Dolomite Problem Research Articles

Dissolved sulfide-catalyzed precipitation of disordered dolomite: Implications for the formation mechanism of sedimentary dolomite

Dolomite is a common mineral in the rock record. However, the rarity of modern dolomite and the notorious difficulty in synthesizing dolomite abiotically under normal Earth-surface conditions result in the long-standing “dolomite problem” in sedimentary geology. Some modern dolomites are associated with sediments where microbial sulfate reduction is active; however, the role of sulfate-reducing bacteria in dolomite formation is still under debate. In this study, we tested the effect of dissolved sulfide on the precipitation of Ca–Mg carbonates, which has been never explored before although dissolved sulfide is one of the major products of microbial sulfate reduction. Our results demonstrated that dissolved sulfide with a concentration of as low as several millimoles can enhance the Mg2+ incorporation into the calcitic structure, and promote the crystallization of high magnesian calcite and disordered dolomite. We also conducted seeded precipitation in experimental solutions containing dissolved sulfide, which showed that calcite seeds can inhibit the precipitation of aragonite and monohydrocalcite (CaCO3·H2O), and induce more Mg2+ incorporation. We propose that accumulated dissolved sulfide in pore waters in organic-rich sediments may trigger the precipitation of disordered dolomite which can be considered as a precursor of some sedimentary dolomite. Our adsorption experiments revealed a strong adsorption of dissolved sulfide onto calcite faces. We suggest that adsorbed dissolved sulfide can lower the energy barrier to the dehydration of Mg2+–water complexes on the growing carbonate surfaces. This study sheds new light on understanding the role of sulfate-reducing bacteria in dolomite formation and the formation mechanism of sedimentary dolomite.

First discovery of dolomite and magnesite in living coralline algae and its geobiological implications

Abstract. Dolomite is a magnesium-rich carbonate mineral abundant in fossil carbonate reef platforms but surprisingly rare in modern sedimentary environments, a conundrum known as the "Dolomite Problem". Marine sedimentary dolomite has been interpreted to form by an unconfirmed, post-depositional diagenetic process, despite minimal experimental success at replicating this. Here we show that dolomite, accompanied by magnesite, forms within living crustose coralline alga, Hydrolithon onkodes, a prolific global tropical reef species. Chemical micro-analysis of the coralline skeleton reveals that not only are the cell walls calcitised, but that cell spaces are typically filled with magnesite, rimmed by dolomite, or both. Mineralogy was confirmed by X-ray Diffraction. Thus there are at least three mineral phases present (magnesium calcite, dolomite and magnesite) rather than one or two (magnesium calcite and brucite) as previously thought. Our results are consistent with dolomite occurrences in coralline algae rich environments in fossil reefs of the last 60 million years. We reveal that the standard method of removing organic material prior to Xray Diffraction analysis can result in a decrease in the most obvious dolomite and magnesite diffraction patterns and this may explain why the abundant protodolomite and magnesite discovered in this study has not previously been recognized. This discovery of dolomite in living coralline algae extends the range of palaeo-environments for which biologically initiated dolomite can be considered a possible source of primary dolomite.

Presence of sulfate does not inhibit low-temperature dolomite precipitation

The hypothesis that sulfate inhibits dolomite formation evolved from geochemical studies of porewaters from deep-sea sedimentary sequences and has been tested with hydrothermal experiments. We examined the sulfate inhibition factor using aerobic culture experiments with Virgibacillus marismortui and Halomonas meridiana, two moderately halophilic aerobic bacteria, which metabolize independent of sulfate concentration. The culture experiments were conducted at 25 and 35 °C using variable SO 4 2− concentrations (0, 14, 28 and 56 mM) and demonstrate that halophilic aerobic bacteria mediate direct precipitation of dolomite with or without SO 4 2− in the culture media which simulate dolomite occurrences commonly found under the Earth's surface conditions. Hence, we report that the presence of sulfate does not inhibit dolomite precipitation. Further, we hypothesize that, if sedimentary dolomite is a direct precipitate, as in our low-temperature culture experiments, the kinetic factors involved are likely to be quite different from those governing a dolomite replacement reaction, such as in hydrothermal experiments. Consequently, the occurrence and, presumably, growth of dolomite in SO 4 2−-rich aerobic cultures may shed new light on the long-standing Dolomite Problem.

