Dolomite Problem Research Articles

Magnesium geochemistry of authigenic carbonate at marine cold seep

Cold seeps, featured by their extremely methane-rich sedimentary environments, play a significant role in the geological history and are common in marine sediments across the seafloor. Primary dolomite, possibly mediated by microorganisms, can be widely discovered in methane-rich environments. Hence, cold seeps may provide new insights into the ‘dolomite problem’, which has confused geologists for decades. Magnesium isotope geochemistry of seep carbonates contributes to the understanding of the dolomite formation mechanism in marine environments. In this paper, magnesium geochemical characteristics of carbonates in modern sediments are summarized, along with rare researches on magnesium isotopes of seep carbonates. Methane vigorously interacts with sulfate by anaerobic oxidation of methane at cold seeps, producing vast amounts of dissolved sulfide which can significantly promote dolomitization of seep carbonates. Compared with temperature, alkalinity, mineralogy, etc., the competition between rapid carbonate precipitation rates and aqueous ligands may be the main factor of the magnesium fractionation at cold seeps, which is controlled by the kinetic effect. The range of magnesium isotopes of seep carbonates is narrow (from -3.46‰ to -2.36‰), and an upper limit of magnesium content seems to exist. This characteristic may be a good indicator for identifying dolomitization related to anaerobic oxidation of methane. Whereas, mechanisms of magnesium isotope fractionation and dolomitization at cold seeps remain unclear, necessitating more natural samples tested, stimulated calculation and laboratory experiment.

Sedimentary dolomite in Western Australia and the dolomite problem: Genesis of channel and playa uranium deposits

ABSTRACTQuaternary to Recent sedimentary dolomite in groundwater calcrete and saltmarsh sediments of Lake Way and Lake Maitland, Western Australia, provide new information about the formation of low‐temperature dolomite. Dolomite can form through numerous pathways depending on the depositional and diagenetic environment. Many pathways involve microbial processes and/or the presence of a nucleation substrate which help overcome kinetic barriers preventing precipitation in the laboratory. Petrographic, mineralogical, hydrogeochemical and stable isotopic data in this study reveal the importance of Mg‐clays as nucleation sites for dolomite precipitation in a range of aquifer environments. There is also a close association between authigenic Mg‐clays, dolomite and the potassium–uranyl–vanadate ore mineral carnotite in channel and playa uranium deposits. It is interpreted that evaporation‐driven precipitation of dolomite establishes a positive feedback loop promoting the dissociation of aqueous uranyl–carbonate complexes and concomitant increase in carnotite saturation. Critical to this model is the presence of authigenic Mg‐clays because they facilitate dolomite precipitation and promote carnotite nucleation by concentrating potassium ions on clay surfaces via adsorption. This Mg‐clay–dolomite–carnotite relationship is widespread throughout Western Australian channel and playa uranium deposits and has been observed in similar deposits in Namibia and Botswana. In addition to this economic implication, Mg‐clay mediated nucleation of dolomite potentially has global relevance as a precipitation mechanism for low temperature dolomite in sedimentary deposits where detrital and/or authigenic Mg‐clays are present. Maturation of sedimentary dolomite from disordered high‐calcian dolomite to ordered low‐calcian dolomite occurs very early in the meteoric realm making it resistant to alteration during burial diagenesis. Diagenetic resistance may be further increased by early meteoric silicification related to the degradation of associated Mg‐clays. These findings indicate that Mg‐clay associated sedimentary dolomite has potential to retain a primary isotopic signature indicative of its origin, making it a useful recorder of palaeoenvironmental conditions.

Dolomite formation during penecontemporaneous subaerial diagenesis: evidence from modern dolomite crusts forming in lagoon Brejo do Espinho, Brazil

