Mixing Zone Model Research Articles

Characterization of Miscellaneous Effluent Discharges from a Mobile Offshore Drilling Unit to the Marine Environment.

A study was performed to evaluate the potential biological impacts from 8 different miscellaneous discharges from an oil and gas mobile offshore drilling unit (MODU) including deck drainage, desalination unit waste, boiler blowdown, fire control system test water, noncontact cooling water, and bilge water. Samples were evaluated for toxicity using a rapid (<1 h) initial screening test (echinoderm [Dendraster excentricus] fertilization test), and if toxicity was found, further testing was conducted using 3 chronic whole-effluent toxicity tests. This additional testing included the embryo larval development 72-h echinoderm (D. excentricus); 7-d mysid (Americamysis bahia) survival, growth, and fecundity invertebrate test; and 7-d topsmelt (Atherinops affinis) survival and growth fish test. Toxicity identification evaluations were performed on 3 discharges that consistently elicited a toxic response during whole-effluent toxicity testing. To place the results of the toxicity testing into the context of environmental risk, the spatial extent of potential biological effects was investigated using the CORMIX mixing zone model. The output of the modeling indicated that discharge of selected effluents did not result in concentrations, or duration of exposure, that would elicit toxic effects to organisms living in the surrounding environment. The present study provides a comprehensive data set that was used to characterize potential toxicity and environmental risk of MODU "miscellaneous discharges" which could help inform future risk assessments of these discharges. Environ Toxicol Chem 2019;38:2811-2823. © 2019 SETAC.

The Effect of Dolomitization on Mergi Formation Sequences (Cenomanian- Early Turonian ) at The Type Section, Northern Iraq

In order to study the effect of dolomitization on Mergi Formation sequences (Cenomanian – Early Turonian), Field and petrographical examinations were carried out on fourty six meters (46m) of the type section of this formation. It has been observed that the formation is highly affected by diagenesis processes, especially dolomitization. On the other hand, the effects of other processes such as. cementation, dissolution, micritization and compaction in its both types, physical and chemical were less than dolomitization. Dolomite crystals appeared as rhomb-shaped euhedral to subhedral. The main diagnosed fabrics were Aphanotopic, Contact-rhomb porpherotopic, fogged mosaic, Sutured mosaic, Saddle dolomite and Cloudy-centered clear-rimmed dolomite fabrics. It is concluded that formation of dolomite was by the mechanism of mixing zone model and subsurface dolomitization model.(.

Dolomitization of the Upper Jurassic carbonate rocks in the Geneva Basin, Switzerland and France

The Upper Jurassic carbonates represent important potential targeted reservoirs for geothermal energy in the Geneva Basin (Switzerland and France). Horizons affected by dolomitization, the focus of the present study, are of particular interest because they proved to be productive in time-equivalent deposits currently exploited in Southern Germany. The study is based on sub-surface samples and outcrops in the Geneva Basin. Petrographic analyses allowed to constrain the paragenesis of the Upper Jurassic units prior to discussing the cause(s) and effect(s) of dolomitization. Data reveal that the facies are affected by early and late diagenesis. All samples show at least two stages of burial blocky calcite cementation with the exception of those from the sub-surface, which display an incomplete burial blocky cementation preserving primary intercrystalline porosity. Dolomitization affected all units. The results point to an early dolomitization event, under the form of replacement dolomite. Dedolomitization, through calcitization and/or dissolution, is an important process, creating secondary pore space. Results of the present study favor a reflux model for dolomitization rather than the mixing-zone model suggested in earlier work. However, considering the geodynamic context, other dolomitization models cannot be excluded for the subsurface. The presence of secondary pore space might contribute to the connectivity of the porous network providing enhanced reservoir properties. These results are a first step towards a better understanding of the diagenetic history of the Upper Jurassic in the Geneva Basin. Moreover, it provides a reasonable framework for further geochemical analyses to constrain the nature and timing of fluid migration. The paragenesis and the dolomitization model hold the potential to help in ongoing exploration for geothermal energy beyond the Geneva Basin.