Dolomite Mountains and the origin of the dolomite rock of which they mainly consist: historical developments and new perspectives

AbstractBeginning in the late 18th Century, the Dolomite Mountains in Northern Italy have been the location for major sedimentological developments, from the discovery of the mineral dolomite to the formulation of the coral‐reef hypothesis to explain the origin of the massive dolomite structures that define the splendid scenery of the region. Further, the Dolomite Mountains have inspired voluminous research into the origin of dolomite, questioning whether dolomite is a primary precipitate or a secondary replacement product. Recently, with the recognition that microbes can mediate dolomite precipitation, a new geomicrobiological approach, combining the study of modern natural environments with bacterial culture experiments, is now being used to calibrate or interpret microbial evidence derived from the dolomite rock record. This three‐pronged methodology applied to the study of dolomite formation holds great promise for future research into the ‘Dolomite Problem’ and provides a new impetus to revisit the Dolomite Mountains in the 21st Century.

Dolomite in caves: Recent dolomite formation in oxic, non-sulfate environments. Castañar Cave, Spain

Dolomite is a common mineral in the rock record but rare in recent superficial environments. Where it does occur, it is related to anoxic, sulfate-rich environments and microbial activity. The occurrence of some dolomite deposits in caves, however, indicates that dolomite formation is also possible in oxic, non-sulfate settings. Dolomite is forming at 17 °C and in oxic-vadose conditions on the host rocks and aragonite speleothems of the Castañar Cave, Cáceres, Spain. It appears as spheroids and dumbbells 50–300 μm in diameter that internally consist of micron-sized rhombic to rounded crystals. Initially this dolomite is Ca-rich, non-stoichiometric and poorly ordered. Mg-rich solutions allow the precipitation of metastable Mg-rich carbonates, such as huntite. This soon transforms into this Ca-rich dolomite, which later “ages” to form a more stoichiometric dolomite. These dolomites show similarities to those grown under anoxic, sulfate-rich conditions and their presence in caves provides a different setting that may contribute to the understanding of the “dolomite problem”, including their initial formation and later recrystallization processes.

Low temperature behaviour of natural saline fluid inclusions in saddle dolomite (Paleozoic, NW Spain)

AbstractThe origin of many dolomites is still a matter of debate because of many possible chemical and hydrological conditions of formation. Fluid inclusion studies have been applied in order to improve knowledge about paleofluids responsible for the precipitation of dolomite, and used to define temperatures and salinities. The combination of Raman Spectroscopy and microthermometry is tested here to improve the analytical method to identify the main ion species present in individual inclusions. Natural samples of saddle dolomite from the Cambrian Láncara Fm., Cantabrian Mountains (NW Spain), contain zoned crystals with two‐phase aqueous fluid inclusions (liquid‐rich). The most stable phase assemblage in these inclusions at −150 °C consists of ice, hydrohalite and an unknown salt hydrate. The latter melts between −47 and −41 °C, probably representing a eutectic temperature. Subsequently, ice melts in the range of −32.5 to −29 °C and, finally, hydrohalite melts between −9 and −3.5 °C. Salinities can be calculated in the fluid system H2O–NaCl with addition of another salt, either CaCl2 or MgCl2, and result in 7.5–10.6 eq. mass% NaCl and 17.0–21.0 eq. mass% CaCl2. Dependent on the rate of cooling runs, three different types of metastability may occur, i.e. the absence of hydrohalite, the unknown salt‐hydrate is not formed, and the nucleation of only ice. Salinity calculations from those melting temperatures differ substantially from equilibrium behaviour values. The unknown salt‐hydrate needs to be further specified by comparison to standard solutions. The method gives an opportunity to characterize the major compounds in complex fluid systems active during dolomitization, thus contributing to a better understanding of the ‘dolomite problem’.