The early lithification of carbonate mud during the subaerial exposure stage under semiarid conditions has been proposed to facilitate dolomite formation. However, how the biogeochemical processes during subaerial diagenesis promote dolomite formation remains unclear. Here, we employ a multiproxy approach to investigate the process of dolomite formation by analysing modern dolomite crusts forming in lagoon Brejo do Espinho. Petrological analysis reveals that the crusts consist of coexisting high-Mg calcite and dolomite. Low Fe and Mn concentrations indicate the formation of dolomite under oxic conditions, whereas a higher Sr concentration in well-lithified crust suggests primary bacterial-induced dolomite precipitation. The Mg isotopic composition of the crusts exhibits a lighter value than that of modern sabkha dolomite, suggesting different dolomitization processes and Mg sources. The more negative δ 13 C values of the crusts than those of the unlithified carbonate mud in lagoon Brejo do Espinho indicate the incorporation of 13 C-depleted organic carbon. The biogeochemical processes related to decaying organic matter during subaerial diagenesis generate partially oxidized organic matter that promotes Mg 2+ dehydration and enhances the dissolution of primary high-Mg calcite, ultimately triggering the transition of high-Mg calcite to dolomite and/or the direct precipitation of dolomite. The ancient ‘dolomite factory’ operated through the cyclic deposition of carbonate sediments and penecontemporaneous subaerial diagenesis. Thematic collection: This article is part of the Towards unravelling the ‘Dolomite Problem’: new approaches and novel perspectives collection available at: https://www.lyellcollection.org/topic/collections/towards-unravelling-the-dolomite-problem

Dolomite problem may be solved

Dolomite problem may be solved

Thermodynamic and Kinetic Studies of Dolomite Formation: A Review

The “dolomite problem”, which has confused scientists for nearly two centuries, is an important fundamental geological problem. The mineralogical characteristics of carbonate minerals show that the dolomite structure consists of an ordered arrangement of alternating layers of Ca2+ and Mg2+ cations interspersed with CO32− anion layers normal to the c-axis. The dolomite structure violates the c glide plane in the calcite structure, which means that dolomite has R3¯ space group symmetry. The ordered dolomite has superlattice XRD reflections [e.g., (101), (015) and (021)], which distinguish it from calcite and high-Mg calcite. The calculation of thermodynamic parameters shows that modern seawater has a thermodynamic tendency of dolomite precipitation and the dolomitization reaction can be carried out in standard state. However, the latest thermodynamic study shows that modern seawater is not conducive to dolomitization, and that seawater is favorable for dolomitization in only a few regions, such as Abu Dhabi, the Mediterranean and the hypersaline lagoons in Brazil. The kinetic factors of dolomite formation mainly consist of the hydration of Mg2+, the presence of sulfate and the activity of carbonate. Current studies have shown that the presence of microorganisms, exopolymeric substances (EPS), organic molecules, carboxyl and hydroxyl functional groups associated with microorganisms and organic molecules, clay minerals with negative charges and dissolved silica facilitate magnesium ions to overcome hydration and thus promote Mg2+ incorporation into growing Ca-Mg carbonates. Similarly, the metabolic activity of microorganisms is conducive to the increase in alkalinity. However, the inhibitory effect of sulfate on dolomite formation seems to be overestimated, and sulfate may even be a catalyst for dolomite formation. Combining the carbonate crystallization mechanism with thermodynamic and kinetic factors suggests that the early stage of dolomite precipitation or the dolomitization reaction may be controlled by kinetics and dominated by unstable intermediate phases, while metastable intermediate phases later transform to ordered dolomite via an Ostwald’s step rule.

A fluctuating solution to the dolomite problem.

Episodes of dissolution and crystal growth stoke the formation of a common carbonate mineral.

Organomineralization of dolomite in hypersaline microbial mats from Qatar sabkhas visualized by TEM & STXM