Dolomitization Mechanism Based on Petrography and Geochemistry in the Shotori Formation (Middle Triassic), Central Iran

Middle Triassic carbonate sequences of Shotori Formation have a thickness of 70 m and are deposited Robat-e-Kalmard region of Tabas city in Central Iran basin. Gradationally and conformably overlying Sorkh shale Formation, Shotori Formation, mostly composed of medium to thick dolomites (50 m), interbeded with thin lime and sandstones, is disconformable by a laterite horizon at its upper boundary. This Formation mainly consists of fine-to-coarsely crystalline dolomites. According to petrographic (fabric and grain size) and geochemical (elemental analysis of Ca, Mg, Na, Sr, Fe, Mn) evidence, five various types of dolomites were recognized in Shotori Formation. This variety results from early and late diagenetic processes, triggering a change in dolomitizing fluids and thereby forming various dolomites. Geochemical studies have revealed that the dolomites of Shotori Formation have formed under meteoric diagenesis and reducing conditions. Various dolomitization mechanisms are proposed for various types of dolomites; that is to say, Sabkha model is considered for type 1 dolomite, mixing zone model for type 2 and 3 dolomites and burial model for type 4 and 5 dolomites.

CFD modelling of marine discharge mixing and dispersion

Many municipal and industrial outfalls release effluent into water bodies. Temperatures and/or salinities of the effluent and receiving water may be different, and other dissolved/suspended constituents may be present. Away from the outfall, mixing processes usually reduce constituent concentrations to acceptable levels for local water quality. However, at the outlet, concentrations may be sufficient to cause environmental concern. Accurate dispersion prediction is therefore important. Two-stage modelling approaches are typically used: (1) a near-field dilution assessment, based on mixing zone or empirical models; and (2) a mid- /far-field dispersion assessment, using hydrodynamic models. As computational and numerical methods improve, computational fluid dynamics (CFD) can increasingly model dispersion across both regions. These methods require validation of the underlying discretisation and turbulence schemes. Preliminary validation is presented for near-field simulations of buoyant and dense jets. Buoyant jet predictions compare well with established results. Preliminary simulations under-predict entrainment into the dense jet, overly-predicting near-field concentrations.

Microbial mixing zone dolomitization and karst development within Isla de Mona Dolomite, Isla de Mona, Puerto Rico

Modification of the mixing zone model to include microbial processes that promote dolomitization explains many of the petrographic, isotopic, and outcrop observations found on Isla de Mona. Large flank margin caves at the contact between the Isla de Mona Dolomite and the capping Lirio Limestone formed during prolonged, likely episodic, periods with a stable lens position. These periods would produce a stable halocline. The halocline is hypothesized to be the zone of dolomitization in the microbial mixing zone model. Collection of organics at the density interface of the halocline coupled with sulfate reduction by sulfate-reducing bacteria would establish a geochemical environment that promotes dolomitization of the precursor limestone. This microbial community and geochemical environment would overcome the kinetic and thermodynamic barriers associated with the previous mixing zone model.

Episodic Fluid Flow and Dolomitization By Methane-Bearing Pore Water of Marine Parentage In An Accretionary Prism Setting, Barbados, West Indies