Structure and reactivity of the dolomite (104)–water interface: New insights into the dolomite problem

The structure and reactivity of the dolomite (104)–water interface was probed in situ with high resolution X-ray reflectivity and surface force microscopy at room temperature. Measurements in stoichiometric solutions alternating between saturated and supersaturated (log IAP/K = 2.3) conditions show that the dolomite surface termination readily changes in response to solution composition, but these changes are self-limiting and partially irreversible. The freshly cleaved dolomite (104) surface in contact with the saturated solution has a stoichiometric termination, a distinct surface hydration layer and small surface structural displacements, similar to those observed previously at the calcite–water interface. After reaction with supersaturated solutions dolomite is terminated by a two-layer thick Ca-rich film with substantial structural displacements of the cations. With subsequent exposure to a saturated solution this surface was transformed to an interfacial structure different from the freshly cleaved surface, having a reduced density of the outermost surface layer and a Ca-rich second layer. These results provide new insight into the lack of dolomite growth in modern carbonate environments (i.e., the “dolomite problem”), suggesting that this behavior is associated with a combination of thermodynamic and kinetic factors, including (1) growth of compositionally modified epitaxial Ca X Mg 2− X (CO 3) 2 layers having thicknesses limited by lattice strain, (2) slow incorporation of Mg during layer growth, and (3) partial irreversibility of surface reactions.

Sedimentary dolomite: a reality check

Abstract The failure to precipitate dolomite experimentally at low temperatures or from seawater in which it is both supersaturated and the most thermodynamically favoured carbonate phase, together with its unequal distribution through geological time relative to limestone, are all aspects of the ‘dolomite problem’, a subject of continuing controversy. A plethora of physicochemical models has been invoked to explain sedimentary dolomite formation, none of which satisfactorily addresses the basic problem of how kinetic barriers are overcome. These barriers are related to the disproportionate distribution of the component ions of dolomite, cation hydration and ion complexing in seawater. Competing claims for the effectiveness of sulphate as an inhibitor to dolomite formation further confuse the debate, although there are many reports of modern dolomite associated with bacterial sulphate reduction. The uppermost sediments in some lakes of the Coorong region of South Australia comprise almost 100% dolomite, and afford an ideal opportunity to study this association. Samples of lake waters taken during late evaporative stages of several shallow hypersaline dolomitic lakes showed high initial sulphate concentrations, high pH and high carbonate alkalinities. Pore waters from unlithified lake sediment cores directly below the lake-water sample sites showed a substantial and progressive decrease in sulphate concentrations with depth, coupled with an exponential increase in carbonate concentrations, through the sulphate-reduction zone. By the end of the evaporative cycle, sulphate was entirely removed. High bacterial counts on cultures from the sediment cores, and sulphur isotope values consistent with ‘bacterial’ fractionation in lake waters, indicate that the chemical changes in ambient water chemistry can be related to active bacterial sulphate reduction. Laboratory experiments using sulphate reducers cultured from the lake sediments and simulating the anoxic microbiogeochemical environment of the lakes, have resulted in the precipitation of dolomite, demonstrating that bacterial sulphate reduction in the Coorong lakes modifies lake-water and pore-water chemistry so that dolomite precipitation is kinetically favoured. Given the wide spatial and temporal distribution of sulphate-reducing bacteria, and their frequent association, both past and present, with cyanobacteria, it is likely that this process was more widespread in the geological past when dolomite was found in far greater abundance than limestone. Bacterial sulphate reduction may thus have played an important role in dolomite formation throughout the geological record.

