The timing of dolomitization is difficult to constrain. Many studies suggest that it is early, but the rates at which dolomitization occurs in nature are rarely defined. This study aims to pinpoint the timing of dolomitization in sedimentary environments using detailed petrography and sedimentological observations from two near-surface research cores in the lower part of the Dam Formation (lower Dam) in southwestern Qatar. The lower Dam, deposited in a shallow-marine-estuarine setting with frequent sea-level fluctuations, presents strong sedimentological and stratigraphic constraints for the timing of dolomitization. The highly variable environmental conditions resulted in deposition of marine to brackish-water carbonates, land-derived siliciclastics, and intermittent subaerial exposure, creating a high-frequency cyclicity that allows for cycle-by-cycle diagenetic analysis in stratigraphically distinct depositional units. Observations from core and thin-section petrography permit the identification of five distinct lithofacies, which are classified as either carbonate-dominated or siliciclastic-dominated lithotypes. Dolomite, the most abundant mineral in the carbonate-dominated intervals, occurs as a fabric-retentive very fine crystalline (VFxn) or isopachous dolomite. Crosscutting relationships in each stratigraphically distinct depositional unit indicate that replacive dolomite predates all other diagenetic mineral phases, including palygorskite, illite, quartz cement, pyrite, and exposure-related blocky and poikilotopic calcite cements. Diagenetic calcite cements are most abundant immediately below exposure surfaces, gradually decrease in abundance down section, and are generally absent directly above exposure surfaces. Collectively, these observations imply that blocky and poikilotopic calcite cements postdate dolomitization, supporting a model where dolomitization of a given depositional cycle occurred syndepositionally, before the deposition of the overlying cycle. With an estimated deposition rate of the Dam Formation of between similar to 5.6 +/- 1.4 and 3.3 +/- 0.9 cm kyr(-1), dolomitization in some cycles likely occurred in similar to 5.4 +/- 1.4 to 9.1 +/- 2.3 kyr, consistent with findings from Holocene dolomite studies, and considerably shorter (< 100x) than those extrapolated from high-temperature laboratory experiments. The findings from this study imply that sedimentary carbonates can be dolomitized relatively rapidly on a cycle-by-cycle basis.

Dolomitization is described as a stepwise replacement of CaCO3 via a dissolution-precipitation process that first involves formation of a metastable very-high-magnesium calcite (VHMC), which is then replaced by ordered, stoichiometric dolomite. Laboratory experiments consistently report a long induction period during which no mineral products are detected with X-ray diffraction. In contrast to previous studies, the current study investigates the induction period using a combination of fluid chemistry, powder X-ray diffraction (XRD), scanning electron microscopy (SEM), and data on in situ SEM-EDS elemental composition from dozens of high-temperature dolomitization experiments conducted over the range of 150-200 degrees C. Consistent with previous experiments, XRD data show a prolonged induction period before Ca-carbonate reactants are replaced by VHMC, which are then replaced by dolomite. The fluid-geochemistry data show, however, that fluid Mg/Ca ratios decrease and Sr/Ca ratios increase immediately after the onset of experimental conditions and continue along linear trajectories until most of the CaCO3 reactants are consumed. These observations are consistent with calcite dissolution and VHMC precipitation during the first stages of the mineral-replacement reaction. SEM and SEM-EDS observations align with the fluid-geochemistry data in that they show VHMC crystals on calcite reactants, hours before VHMC is detected by XRD. Collectively, these data indicate VHMC nucleation and growth is not significantly inhibited, but rather that VHMC forms at a low rate. A revised three-stage model of dolomitization is presented whereby there no induction period for VHMC. In this model, stage one is characterized by CaCO3 replacement by VHMC, which proceeds slowly at first and then rapidly. Stage two is characterized by VHMC replacement by poorly ordered dolomite, and three is characterized to ordered dolomite.