Abstract Barbados is a small island located on the crest of an accretionary prism at the eastern edge of the Caribbean Sea. Dolomite occurs in relatively small amounts in Pleistocene carbonates and in the underlying Oligocene–Miocene chalk, which also contains a few small caves, distinctive authigenic calcites, and oil that ascended through a dense networks of fractures. The stable isotopes of calcites form a positive trend, while those of the dolomites form a negative trend. Dolomite δ18O values range from about +1 to +8‰ PDB, and δ13C values range from about +2 to −25‰ PDB. Sr-isotope ratios range from 0.7079 to 0.7094, which corresponds to a secular range from the Oligocene to the Holocene. The data presented in this study support the notion that the Barbados accretionary wedge has undergone a prolonged history of compression with concomitant expulsion of fluids that varied through time and space. At times these fluids were diluted relative to seawater and also undersaturated with respect to calcite, which resulted in limited dissolution along fractures and in their immediate vicinity. At other times the expelled pore fluids were prone to limited dolomitization, forming pods of dolomite. At yet other times the expelled fluids formed calcite as interstitial cements and around submarine points of exit in what appears to be “cold seeps” with calculated temperatures as low as +4 °C. The expelled fluids were chemically slightly modified seawater that contained thermogenic methane. Calculated temperatures of dolomitization range between +4 and +25 °C, which suggest fluid expulsion from sub-sea depths of zero to about –650 m. A temperature of +4 °C is the lowest ever determined for dolomite formation. The Sr-isotope data suggest that the hypogene fluids were expelled episodically between about 8 and 2 My ago. This study provides support for the hypothesis that dolomitization can result from fluid expulsion in an accretionary prism setting. However, the volumes of dolomite formed in Barbados are small and probably are similarly small in most other accretionary prism settings. Thus, regionally extensive, pervasive dolomitization is not to be expected from the dolomitizing process identified here. If elevated to the status of a model, dolomitization via fluid expulsion from an accretionary prism would be comparable in importance to the mixing zone model of dolomitization.

Coupling bacterial decay and hydrodynamic models for sewage outfall simulation

The evaluation of wastewater concentrations in coastal environments has to be accurate for water quality control. The description of the physical process in a comprehensive model is a complex task. Due to the different spatial and temporal scales involved, the use of different numerical models is required: far-field hydrodynamic models; near-field and pollutant transport models. In this paper, a coupled application of these models is described. Capabilities and limitations of modelling techniques applied to wastewater mixing and dispersion from submerged multiport discharges are discussed for environmental impact analysis. The recommended procedure combines a near-field mixing zone model, such as NRFIELD, with the Lagrangian far-field flow and water quality models within SisBaHia® (http://www.sisbahia.coppe.ufrj.br/). The results showed that the model coupling improves the description and analysis of treated wastewater discharges over large scales. This allows a more realistic evaluation of the environmental impact due to domestic sewage discharges in coastal waters.

Ascending Freshwater-Mesohaline Mixing: A New Scenario for Dolomitization

Abstract The process of mixing-zone dolomitization has received enough criticism in recent years to suggest that it no longer be considered viable for making large amounts of dolomite. This study proposes an alternative model that still requires fluid mixing. In this model, ascending flow of freshwater mixes with evaporated seawater, leading to extensive dolomitization. The new model is constrained by data from upper Miocene strata of southeast Spain, where dolomite yields fluid-inclusion Tm-ice data that range from −0.2 to −2.3 °C, indicating precipitation from fluids of 4 to 43 ppt seawater salt equivalent, thus confirming fluid mixing. The δ13C and δ18O of the dolomite ranges from −4.5 to +3.0‰ VPDB and +0.9 to +6.0‰ VPDB, respectively, and displays positive covariance. Although such positive covariation may represent fluid mixing, the mixing of two dolomites during sampling, or an originally homogeneous dolomite that has been partially altered (recrystallized), fluid-inclusion data and cathodoluminescence petrography disprove the mixture during sampling and partial-alteration hypotheses. Modeling of the most positive dolomite δ18O values indicates an evaporated seawater endmember of 43 ppt salinity, identical to that of the most saline fluid inclusions. Because paleotopography is preserved in the study area, the spatial variation in dolomite geochemistry can be mapped to evaluate if it is consistent with the hydrogeology of a typical mixing-zone model or an alternative. The data reveal that both downdip and updip areas were influenced by high-salinity and low-salinity fluid endmembers, and that stratigraphically lower units have more depleted δ13C and δ18O than upper units. These results are consistent with a model of upward flow of freshwater that provides the mechanism of mixing of freshwater and evaporated seawater. Upward flow and degassing of CO2 from loss of pressure contributed to nearly platform-wide dolomitization in this succession, and may have analogs in other ancient successions.