Microbial precipitation of dolomite in methanogenic groundwater

We report low-temperature microbial precipitation of dolomite in dilute natural waters from both field and laboratory experiments. In a freshwater aquifer, microorganisms colonize basalt and nucleate nonstoichiometric dolomite on cell walls. In the laboratory, ordered dolomite formed at near-equilibrium conditions from groundwater with molar Mg:Ca ratios of <1; dolomite was absent in sterile experiments. Geochemical and microbiological data suggest that methanogens are the dominant metabolic guild in this system and are integral to dolomite precipitation. We hypothesize that the attached microbial consortium reacts with the basalt surface, releasing Mg and Ca into solution, which drives dolomite precipitation via nucleation on the cell wall. These findings provide insight into the long-standing dolomite problem and suggest a fundamental role for microbial processes in the formation of dolomite across a wide range of environmental conditions.

Hydrous calcium magnesium carbonate, a possible precursor to the formation of sedimentary dolomite

Material of composition Ca1.056 Mg0.944 (SO4)0.035 (CO3)1.965 ⋅0.26H2O has been synthesised from solutions of Ca(NO3)2, MgSO4 and Na2CO3. X-ray diffraction and infrared spectroscopy show this to be identical to "protodolomite" found in nature, and that the structure is related to dolomite but is hydrous, retaining water to beyond 200°C on heating. This hydrous magnesium calcium carbonate grows as spherules, with a distinct core-rim structure apparent in optical microscopy. It seems likely that in low-temperature environments (around 80°C) a dolomite crystal grows in nature via this or a similar hydrous intermediate phase, which may hold the solution to the "dolomite problem".

Surface–water interactions in the dolomite problem

Atomistic computer modelling methods are used to investigate the effect of the strong magnesium–water interaction on the crystal growth of dolomite MgCa(CO3)2. We find that the main dolomite cleavage plane under aqueous conditions is less stable than its counterparts in either pure calcite CaCO3 or magnesite MgCO3. Molecular dynamics simulations of the incorporation of CaCO3 and MgCO3 at calcite growth steps show that, on thermodynamic grounds, magnesium is easily incorporated into a growing calcite crystal, but will then inhibit incorporation of further calcium, which is required for dolomite formation. In addition, segregation of surface magnesium ions into the bulk calcite crystal is energetically unfavourable when the surface is hydrated, but will occur if the water is removed. However, incorporation of magnesium into the bulk crystal of ikaite, a calcium carbonate hexahydrate and precursor of calcite, is energetically feasible and highly site specific, with an optimum uptake of 50% magnesium. Hence, we suggest that hydrous Ca–Mg carbonate phases may be successful as precursors in the laboratory preparation of dolomite.

Dolomite formation and biogeochemical cycles in the Phanerozoic

SummaryBased on present knowledge of the purely chemical controls on the kinetics of massive dolomite formation, the abundance and distribution of dolomite throughout the Phanerozoic remains an enigma, sometimes referred to as the ‘dolomite problem'. Comparing dolomite abundance to secular variation in seawater chemistry indicates that some changes in seawater chemistry are more likely to have resulted from extensive dolomitization rather than to have caused it. The recently formulated microbial dolomite model provides the opportunity to view the geological history of dolomite formation from a new perspective. A biogeochemical approach to the ‘dolomite problem' reveals a plausible connection between Phanerozoic geochemical cycles and dolomite formation. In particular, periods of more extensive dolomitization broadly correlate with diverse indicators of decreased oxygen levels in the atmosphere and oceans. Lowered oxygen levels would have fostered a more active community of anaerobic microbes, including sulphate‐reducing bacteria, which in turn could have led to more extensive dolomitization of marine carbonates.

