysis of alteration products ranging from high‐grade metamorphic overprint of marbles at temperatures of 700°C in the contact aureole to moderately altered limestones in the platform interior at temperatures <100°C, allows the observation of processes that commonly occur along vertical (prograde) gradients from shallow burial to metamorphism at depths >20 km. Second, under rock‐buffered conditions, and irrespective of metamorphic to diagenetic fluid−rock interactions, both marbles, and low‐temperature hydrothermal dolomites have conservative marine δ13C and δ18O values. The fact that metamorphism and hydrothermal dolomitization of precursor limestones and early diagenetic dolostones did not per se reset environmental proxy data is of interest for those concerned with carbonate archive research in Earth's deep time. [ABSTRACT FROM AUTHOR]