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Dolomite nucleation and subsequent crystallization are kinetically-controlled processes. Modern dolomite-forming environments provide clues to the trigger factors that facilitate dolomite formation under Earth surface conditions. It has been documented that certain types of benthic microorganisms promoted the precipitation of protodolomite from sediment pore waters. As protodolomite is thought to be a possible precursor of sedimentary ordered dolomite, microbial mediation has thus been suggested as one interpretation of the occurrence of dolomite in modern sediments. To date, however, it is still unclear whether planktonic microorganisms could directly initiate protodolomite crystallization in the upper water column of present dolomite depositing environments. In this study, we report on the occurrence of authigenic protodolomite in the upmost sediments of a high-sulfate, Chinese inland saline lake (Lake Jibuhulangtu Nuur). This lake was therefore considered to be a natural laboratory to test the catalytic effect of planktonic aerobic heterotrophic bacteria on protodolomite formation. Laboratory mineralization experiments were conducted in a liquid medium that mimicked the ion concentrations and pH condition of lake surface water. The incubation experiments showed that aragonite formed in the abiotic systems, while protodolomite predominantly occurred in the bioreactors using either an enrichment culture or pure isolates of aerobic heterotrophic and halophilic bacteria from lake water. The resulting microbially-induced protodolomite crystals displayed spherical morphology and had MgCO 3 composition ranging from 42.7 mol% to 47.1 mol%. These protodolomite spherulites were formed by aggregation of randomly-distributed nano-crystals. Compared to synthetic abiotic protodolomite, microbially-induced protodolomite contained considerable amounts of organic matter, which might occur as intracrystalline inclusion or was located between nano-crystals of protodolomite spherulite. Our results support the emerging view that dissolved sulfate is not an inhibitor for the formation of low-temperature (proto-)dolomite. The presence of organic matter intimately associated with dolomite crystals may serve as a hallmark indicative of a biotically induced origin for some types of dolomite. [ABSTRACT FROM AUTHOR]

Abstract The origin of sedimentary dolomite is a subject of long-standing enigma that still awaits resolution. Previous studies have shown that room temperature synthesis of abiotic dolomite is rarely achieved and primary (proto-)dolomite precipitation is closely associated with microbial activities. In this study, we demonstrate through laboratory carbonation experiments that highly negative-charged clay minerals (as indicated by the values of zetal potential) such as illite and montmorillonite can aid the precipitation of abiotic proto-dolomite under ambient conditions, whereas nearly-neutral charged kaolinite exerts negligible influence on such process. In comparison to montmorillonite, illite has higher surface-charge density, thus is more effective in catalyzing proto-dolomite precipitation. Furthermore, the signal of proto-dolomite in carbonate neoformations is enhanced with increasing concentrations of illite or montmorillonite. On the basis of these results, we suggest that clay minerals catalyze dolomite formation perhaps via electrostatic binding of Mg2+ and Ca2+ ions and simultaneous desolvation of these strongly hydrated cations, a crucial step for dolomite crystallization. The resulting proto-dolomites display the morphologies, textures, and structures similar to those of biogenic dolomite reported before, which are considered precursors of ordered sedimentary dolomite. Therefore, our results offer a possible route to authigenic dolomite found in sedimentary environments. [ABSTRACT FROM AUTHOR]

• Transformation of protodolomite by dry heating is investigated under laboratory conditions. • Both biotic and abiotic protodolomites contain a significant amount of water molecules. • The water content of protodolomites is strongly synthetic temperature dependent. • The protodolomite-to-dolomite transformation can proceed in the open diagenetic system. • Structural water within protodolomite might play an important role in the protodolomite-to-dolomite transformation. The genesis of sedimentary dolomite remains an unresolved issue. Protodolomite has been considered as a metastable precursor for some sedimentary dolomites. Through laboratory experiments, much has been learnt about the transformation of protodolomite into dolomite under hydrothermal conditions mimicking those in open diagenetic systems. However, it is still unclear whether such mineral transformation could proceed in closed diagenetic systems, in which the supply of externally-derived fluids is often limited. Here through dry-heating experiments we demonstrated that low-temperature protodolomite converts into dolomite in the absence of external fluid. The starting materials for the recrystallization reactions included two types of protodolomite: biotic protodolomite and its abiotic counterpart. Biotic protodolomite was synthesized by means of a halophilic bacterium at 30 °C. Since the synthesis of abiotic protodolomite normally requires higher temperatures than biotic ones, the abiotic protodolomite samples used herein were prepared at 60 °C and 100 °C. These protodolomites were spherical in shape and composed of nano-globular subunits. Our protodolomite samples contained considerable structural water in the range of 1.4-7 wt%. The water content of protodolomites was linearly correlated with their synthesis temperature, that is, biotic protodolomite had a higher amount of water than its abiotic counterparts. The protodolomite samples were then dry-annealed at temperatures of 100 to 300 °C for two months. The results indicated that the rate of protodolomite-to-dolomite transformation was higher in the reactors using biotic protodolomite than those using abiotic protodolomites. This conversion was likely triggered by the dehydration of structural water within protodolomite. The resulting dolomite mostly retained spherical morphology, whereas its nanosized subunits tended to become rhombohedral. Calcite neoformation was also found to accompany the dolomite formation. Our findings suggest that structural water within protodolomite is an overlooked internal fluid and it might have an impact on the genesis of sedimentary dolomite during burial diagenesis. [ABSTRACT FROM AUTHOR]