Deep insight into the low-temperature mineralization mechanism of dolomite in sediments has remained elusive. This issue is popularly termed “The Dolomite Problem” due to its multifactorial nature. Dolomite has been observed to mineralize in the exopolymeric substances produced by microbial mat communities (Bontognali et al. 2013), where productivity is high. One working hypothesis suggests that degrading organic matter in hypersaline environments releases the necessary component ions, increasing saturation with respect to dolomite (DiLoreto et al. 2019, Dupraz et al. 2009, Petrash et al. 2017). Other models suggest a dissolution-reprecipitation reaction of calcite to dolomite (Rivers 2023). High-resolution micro-spectroscopy techniques (such as transmission electron microscopy, TEM; and scanning transmission X-ray microscopy, STXM) can be used to determine chemical changes in crystals nucleating in a matrix, however to date very little studies have focused on observing dolomite mineralization at the nano-scale. The present study investigates microbial mats collected from hypersaline salt flats in the Persian gulf at micro- to nano-meter scales using high-spatial and -energy resolution TEM (Thermo Scientific Talos 200X at the Canadian Centre for Electron Microscopy) and STXM (PolLux Beamline at the Swiss Light Source at Paul Scherrer Institut), specifically to see changes in carbonate mineralization due to interactions with organic matter. C, Ca and O elemental maps of carbonate crystals were obtained with EDXS (energy-dispersive X-ray spectroscopy) in TEM. These crystals were also indexed by SAED (selected area electron diffraction in TEM). Fine spectral signatures (near-edge X-ray absorption fine structures, or NEXAFS) at the C K-edge (280-290 eV) and Ca L2,3-edge (344-356 eV) in STXM were used to determine the chemical identity of carbonate minerals and surrounding organic matter of the microbial mats. The results of the study show that dolomite nucleates in close association with the organic matter of the mats, where degradation is highest (defined in our adjacent study as the increase in C:N ratio). In TEM, polycrystalline dolomite is seen mineralizing in the matrix of the microbial mat organic material (Fig. 1). In STXM, the identity of the carbonate mineral changes from calcite on the outside to dolomite on the inside of the microbial mat particle (Fig. 2). In addition, our microsensor observations of elevated H2S concentrations, surface oxygenation from oxygenic phototrophy, high reduction potential, high organic carbon, high Mg:Ca ratio and high organic matter degradation (by C:N ratio) in each of the studied microbial mats confirms that the ideal dolomite mineralization conditions according to models of the dolomite problem are present in each case.

Lithium isotope composition of Ediacaran dolostones from the Nuccaleena and Doushantuo formations

The end of the Cryogenian glaciations undoubtedly affected the chemistry of the Neoproterozoic oceans, with potential consequences for the evolution of life; the duration and extent of this influence are poorly constrained. Lithium (Li) isotopes in carbonates can be used to investigate past weathering events and riverine input into the oceans. Here, we report the Li isotope (δ7Li) composition of the Ediacaran cap dolostones (Marinoan) from the Nuccaleena Formation, South Australia and the Doushantuo Formation, South China to investigate changes in weathering during the aftermath of the Marinoan glaciation. The origin of dolomite formation is still hotly debated, and the inability to precipitate dolomite at ambient temperatures (the ‘dolomite problem’) has not yet been resolved. The dominant hypothesis for the presence of marine dolomite is that it is of secondary origin, owing to diagenetic replacement of calcium carbonate, but a second hypothesis is that it forms as a result of primary marine deposition; both are plausible hypotheses. Using carbon isotopes (δ13C) and ratios of manganese and strontium (Mn/Sr), we suggest that diagenesis may not have significantly altered the δ7Li composition of the primary dolomite, but this cannot be discounted without further diagenetic modelling. As a result, we cannot infer the δ7Li composition of the Ediacaran oceans, as further work must be done to address the impact of diagenesis on the δ7Li values. Nevertheless, this dataset contributes to a Li isotope chemostratigraphic record of the Proterozoic, which is a key element to understanding the emergence of complex life.