Surface Water Quality at Boete Slimes Dam, AngloGold Ashanti Ltd Obuasi

Mining activities around the Boete Slimes Dam (BSD) of AngloGold Ashanti, Obuasi Mine, and its likely impact on surface water quality around the BSD, was investigated. Analyses of samples taken from Pompo stream and Kwabrafo stream and the confluence of the two streams draining the Boete Slimes Dam area showed that Conductivity, Total Suspended Solids (TSS), Total Dissolved Solids (TDS), Sulphate (SO42-), Cyanide (CN), and Arsenic (As) had values higher than EPA Guideline values,whiles Zinc (Zn), Lead (Pb) and Iron (Fe) were within acceptable range. Comparison of calculated and measured levels of conservative parameters at the confluence of the two streams using the mixing zone model corresponded well with a regression coefficient of between 0.79 and 0.93. The paper concluded that there is negative impact of the activities at Boete on the quality of Kwabrafo stream. The study also revealed that the company could save at least $1776 per sampling point annually since sampling at the confluence of the two streams could be suspended.

Comparing four mixing zone models with brine discharge measurements from a reverse osmosis desalination plant in Spain

Nuevo Canal de Cartagena (Spain) desalination plants discharge hypersaline effluent through a submarine outfall pipeline, creating a negatively buoyant brine jet. Many near-field mixing models are used in the prediction and management of brine discharges, but they have rarely been compared with field salinity measurements obtained directly inside the brine jet. Two divers obtained these field measurements and compared them with CORMIX1, CORJET, MEDVSA and UM3 mixing zone model predictions. In general, each model was quite conservative in its results, except UM3, whose prediction presented the best approximation to measured data. It is concluded that direct field measurements should be essential when testing the accuracy of current models or developing new near-field mixing models.

Optimal expansion of a coastal wastewater treatment and ocean outfall system under uncertainty (I): simulation analysis

An ocean outfall system is an integrated engineering system in which a coastal wastewater treatment plant (WWTP) is connected with a pipeline that discharges wastewater effluent into a body of water. Safety factors to accommodate for uncertainty in modelling of such systems often result in a degree of wastewater treatment that is not cost-effective. This paper presents the first part of an integrated simulation and optimisation analysis to resolve some such uncertainties in the Cornell mixing zone expert system (CORMIX) model by testing it against a laboratory flume. CORMIX is a United States Environmental Protection Agency (USEPA)-supported mixing zone model for environmental impact assessment of regulatory mixing zones resulting from continuous point source discharges. This model especially emphasises prediction of plume geometry, flow classification, and dilution characteristics within an initial mixing zone of a receiving water body so that compliance with regulatory constraints can be evaluated. With the aid of a flume test, such information may be used as stochastic inputs to an optimisation model to assess the cost-effectiveness of discharge treatment. A total of 19 dye-study scenarios were designed and carried out for single-port (co-flow or counter-flow) and multi-port experiments according to the physical features of the outfall pipe to be expanded in the coastal environment of Taiwan Strait. The flume test showed a good agreement between CORMIX1/CORMIX2 models and the dye-study data. This approach is a stepwise, flexible technique allowing for continuous feedback of the varying design and operating conditions and the observed impacts on coastal waters and may be used to identify the optimal expansion strategy of an integrated coastal WWTP and ocean outfall system such as the case study presented for Kaohsiung, South Taiwan.