The dolomite problem; control of precipitation kinetics by temperature and saturation state

The mineral dolomite and the uncertainties surrounding its origin have attracted the attention of earth scientists for over a century. The core of the dolomite is the apparent paradox posed by the paucity of dolomite in modern marine depositional environments versus its relative abundance in the sedimentary rock record. Solving this problem requires knowledge of the conditions under which the mineral forms and the rate of precipitation under those conditions. As a working hypothesis, it is suggested that the precipitation rate of dolomite may be quantified and modeled in a manner similar to other carbonate minerals through application of a rate law that represents the rate as a simple function of saturation index, r = k (Omega - 1) n . This hypothesis is tested in a series of experiments by measuring the steady state rate of dolomite precipitation in a dolomite-seeded flow reactor through analysis of reacted fluid chemistry. By varying temperature from approx 100 degrees to 200 degrees C and saturation index (Omega ) from near saturation to approximately 100, sufficient data were collected to solve for the reaction order and Arrhenius rate constant (k = A exp {-(epsilon A /RT)}) of this rate law. The dolomite produced in these experiments was variable in composition but typically a calcium-rich protodolomite, forming syntaxial overgrowths on the seed material. At the highest supersaturations obtained, formation of distinct nucleation centers was observed. These experiments do confirm a strong temperature dependence for the precipitation reaction (activation energy epsilon A = 31.9 kcal mol (super -1) ) and moderate dependency on saturation index (n = 2.26, log A = 1.05). The experimental findings of this paper suggest that the abundance of dolomite in the sedimentary rock record reflects, at least in part, environmental changes in temperature and seawater chemistry over geologic time.

Tentative kinetic model for dolomite precipitation rate and its application to dolomite distribution

The ‘dolomite problem’ has a long history and remains one of the most intensely studied and debated topics in geology. Major amounts of dolomite are not directly forming today from seawater. This observation has led many investigators to develop geochemical/hydrologic models for dolomite formation in diagenetic environments. A fundamental limitation of the current models for the growth of sedimentary dolomite is the dearth of kinetic information for this phase, in contrast to that available for calcite and aragonite. We present a simple kinetic model describing dolomite growth as a function of supersaturation using data from published high temperature synthesis experiments and our own experimental results. This model is similar in form to empirical models used to describe precipitation and dissolution rates of other carbonate minerals. Despite the considerable uncertainties and assumptions implicit in this approach, the model satisfies a basic expectation of classical precipitation theory, i.e., that the distance from equilibrium is a basic driving force for reaction rate. The calculated reaction order is high (~ 3), and the combined effect of high order and large activation energy produces a very strong dependence of the rate on temperature and the degree of supersaturation of aqueous solutions with respect to this phase. Using the calculated parameters, we applied the model to well-documented case studies of sabkha dolomite at Abu Dhabi (Persian Gulf), and organogenic dolomite from the Gulf of California. Growth rates calculated from the model agree with independent estimates of the age of these dolomites to well within an order of magnitude. A comparison of precipitation rates in seawater also shows the rate of dolomite precipitation to converge strongly with that of calcite with increasing temperature. If correct, this result implies that dolomite may respond to relatively modest warming of surface environments by substantial increases in accumulation rate, and suggests that the distribution of sedimentary dolomite in the rock record may be to some extent a temperature signal.

Microbial Mediation of Modern Dolomite Precipitation and Diagenesis Under Anoxic Conditions (Lagoa Vermelha, Rio de Janeiro, Brazil)

ABSTRACT Dolomite, despite its thermodynamic stability and abundance in the ancient rock record, is rarely found forming in Holocene environments. This enigma is frequently called the Dolomite Problem. The recent discovery of modern dolomite formation in Lagoa Vermelha, a shallow-water isolated coastal lagoon east of Rio de Janeiro, Brazil, provides a new environment to investigate the factors promoting dolomite precipitation under earth surface conditions. Lagoa Vermelha serves as a natural laboratory in which the dolomite formation process was studied using an integrated hydrologic, geochemical, and sedimentological approach. The results of this study indicate that Ca-dolomite precipitation occurs under anoxic hypersaline conditions within a black sludge layer directly overlying the water/se iment interface. With deposition, the dolomite undergoes an ageing process, whereby increased ordering of the crystal structure occurs. Both the initial precipitation and subsequent early diagenesis are strongly mediated by microbial activity. In fact, using sulfate-reducing bacteria cultured from Lagoa Vermelha samples, a highly ordered dolomite has been produced in the laboratory at low temperatures. These experimental results combined with the study of the natural environment mandate that a microbial factor be added to the list of factors capable of causing dolomite precipitation. Considering the Lagoa Vermelha system, we propose a new actualistic model for dolomite formation, which we call the microbial dolomite model.