Warm acidified seawater: a dolomite solution

ABSTRACTThe “dolomite problem” is the product of two distinct observations. First, there are massive amounts of ancient marine limestone (CaCO3) deposits that have been replaced by the mineral dolomite (MgCa(CO3)2). However, recent (Holocene and Pleistocene) marine deposits contain relatively minuscule amounts of dolomite, although the occurrence of small quantities of dolomite is observed in many modern settings, from deep marine to supratidal. Second, low-temperature synthesis of dolomite in laboratory settings has been elusive, particularly in comparison to the ease with which common marine calcium carbonate minerals (aragonite and calcite) can be synthesized. Since low-temperature solid-state diffusion can be discounted as a method for Mg incorporation into calcium carbonate (as it operates on time scales too long to matter), the replacement of CaCO3 by dolomite is one of dissolution followed by precipitation. Therefore, an often overlooked but required factor in the replacement of limestone by dolomite is that of undersaturation regarding the original calcium carbonate mineral during replacement. Such conditions could conceivably be caused by rapid dolomite growth relative to aragonite and calcite dissolution–precipitation reactions, but laboratory studies, modern systems analyses, and observations of ancient deposits all point to this possibility being uncommon because dolomite growth is kinetically inhibited at low temperature. Pressure solution by force of dolomite crystallization is a second possible driver for CaCO3 undersaturation, but requires a confining stress most likely attained through burial. However, based on petrographic observations, significant amounts of ancient dolomite replaced limestone before burial (synsedimentary dolomite), and many such platforms have not suffered any significant burial. Because these possibilities of undersaturation caused by dolomite precipitation and crystal growth can be largely discounted, the undersaturation required for “dolomitization” to proceed is most likely to be externally forced. In modern natural systems, undersaturation and selective CaCO3 dissolution in marine porewaters is very common, even in warm-water environments, being forced by the breakdown of organic matter. Such dissolution is frequently attended, to varying degrees, by precipitation of a kinetically-less-favored but thermodynamically more stable phase of CaCO3. Laboratory studies as well as observations of modern systems show that when undersaturation is reached with respect to all common marine CaCO3 phases, dolomite assumes the role of this kinetically-less-favored precipitate. This degree of undersaturation is uncommon in modern shallow marine pore systems in warm-water settings, but it was more common during times of elevated atmospheric CO2, and ocean acidification. Furthermore, because oxidation of organic matter drives dolomite formation, near-surface organic-rich deposits such as the remains of microbial mat communities, were more predisposed to dolomite replacement in the acidified oceans of the ancient past relative to contemporaneous deposits that contained less organic matter. These observations lend to a more harmonious explanation for the abundance and occurrence of dolomite through time.

Dichotomous effects of oxidative metabolisms: A theoretical perspective on the dolomite problem

Dichotomous effects of oxidative metabolisms: A theoretical perspective on the dolomite problem

A DOM regulation model for dolomite versus calcite precipitation in the Ediacaran ocean: Implications for the “dolomite problem”

The Ediacaran Period (∼635–539 Ma) witnessed the largest negative carbonate carbon isotope (δ13Ccarb) excursion in Earth history (to −12‰ VPDB), known as the Shuram Excursion (SE), which has been attributed to oxidation of a massive dissolved organic matter (DOM) reservoir, yielding highly 13C-depleted oceanic dissolved inorganic carbon (DIC). Limestone and dolostone are intimately interbedded in the Ediacaran Doushantuo Formation (South China) as well as in coeval successions globally, suggesting frequent shifts between calcite and dolomite precipitation in the Ediacaran ocean. In this study, we compiled paired Mg/(Mg + Ca) and δ13Ccarb data for 991 well-preserved (i.e., minimally altered) carbonate-rich samples from 17 SE-associated sections with a wide global distribution. δ13Ccarb compositions vary among the dolomite-dominated (−10‰ to +7‰), calcite-dominated (mainly in −10‰ to −5‰), and mineralogically mixed samples (with a bimodal distribution of −10‰ to −5‰ or +4‰ to +8‰). Inspired by the recent DOM catalyzation hypothesis and the highly-stratified redox model as well as the large DOM reservoir hypothesis for the Ediacaran ocean, we propose a “DOM regulation model” to account for controls on the precipitation of dolomite versus calcite in the Ediacaran ocean and, thus, for observed Mg/(Mg + Ca)-δ13Ccarb patterns. In this model, calcite was precipitated under low-DOM conditions, which existed in oxic surface waters and the nearshore part of a mid-depth DOM-anaerobic oxidation zone. The former had δ13CDIC > +4‰, buffered by atmospheric CO2, whereas the latter had δ13CDIC decreasing from +4‰ to −10‰ due to increasing contributions of DOM oxidation. On the other hand, dolomite precipitation was catalyzed by high-DOM conditions, which existed in distal anoxic deep waters and the offshore part of the DOM-anaerobic oxidation zone. The former had δ13CDIC > +4‰, ultimately buffered by atmospheric CO2, whereas the latter had δ13CDIC increasing from −10‰ to +4‰ due to decreasing contributions of DOM oxidation. Our model is well supported by coexisting redox data. Our study highlights a possible link between massive dolomite formation and the existence of a large DOM reservoir in the Ediacaran ocean, providing an alternative perspective on the origin and widespread distribution of dolomite in deep-time marine systems (i.e., the “dolomite problem”).