Three-Dimensional Numerical Modeling of Initial Mixing of Thermal Discharges at Real-Life Configurations

A three-dimensional Reynolds-averaged Navier–Stokes computational fluid dynamics (CFD) model is developed for simulating initial mixing in the near field of thermal discharges at real-life geometrical configurations. The domain decomposition method with multilevel embedded overset grids is employed to handle the complexity of real-life diffusers as well as to efficiently account for the large disparity in length scales arising from the relative size of the ambient river reach and the typical diffuser diameter. An algebraic mixing length model with a Richardson-number correction for buoyancy effects is used for the turbulence closure. The governing equations are solved with a second-order-accurate, finite-volume, artificial compressibility method. The model is validated by applying it to simulate thermally stratified shear flows and negatively buoyant wall jet flows and the computed results are shown to be in good overall agreement with the experimental measurements. To demonstrate the potential of the numerical model as a powerful engineering simulation tool we apply it to simulate turbulent initial mixing of thermal discharges loaded from both single-port and multiport diffusers in a prismatic channel and a natural river. Comparisons of the CFD model results with those obtained by applying two widely used empirical mixing zone models show that the results are very similar in terms of both the rate of dilution and overall shape of the plumes. The CFD model further resolves the complex three-dimensional features of such flows, including the complex interplay of the ambient flow and thermal discharges as well as the interaction between each of discharges loaded from multiple ports, which are obviously not accessible by the simpler empirical models.

Upper Jurassic thrombolite reservoir play, northeastern Gulf of Mexico

In the northeastern Gulf of Mexico, Upper Jurassic Smackover inner ramp, shallow-water thrombolite buildups developed on paleotopographic features in the eastern part of the Mississippi Interior Salt basin and in the Manila and Conecuh subbasins. These thrombolites attained a thickness of 58 m (190 ft) and were present in an area of as much as 6.2 km2 (2.4 mi2). Although these buildups have been exploration targets for some 30 yr, new field discoveries continue to be made in this region. Thrombolites were best developed on a hard substrate during a rise in sea level under initial zero to low background sedimentation rates in low-energy and eurytopic paleoenvironments. Extensive microbial growth occurred in response to available accommodation space. The demise of the thrombolites corresponded to changes in the paleoenvironmental conditions associated with an overall regression of the sea. The keys to drilling successful wildcat wells in the thrombolite reservoir play are to (1) use three-dimensional seismic reflection technology to find paleohighs and to determine whether potential thrombolite reservoir facies occur on the crest and/or flanks of these features and are above the oil-water contact; (2) use the characteristics of thrombolite bioherms and reefs as observed in outcrop to develop a three-dimensional geologic model to reconstruct the growth of thrombolite buildups on paleohighs for improved targeting of the preferred dendroidal and chaotic thrombolite reservoir facies; and (3) use the evaporative pumping mechanism instead of the seepage reflux or mixing zone models as a means for assessing potential dolomitization of the thrombolite boundstone.

Discussion of “Sensitivity Analysis and Comparative Performance of Outfalls with Single Buoyant Plumes” by Maria A. Economopoulou, Alexia A. Economopoulou, and Alexander P. Economopoulos

The discussers’ criticism focuses on the validity of the literature models selected by the writers, and not on the analysis performed in the paper with the use of selected models. The discussers consider the analysis in the paper incomplete as the predictions of the literature models used are not in agreement with those of CORMIX, a USEPA-approved computer model. However, the models used in the paper are well established in literature, and their results are carefully validated with the Visual Plume (VP), a widely used USEPA computer model (Frick et al. 2003). Based on the above, the underlined issue raised by the discussers is the differences in the predictions between CORMIX on one hand, and the VP and other well-established models on the other. It appears rather odd that the discussers, who happen to be the developers of CORMIX, direct their criticism against the users of alternative models; this is hardly the most effective way to resolve the plume modeling validation issue. Nonetheless, the writers feel compelled to defend their selection of the literature models used in the paper for analysis and validation, and for this reason they note the following. The discussers state that the analysis is incomplete, elaborating further that the relations of the models selected and used in the paper, as well as in the VP UM3 and VP NRFIELD models used for validating the results, do not consider the gravitational density current flow that develops after initial mixing. They also state that gravitational spreading and mixing is absent from most mixingzone models, “with one important exception—the USEPAapproved CORMIX system.” To prove the importance of these mechanisms, the discussers refer to their previous discussion (Jirka and Doneker 2003) of field dye tests and modeling of the Ipanema Beach outfall (Carvalho et al. 2001), in which density current mixing appears to be important. It is interesting to note that in the above paper (Carvalho et al. 2001) and its closure (Roberts and Carvalho 2003), the predictions of RSB, UM3, and CORMIX were compared with field test results and it was concluded that “with suitable assumptions, all the models reasonably predict near field dilution,” and that “CORMIX grossly underestimates the observed thicknesses” of the spreading layer. Furthermore, the discussers’ statement that the models do not include gravitational spreading is incorrect for NRFIELD. NRFIELD does include spreading and the mixing in the spreading layer, as dealt with at length in Roberts and Carvalho (2003). One should recognize that plume mixing involves complex physical phenomena, not all of which can be reliably modeled at