Microbial mediation as a possible mechanism for natural dolomite formation at low temperatures

DOLOMITE (CaMg(CO3)2) is a common carbonate mineral which is found in much greater abundance in ancient rocks than in modern carbonate environments. Why this is so remains a mystery. Over the past 30 years, dolomite formation has been observed in several modern environments, and various thermodynamic, kinetic and hydrological factors have been proposed to explain its formation1,2. But attempts to precipitate dolomite at low temperatures in the laboratory have been unsuccessful3,4, and the 'dolomite problem' remains a source of controversy in sedimentary geology5-7. Here we describe experiments in which a ferroan dolomite with a fairly high degree of cation order was precipitated in the presence of sulphate-reducing bacteria from the Desulfovibrio group. We propose that the direct mediation of these anaerobes can overcome the kinetic barrier to dolomite nucleation, and that they may play an active role in the formation of this mineral in natural environments.

A stable-isotope study of magnesites from the Rum Jungle Uranium Field, Australia: Implications for the origin of strata-bound massive magnesites

The contrast between the scanty magnesite deposition in modern sedimentary environments and the large strata-bound magnesites of the past constitutes a “magnesite problem” which, unlike the “dolomite problem”, has so far attracted little attention from the geologists. Magnesite beds up to 700 m thick from the Lower Proterozoic rocks of the Rum Jungle Uranium Field, Northern Territory of Australia, offer attractive circumstances for studying the isotope systematics of coarse-crystalline magnesite and coexisting dolomite. Magnesites yield δ 18O and δ 13C compositions of + 11.2 to + 14.6‰ (SMOW) and + 1.0 to + 5.8‰ (PDB), respectively. Dolomites yield δ 18O- and δ 13C- values of + 11.7 to + 21.9‰ and − 4.0 to + 2.9‰, respectively, and show an 18O enrichment gradient away from the magnesite. Fluid-inclusion homogenization temperatures, used in conjunction with the isotope determinations and an approximate magnesite-water equation, indicate that magnesites were formed in the temperature range of 100–200°C from fluids that became progressively enriched in 18O. The results support an epigenetic-metasomatic replacement model of dolomite by magnesite under the influence of invasive hot Mg-rich basinal fluids. Strata-bound massive magnesites, which abound particularly in Archean-Proterozoic rock sequences, generally preserve the sedimentary structures but do not retain primary isotopic compositions and are therefore unsatisfactory indicators of oceanic isotope evolution and ocean paleotemperatures.