Microfabrics and organominerals as indicator of microbial dolomite in deep time: An example from the Mesoproterozoic of North China

Both laboratory experiment and modern environment studies show that primary dolomite can precipitate at low temperature (≤60 ℃) only when microbes were involved. Vast dolostones in mid-Proterozoic have long been suspected to be the results of microbial mediation, but direct evidence is rare. To reveal their origin, an integrated study was conducted on the Mesoproterozoic Wumishan Formation (∼1.48 Ga) of North China using multiple techniques. The results show that the Wumishan dolostone contains abundant multiscale organominerals that are closely associated with fossilized remnants of extracellular polymeric substances (EPS) and putative bacteria fossils. These microfabrics are strikingly similar to those in dolomite precipitates produced in laboratory culture and modern sabkha environment, suggesting microbial origin. Nanoglobules (60–200 nm) preferentially attach to fossilized EPS filaments, and tend to merge into polyhedrons (3–8 μm) that in turn coalesce into microspheres (20–50 μm). EDS analysis revealed a successive decrease in carbon from nanoglobules to microspheres with the increases of Mg and Ca contents, implying an increased mineralization. Nanoglobules may have derived from EPS degradation during nucleation, and served as seeds for subsequent growth of carbonate polyhedrons. Microsphere, as a basic building block in dolostone, can transform into rhombohedral crystal via neomorphogenesis during early diagenesis. Petrographic and XRD analyses show that the dolostone is dominated by ordered dolomite and was largely formed in peritidal environment with thriving microbial community. Geochemical analyses of iodine species, redox sensitive elements, Ce anomalies, and C–O isotopes in the Wumishan dolostone suggested an environment predominated by anoxic to suboxic conditions, with low P but high Si contents in seawater.Our study offered direct evidence of microbial origin for the Mesoproterozoic dolostone, providing new insights into the “Dolomite Problem”. The massive development of microbial dolostone in Mesoproterozoic points to a specific ocean chemistry, where low oxygen and active bacterial metabolisms may have played crucial roles in precipitating dolomite. The co-existence of abundant microbial components and multiscale organominerals may be taken as textural evidence for microbial dolomites and signatures of life-environment interactions in deep time.

Non‐classical crystallization of very high magnesium calcite and magnesite in the Coorong Lakes, Australia

AbstractThe 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.

Microbialites of terminal Ediacaran in the Upper Yangtze Platform, China: From mesoscopic to nanoscale

Dolomitic microbialites were developed extensively during the Neoproterozoic and record widespread oxygenation of the oceans and the accompanying rise of animal life. Primary precipitation of dolomite cements has now been recognized in both Cryogenian and Ediacaran periods, although the origin of matrix dolomite is debated. The terminal Ediacaran Dengying Formation (551–541 Ma) in the Upper Yangtze Platform of China contains massive dolomite. Particularly delicate microbial structures are preserved in the second member of the formation (Z 2 dn 2 ). This study aimed to elucidate the origin of matrix dolomite in these microbialites, the main mesoscale features of which are stromatolites, thrombolites and composite microbialites. The stromatolites typically have planar and low-relief planar–domal forms, with distinct laminated microstructure alternated with diffuse laminated microstructures. The thrombolites comprise a variety of mesoclot structures and cavities, which are rimmed by isopachous fringes of fibrous cement and filled by late sparry cement. The composite microbialites have clotted and laminated textures of various styles that are intermingled at the sub-centimeter scale. These microbialites are inferred to have been deposited in a subtidal to intertidal setting. Abundant nanoscale dolomite biomorphs were observed with scanning electron microscopy, including single nanoscale spheroids and aggregates of nanometer filaments with granulated, micro-rose shaped, micro-sized foliage-like, and trumpet-like forms, most of which are presumably of biogenic origin. These nanoscale microbial textures act as archives of ancient microbial activity that may have produced abundant extracellular polymeric substance, providing sites for the nucleation and maturation of dolomite. This process may have been facilitated by Mn–S redox cycling and resulted in precipitation of primary ferroan dolomite in the matrix. • Novel study on the old “dolomite problem” from the view of microbialite; • Stromatolite, thrombolite and composite microbialite are all composed of dolomite; • Microbial features of Ediacaran dolomite were displayed from mesoscopic to nanoscale; • Delicate nanoscale microbial biomorphs of dolomite observed under SEM.