• Microbially-derived carboxylic acids can enhance the incorporation of Mg2+ into growing Ca-Mg carbonates. • Citric acid is more effective in loading Mg2+ into Ca-Mg carbonates than succinic acid. • Disordered dolomite can form under the ambient temperature with aid of succinic and citric acids. • Solution salinity is important in the formation of disordered dolomite. The genesis of dolomite is a contentious issue partly due to the difficulty in its synthesis at ambient temperature. Certain types of microorganisms have been demonstrated to be effective in promoting the precipitation of disordered dolomite, an important precursor of sedimentary ordered dolomite. In contrast to a growing body of research on the catalytic role of microbial exopolymers in the crystallization of disordered dolomite, the role of other microbial exudates (e.g., carboxylic acids) remains unknown. To fill in this knowledge gap, precipitation experiments, mimicking the carbonation process within microbial mats, were conducted in saline solutions containing 0–30 mM succinic acid or citric acid, which are commonly produced by microbes. The starting salinities of experiment solutions were set to 35‰ and 70‰, in order to evaluate the effect of solution salinity on dolomite formation. Our results showed that both succinic acid and citric acid enhanced the incorporation of Mg2+ into growing Ca-Mg carbonates. Solution salinity also played a positive role in enhancing Mg signature in Ca-Mg carbonates. Disordered dolomite with 40.92 mol% MgCO 3 was detected under the conditions of 30 mM succinic acid and 70‰ salinity, whereas in other reactors with succinic acid, Mg-calcites formed. Citric acid was more effective in loading Mg2+ into Ca-Mg carbonates compared with succinic acid, as evidenced by the predominant occurrence of disordered dolomite with MgCO 3 content ranging from 40.91 mol% to 46.75 mol% in most conditions tested. The results of this study have implications for the formation mechanism of sedimentary dolomite. [ABSTRACT FROM AUTHOR]

The origin of dolomite has long puzzled geologists. It has recently been documented that sulfate-reducing bacteria (SRB) are capable of catalyzing the formation of protodolomite, a previously proposed precursor of ordered sedimentary dolomite. However, the catalytic mechanism of SRB remains incompletely understood. This experimental study is aimed at probing the effect of capsule extracellular polymeric substances (capsule EPS) from SRB on the crystallization of protodolomite in vivo. The capsule EPS tested herein was isolated from a protodolomite-mediating SRB, Desulfotomaculum ruminis , and added into a solution wherein the degree of oversaturation was close to the growth medium of D. ruminis at stationary phase. The solid products were characterized with X-ray diffraction (XRD), Raman spectroscopy and, scanning and transmission electron microscopy (SEM and TEM). Our results indicated that aragonite emerged in the reactors without capsule EPS, while Ca-Mg carbonates (Mg-calcite and protodolomite) were produced in the systems amended with capsule EPS. The incorporation amount of Mg2+ in Ca-Mg carbonates was enhanced with the increasing concentration of capsule EPS. The predominant occurrence of protodolomite was found in the reactor with 140 mg/L capsule EPS. These resulting protodolomites were spherical in shape, and composed of numerous nano-particles. The catalytic influence of capsule EPS on the precipitation of protodolomite might be attributed to their strong Mg2+ binding capacity, potentially diminishing Mg-hydration, which is a potent inhibitor of protodolomite crystallization. The results of Fourier transformation infrared (FT-IR) spectra showed that Mg2+ was bonded with carboxyl and hydroxyl groups on capsule EPS. This inferred adsorption capacity of capsule EPS was also supported by new calculations of complexation chemistry between Mg-H 2 O complex and organic compounds present in capsule EPS. [ABSTRACT FROM AUTHOR]