Road salt (e.g., NaCl) application in cold-temperature regions has increased Cl- concentrations in freshwater. Removing Cl- from water is challenging because it is highly soluble. This paper is the first to demonstrate and characterize the removal of Cl- from stormwater via Friedel's salt (Ca4Al2(OH)12Cl2(H2O)4) precipitation (FSP), achieved by adding calcium oxide (CaO) and sodium aluminate (NaAlO2). Laboratory tests were done on Cl- removal with FSP using a range of Ca/Al/Cl ratios in deionized (DI) water and stormwater samples impacted by road salt. Even the lowest doses resulted in significant removal of Cl- with maximum removal rates for comparable Ca/Al/Cl ratios of 72% and 84% in stormwater and DI water, respectively. Temporal experiments in stormwater and DI water indicate that most of the Cl- removal occurred within the first 10 min of reaction time. Potential applications of FSP were demonstrated for two hypothetical scenarios based on a well-characterized, salt-impacted lake. The first scenario indicates that a 30% reduction in stormwater Cl- mass is sufficient to maintain concentrations below 150 mg/L. A second scenario with an initial 300 mg/L lake water concentration shows the potential for implementing FSP in low-flow pump and treat systems for the rehabilitation of road-salt-impaired lakes.

Correlating shallow shelf carbonates and their deep basin equivalents is a perennial challenge in the geosciences, with wide-ranging implications. This hurdle is well illustrated in the Llandovery succession of the Michigan Basin, USA, a 40- to 265-m-thick carbonate interval represented by three lithostratigraphic units: the Cataract, the Burnt Bluff, and the Manistique groups. Although extensively studied at various localities within the basin and across the region, the chronostratigraphic relationships between these units remain unknown. The current study presents a cross-basin chronostratigraphic framework for the Llandovery succession utilizing globally documented carbon (δ13Ccarb) isotope excursions (CIEs). From 10 drill cores and three quarry sites throughout the Michigan Basin, five CIEs were identified and chronostratigraphically constrained using conodont biostratigraphy and conodont 87Sr/86Sr data. The five excursions are interpreted to be the global CIEs: the (1) Hirnantian Isotope Carbon Excursion (HICE; Hirnantian Stage), (2) Early Aeronian, (3) Late Aeronian (Aeronian Stage), (4) Valgu (Telychian Stage), and (5) Ireviken (Sheinwoodian Stage). Most importantly, the HICE and the Ireviken CIEs bracket the Llandovery strata preserved in the basin. The new high-resolution δ13Ccarb data suggest that CIEs can be effectively used to correlate among shallow marine shelf carbonates and their deeper water equivalents. The new chronostratigraphic framework shows that CIE-based time horizons across the Michigan Basin cut across lithostratigraphic unit boundaries, which indicates that these lithostratigraphic units are diachronous in the Michigan Basin. In addition to refining the stratigraphy of the Llandovery succession of the Michigan Basin, particularly the timing of various key sedimentary deposits, the new chronostratigraphic framework can be used to: (1) constrain the timing of various regional tectonic phenomena, (2) identify multiple tectonically driven siliciclastic sediment pulses in the basin, and (3) predict various stratal relationships that may result in previously unknown stratigraphic traps and, therefore, new hydrocarbon plays within the basin. The results of the current study also show that δ13Ccarb trends across the shelf-to-basin transect are spatially and temporally variable and do not match those reported in Modern carbonate settings, which possibly suggests that such δ13Ccarb trends, to some extent, reflect variations in water circulation and water mass heterogeneity during deposition.