An Early Holocene Primary Dolomite Layer of Abiotic Origin in Lake Sayram, Central Asia

AbstractThe “dolomite problem” is a long‐standing puzzle in sedimentology and mineralogy. Previous studies have shown that some dolostones are formed by microbes or in hydrothermal–burial environments. Here, we provide a different case in which an abiotic and Ca‐rich dolomite layer precipitated in Lake Sayram, Central Asia, during the early Holocene. The 12‐cm‐thick layer consists of abundant partially ordered dolomite crystals (mean > 50 wt% and maximum = 81 wt%) which have euhedral and rhombohedral shapes and weak cation ordering. Transmission electron microscopy reveals that even in a single crystal, compositions vary among different domains. Mineralogical and isotopic evidence suggests that these dolomites are of primary and abiotic origin. We infer that this dolomite layer might be the product of day and night temperature cycling of shallow lake water under local warming trend. This study provides evidence for abiotic dolomites forming at ambient temperature.

Equilibrium between dolomitization and dedolomitization of a global set of surface water samples: A new theoretical insight on the dolomite inorganic formation mechanism

The dolomite problem has confused scientists for nearly two centuries because the classic saturation index cannot explain the failure of dolomite precipitation in carbonate solutions including 1000-fold super saturation for 32 years and oversaturation of modern seawater for millions of years. Although the dolomite problem is still controversial, the chemistry of a global set of surface water samples reported in dolomitization (N = 54) and dedolomitization (N = 433) is indispensable, and firstly summarized to explore the mechanism of dolomite inorganic precipitation. This study indicates that: (1) saturation index of dolomite in the surface water of dolomitization is closing to dedolomitization, suggesting the failure of SI to reflect the real precipitation driving force of dolomite; (2) the primary difference is that most of water in dedolomitization has negative SI of gypsum, while most of water in dolomitization has positive SI of gypsum; (3) the inherent stoichiometry competition relationship between calcite and dolomite is proposed to explain the incompleteness of classic saturation index model in predicting inorganic dolomitization; (4) A multi-component calcite - dolomite - gypsum phase equilibrium (CDGPE) model is recommended to replace SI in predicting the dolomitization and dedolomitization of a global set of surface waters; (5) CDGPE model is proved reliable and useful in predicting the surface water reported in dolomitization and dedolomitization (including the modern seawater); (6) the evaporation modelling of the modern seawater through the stepwise detraction of water shows that the surface waters (Persian Gulf, Mediterranean, Abu Dhabi, Coorong Region at Southern Australia, and LagoaVermelha at Brazil) observed with abundant dolomitization are all located at the different evaporation paths of modern seawater, suggesting that the dolomitization at these seawater are resulted by the evaporation, which is consistent with the recent finding that the dolomitization event at the oceanic chemocline occurred during the Permian-Triassic transition period when the temperature was hot on the earth surface.

Formation pathway of norsethite dominated by solution chemistry under ambient conditions

Abstract Recently dolomite analogs, including norsethite, kutnahorite, and PbMg(CO3)2, etc., have attracted much attention due to their dolomite-like crystallographic structure and the resulting potential in solving “dolomite problem.” Previous studies indicate that mechanisms that underlie the formation of norsethite exhibit some similarities to pathways of dolomite crystallization. However, the crystallization behavior of norsethite is still poorly understood, and the physicochemical factors regulating the process are not yet fully established. Herein, to determine the relationships between solution chemistry and formation pathway of norsethite, a series of experiments for the synthesis of norsethite from the solutions with different concentrations of Mg2+ and Ba2+ by a CO2 gas-diffusion method was carried out under ambient conditions. The morphology and phase composition of the products were investigated by a range of techniques, including XRD, FESEM, micro-Raman, and FTIR techniques. ICP-AES was used to monitor the evolution of the concentrations of Mg2+ and Ba2+ in the mineralization solutions. Our observations suggest that the formation pathway of norsethite strongly depends on Mg/Ba ratio in solution, and pure norsethite can directly crystallize from the solutions with Mg/Ba ratio ranging in 20–40. This is the first time to report the direct precipitation of dolomite analogs at ambient temperatures. It suggests that direct precipitation is a feasible pathway for cation ordering structure formation under ambient conditions.