NSW ocean outfalls: using near field models to investigate hydrodynamic performance

Currently 35 licensed Sewage Treatment Plants discharge various grades of treated sewage to the ocean via shoreline, extended or deep submerged outfalls along the New South Wales coast. Outfalls range from single pipes located in beach foredunes to multi-port gas-burner type diffusers located 3–4km offshore at depths of 60–80m.The relative hydrodynamic performance of NSW coastal sewage discharges was examined using the United States Environmental Protection Agency’s hydrodynamic mixing zone model CORMIX (Cornell University’s expert mixing zone model). Examination of near field model predictions demonstrates that NSW ocean outfalls fall into three broad categories: 1) hydrodynamically efficient multi-port diffuser type outfalls, 2) moderately hydrodynamically efficient single port shoreline outfalls, and 3) hydrodynamically inefficient single port shoreline outfalls. From the results of this study, multi-port diffuser outfalls are the most efficient in dispersion of sewage-derived pollutants. In some situations, however, a well-designed shoreline or nearshore discharge can achieve moderate to high dilutions. Such outfalls are generally less expensive to construct, operate and maintain compared with multi-port diffuser type outfalls.

Visual Plumes mixing zone modeling software

The US Environmental Protection Agency has a history of developing plume models and providing technical assistance. The Visual Plumes model (VP) is a recent addition to the public-domain models available on the EPA Center for Exposure Assessment Modeling (CEAM) web page. The Windows-based VP adapts, modifies, and enhances the earlier DOS-based PLUMES with a new interface, models, and capabilities. VP is a public platform for mixing zone models designed to encourage the continued improvement of plume theory and models by facilitating verification and inter-model comparison. Some examples are presented to illustrate VP’s new capabilities. One demonstrates its ability, for reasonably one-dimensional estuaries, to estimate background concentrations due to tidal re-circulation of previously contaminated receiving water. This capability depends on the optional linkage to time-series input files that enables VP to simulate mixing zone and far-field parameters for long periods. Also described are the new bacterial decay models used to estimate depth changes in first-order decay rates based on environmental stressors, including solar insolation, salinity, and temperature. The nascent density phenomenon is briefly described as it is potentially important to Outer Continental Shelf (OCS) oil exploration discharges.

Impact of Proposed Burlington and Hamilton Sewage Discharges in Western Lake Ontario

Abstract The alternate strategy of open-lake discharge may alleviate the need for unusually stringent treatment needed to meet water quality goals of the Hamilton Harbour Remedial Action Plan (RAP). The latest update of the RAP recommended a study of the possibility of offshore discharges. A study conducted for the City of Burlington has proposed a location for outfall in Lake Ontario. This paper utilizes a combination of physical limnological data and mathematical models to predict the waste plume characteristics for the proposed outfall in the lake. Near-field dilutions obtained from a mixing zone model show that, for treated effluents with a discharge condition of 2 m3/s at the proposed outfall site at Burlington, the dilution ratios are in the range of 13:1 to 28:1 for weak to moderate currents during summer stratification. Winter dilution ratios increased to 21:1 to 96:1 for moderate currents. The recommended site for open-lake outfall provides acceptable near-field dilutions for treated effluents under typical lake currents and density structure. The extension of outfall to a location farther offshore is only marginally beneficial. With the proposed Burlington outfall location and discharge conditions, no far-field contamination is observed near the beaches or nearby water intakes for typical summer and winter conditions. Thus, this study indicates that by discharging the treated sewage from an outfall in Lake Ontario it is possible to achieve the Hamilton Harbour RAP goals.