Dolomitization; a critical view of some current views

The principal models in vogue today for dolomitization are the mixing zone and the sabkha models. Despite the wide acceptance of these models, there has been little critical assessment of their validity. Such an assessment is the objective of the present paper. A close look at the mixing-zone model (Badiozamani 1973) reveals several serious weaknesses: 1) the range of freshwater-seawater mixtures that meet the Dorag requirement for dolomitization shrinks to a small window if the geologically more realistic disordered dolomite is used in the calculations instead of the ordered dolomite on which the model is based; 2) in none of the known modern coastal mixing zones in limestone or lime sediments has replacement by dolomite been observed; 3) in a number of dolomites interpreted to be of mixing-zone origin, dolomite has precipitated without dissolution of the calcite substrate, evidence that negates the fundamental premise of the Dorag model. In addition to these weaknesses, isotope and trace-element data used to identify mixed-water dolomite are inadmissible because 1) isotopic fractionation factors for dolomites remain unresolved; 2) isotope values (uncorrected for temperature) for a host of dolomites, interpreted to be of different origins, overlap; 3) nonisomorphous trace elements, such as Na, in dolomite cannot, on theoretical grounds, be relied on to identify dolomitizing fluids. Similar overall objections can be brought against Folk and Land9s (1975) schizohaline version of mixing-zone dolomitization. In summary, mixing-zone models have such weak underpinnings that they should be questioned as viable explanations for massive dolomitization. Contemporaneous dolomite formation in modern sabkhas is well documented, but the important question of whether the mechanism of dolomite formation is replacement or direct precipitation remains to be resolved. In the well-studied sabkha at Abu Dhabi, brine chemistry changes have been used as evidence of a replacement origin for the dolomite, but it is shown here that this evidence is far from unequivocal. An alternative origin of direct precipitation of dolomite is offered, an origin in keeping with dolomite precipitation known from other modern sabkhas and saline lakes, and in line with our laboratory experience with dolomite synthesis. Moreover, it is suggested here that, in general, contemporaneous dolomite will form at low temperatures only by direct precipitation, a mechanism that requires conditions of highly super-saturated waters of high Mg/Ca ratio and elevated CO 3 -HCO 3 concentrations. This may explain why modern dolomite is mainly restricted to evaporitic environments, a bias not shared by ancient dolomites. In contrast, replacement dolomite appears to require, at low temperatures, long reaction times > or = 10 4 yr?), a requirement that is mainly to be met in large, regional groundwater-flow systems, both marine and nonmarine. A third dolomitization model considered here is that of Baker and Kastner (1981), based on the experimental finding that sulfate ions inhibit or retard dolomitization. A number of modern sedimentary dolomites are not in accord with this model in that they are forming from brines with large sulfate concentrations, 2 to 70 times that of seawater. Thus, the Baker-Kastner model should be held in abeyance until these serious contradictions are resolved. The current emphasis on mixing-zone and sabkha dolomitization has diverted attention from other promising avenues of approach to the dolomite problem. Four of these avenues, each of which deemphasizes the special water approach, are briefly addressed and are as follows: 1) influence of temperature and time; 2) mass transfer processes; 3) burial diagenesis of epigenetic dolomites; 4) fluid-inclusion studies. At elevated temperatures the dolomite problem essentially disappears (ordered dolomite can be made in the laboratory in days at 100 degrees C). What is more, at temperatures above 60 degrees C, Ca-rich waters become dolomitizing fluids, which makes most natural subsurface waters capable of dolomitization. At low temperatures, time may be the key element, so that seawater will become a major dolomitizing fluid only where stable circulation systems, such as Kohout convention, can drive seawater through carbonate platforms for many thousands to millions of years. This paper has tried to show that currently favored models of dolomitization carry serious uncertainties, enough to warn us to look more critically at the validity of these models, and also, it is hoped, enough to spur new efforts to find new solutions to the problems of dolomitization.

Dolomite formation in the seawater–freshwater interface

The ‘dolomite problem’ long defied solution by geologists largely because it could not be resolved in experiments or by comparative studies with Recent sediments. Most of the known Pleistocene dolomites are from evaporative environments, whereas most of the ancient ones occur in rocks which show no evidence of intertidal or supratidal evaporative conditions. A possible environment for Recent formation of dolomites is the mixing zone between freshwater and seawater. We consider here dolomite formation in the interface mixing zone between freshwater and seawater and suggest that this type of dolomite formation is much more common than was previously indicated.

Zonal Arrangement of the Triassic Rocks of the Western Carpathians: A Contribution to the Dolomite Problem

ABSTRACT The Triassic of the Western Carpathians consists of hypersaline sequence in the north, grading southwards into dolomite and then limestone. This zonal arrangement was due to the development of algal reefs (banks) in the south, with subsequent hypersaline lagoonal evolution in the back-reef (bank) area. It is proposed that the dolomites within the lagoonal sequence originated by a very early diagenetic replacement of an aragonite mud, with clay minerals serving as charged net membranes or seeds for penetrating interstitial brines with high Mg/Ca ratios. It is also proposed that these brines were squeezed out from the lagoonal sequence into the porous algal reefs (banks) during later stages of diagenesis, causing dolomitization of the lagoonal side of the reefs (banks).