Dissolved silica-catalyzed disordered dolomite precipitation

AbstractThere is a great abundance of sedimentary dolomite in the Proterozoic and Lower Paleozoic, but examples of primary dolomite are scarce in the Cenozoic. This discrepancy suggests a poorly understood but dramatic shift in the geochemical system that inhibited dolomite formation. Previous research on microbial-mediated dolomite formation demonstrated that microbial activity could promote disordered dolomite precipitation through the catalytic role of polysaccharides. However, the microbial-mediated model cannot explain some of the Precambrian dolomite for which there is no evidence of microbial origin. Here, we present an abiotic mechanism with dissolved silica catalyzed dolomite precipitation that provides new insight into this long-lasting “dolomite problem.” In this study, we demonstrate that the presence of 1–2 mM of aqueous Si(OH)4 in high Mg:Ca ratio solutions at room temperature will promote disordered dolomite precipitation (with up to 48.7 mol% MgCO3) and inhibit aragonite formation. Dissolved silica in solution also promotes Mg incorporation into the Ca-Mg carbonates. Dissolved silica possesses low-dipole moment and dielectric constant similar to hydrogen sulfide, dioxane, polysaccharide, and exopolymeric substances (EPS), which are catalysts in previously established room-temperature dolomite synthesis. The molecules with low-dipole moment adsorbed on the dolomite surface can lower the dehydration energy barrier of a surface Mg2+-water complex and promote dolomite nucleation and growth. This study provides a new model for abiotic sedimentary dolomite formation, which is likely to be responsible for the significant amount of primary dolomite in Earth history.

Modern dolomite formation caused by seasonal cycling of oxygenic phototrophs and anoxygenic phototrophs in a hypersaline sabkha

The “Dolomite Problem” has been a controversy for over a century, owing to massive assemblages of low-temperature dolomite in ancient rocks with little dolomite forming today despite favorable geochemical conditions. Experiments show that microbes and their exopolymeric substances (EPS) nucleate dolomite. However, factors controlling ancient abundances of dolomite can still not be explained. To decode the enigma of ancient dolomite, we examined a modern dolomite forming environment, and found that a cyclic shift in microbial community between cyanobacteria and anoxygenic phototrophs creates EPS suited to dolomite precipitation. Specifically, EPS show an increased concentration of carboxylic functional groups as microbial composition cycles from cyanobacterial to anoxygenic phototroph driven communities at low-and high- salinity, respectively. Comparing these results to other low-T forming environments suggests that large turnover of organic material under anoxic conditions is an important driver of the process. Consequently, the shift in atmospheric oxygen throughout Earth’s history may explain important aspects of “The Dolomite Problem”. Our results provide new context for the interpretation of dolomite throughout Earth’s history.

The dolomite problem: evidence from 3D modeling, XRD and geochemical data of Zechstein reefs (Upper Permian, Germany)

Three dimensional modeling of the limestone and dolomite distribution in an Upper Permian (Zechstein) stromatolite-bryozoan reef, ca. 500m in diameter and 35m thick (77 drill cores, 172 point data), shows that dolomite occurs as laterally and vertically discontinuous intervals. The prevailing mineral phases are near stoichiometric dolomite and Mg free calcite, respectively (370 XRD and 341 XRF analyses). Both ∂ 13 C and ∂ 18 O (526 analyses) show a spread of ca. 10‰ and covary with the mineralogy, and the heaviest dolomite and calcite ∂ 13 C differ by ca. 1,5‰. Diagenetic modifications caused by flowing meteoric fluids could account for the observed “inverted J” trend of stable and the radiogenic signature of 8 7 Sr/ 86 Sr (23 analyses), but neither vertical nor horizontal gradients occur in the modeled reef. Because dolomite geometries are incompatible with those predicted by fluid flow models, and the limestone-dolomite difference in ∂ 13 C overlaps estimates of isotope fractionation associated with Mg content, the studied dolomite was a depositional Very High Mg Calcite recrystallized to dolomite in a semi-closed diagenetic system rather than a Low Mg Calcite transformed by a dolomitization process. The isotope pattern suggests biogenic fractionation and/or loss of heavy stable and light Sr isotopes during diagenesis.