Difficulties in interpreting fast mixing in the radiometric dating of sediments using210Pb and137Cs

210Pb geochronologies should be validated with independent tracers such as 137Cs. In the cases with constant 210Pb activity in the topmost sediments, the presence of a distinct 137Cs peak within the 210Pb plateau has been used as a definitive demonstration of acceleration (increase in the sedimentation rate in recent years) versus fast mixing. Nevertheless, some limitations can be identified in the use of semiquantitative arguments, and a global understanding of the whole 137Cs activity profile is then required. Particularly, the incomplete mixing within the top sediment zone (described through the Incomplete Mixing Zone model) can explain quantitatively and simultaneously the 137Cs peak and the flattening in the 210Pb activity profile. This is demonstrated using selected examples from literature data. Thus, measured constant 210Pb activities in the top 6 cm of a sediment core from Lake Zurich. Nevertheless, they found 7Be only in the uppermost layer, the distinct 137Cs maximum at 6 cm depth, and undisturbed varves. The fast mixing seemed then opposed to common sense. The constant rate of supply model shows acceleration and it adequately matches the position of the 1963 137Cs peak. Nevertheless it fails to explain the whole 137Cs profile when handling time series of 137Cs atmospheric deposition. Finally, it is shown how the incomplete mixing of the activity (through the pore water) over a certain mass depth at the top sediment, with a finite value of the mixing coefficient, can quantitatively explain the whole activity versus mass depth profiles of 137Cs and 210Pb, and the presence of 7Be only in the uppermost sediment layer. A further validation of these ideas is presented from other literature data.

Dolomitization and isotope stratigraphy of the Vazante Formation, São Francisco Basin, Brazil

The Vazante Formation consists of approximately 1700 m of mainly microbial mats and stromatolitic reefal lenses that were deposited on shallow marine platform and have been entirely dolomitized. Samples representing different dolomite generations were taken from three boreholes covering the entire spectrum of the Vazante carbonates. Dolomites can be classified, based on petrography and geochemistry, into four generations ranging in crystal size between ∼4 μm (almost micritic) and 3 mm and occurring as both replacements and cements. The Sr/Ca molar ratios, calculated for the dolomitizing fluid (0.0006–0.0138), suggest a contribution from a non-marine, possibly meteoric, water component. The δ 18O and δ 13C values vary from −0.1 to −14.3‰ (PDB) and 0.2 to −2.3‰ (PDB), respectively. Fluid-inclusion study suggests that dolomitization must have commenced at a temperature lower than about 50 °C. A mixing-zone model of dolomitization for Dolomites I–III satisfies the constraints from elemental chemistry, δ 18O, 87Sr/ 86Sr and fluid inclusions. The petrographic and chemical criteria of the latest generation, Dolomite IV, reflect conditions of deep burial environment at temperatures above 120–130 °C. The δ 13C values show only small variations at the base of the formation, followed by a major negative plunge (∼4‰) at the top, this plunge correlated with the Sturtian glacial phase. In contrast to C-isotopes, only few samples may have retained their near-primary 87Sr/ 86Sr values of 0.70614–0.70734. The lowest 87Sr/ 86Sr value (0.70614), from fibrous cement in the upper part of the formation, correlates with the negative δ 13C shift and is also consistent with the Sr-isotope signature proposed for the glacial Sturtian seawater. The δ 34S values, obtained from sulfates trapped in carbonates, range between 10.8 and 16.9‰ with a jump to 21.3‰ in the overlying formation. All isotope signals are within the range suggested for the early Neoproterozoic seawater, but the Precambrian baseline is poorly known and additional work is required to confirm this tentative assignment.