ABSTRACTSaddle dolomite is a Ca–Mg carbonate characterized by curved crystal faces, curved cleavage and sweeping extinction under cross‐polarized light. Saddle dolomite and, generally, Ca–Mg carbonates with curved crystal faces, are often assigned to the burial (hydrothermal) realm and serve as proxy archives for hydrogeochemical processes in sedimentary basins and orogens. At present, the physicochemical conditions leading to the formation of these peculiar warped carbonates are unclear, and the mechanisms inducing the curvature of their crystal lattice are debated. This study uses laboratory experiments to assess the factors controlling the formation of carbonates with curved crystal faces as a function of fluid temperature, reactant size, fluid salinity and fluid Mg : Ca. Results show that a range of magnesium calcites with curved surfaces form at elevated temperatures (ca 220°C) from calcium‐rich fluids (Mg : Ca = 0.43) within a wide range of fluid salinities (5 to 40 wt.%). Magnesium calcites that nucleate epitaxially on rhombohedral or saddle dolomite substrates exhibit warped surfaces, while those that nucleate on calcite seeds form flat surfaces. Although the two crystal habits can co‐occur, Mg‐calcites with curved crystal faces (Mg : Ca of 0.35 to 0.40) tend to be more calcium‐rich than those with flat faces (Mg : Ca of 0.58 to 0.74). In experiments with higher fluid temperature (230°C), calcite reactants undergo replacement by dolomite exhibiting planar crystal faces. The results collectively indicate that the formation of Ca–Mg carbonates with warped surfaces is likely related to a combination of geochemical and physical parameters and various threshold limits, as opposed to one specific parameter, for example, elevated fluid temperature, as is commonly reported in the literature. Although most of the present experimental precipitates are disordered magnesian calcites with bent crystal faces, it is reasonable, at the level of a tentative working hypothesis, that these represent precursors of many ancient saddle dolomites commonly found in burial settings.

A significant proportion of marine calcium carbonate sediments are comprised of metastable minerals that are susceptible to diagenetic alterations during burial. These reactions can reset the geochemical signature of sediments and pore fluids and influence elemental cycling in the ocean. However, the timing and mechanisms by which these reactions take place are poorly constrained. This study uses cores drilled on the slope of the Great Bahama Bank to provide quantitative constraints on important diagenetic reactions; namely respiration-driven dissolution, authigenic carbonate mineral formation, and conversion of aragonite to low Mg calcite (LMC). We perform detailed mineralogical characterization using newly acquired, high resolution X-ray diffraction (XRD) data and over 1000 reanalyzed XRD scans, characterize sediments texturally and elementally using electron microprobe data, calculate pore fluid saturation state with respect to aragonite using a Pitzer ion interaction approach, and use pore fluid chemistry and experimental distribution coefficients to predict authigenic carbonate compositions. These data suggest that aerobic organic matter oxidation, enabled by the advection of oxygenated seawater throughout the upper similar to 30 m interval of sediment, causes undersaturation and thus dissolution of biogenic high Mg calcite (HMC) and aragonite. Deeper, anaerobic organic matter oxidation takes over and causes supersaturation, promoting authigenic precipitation in the form of HMC, which in turn decreases pore fluid Mg/Ca and promotes aragonite conversion to LMC. One novel aspect of this study is the identification of three types of calcites using the newly acquired XRD and electron microprobe data. Based on their unique crystallographic characteristics and chemical compositions, these types of calcites are interpreted to represent pelagic biogenic LMC, bank-derived biogenic HMC, and authigenic HMC precipitated from pore fluids. Notably, the composition of the authigenic calcite matches that predicted to precipitate from pore fluids using an empirical Mg partition coefficient in calcite. Calcites that form authigenically and from aragonite via replacement are suggested to recrystallize with burial as evidenced by the decrease in Mg content and micro-strain. This process-based geochemical framework assigns diagenetic processes to a specific depth window within the sediment column and paves the way for a mechanistic understanding of carbonate diagenesis, one that is rooted in thermodynamic and kinetic bases.

ABSTRACT The Eocene Uteland Butte Member of the Green River Formation in the Uinta Basin is characterized by lacustrine carbonate depositional cycles consisting of calcareous shales, limestones, and dolomites that have been interpreted to reflect climatically driven lake level fluctuations. Previous work suggests that dolomitization of three distinct stratigraphic intervals in the Uteland Butte Member—the PZ-1, PZ-1′, and PZ-2—occurred during low lake levels and was driven by a combination of density-driven downward reflux and evaporative pumping of concentrated brines. The current study uses a novel high-resolution mineralogical dataset to evaluate these proposed dolomitization mechanisms. Mineralogical data from three drill cores show that the dolomitized intervals are characterized by variations in dolomite abundance (relative to calcite), dolomite stoichiometry, and cation ordering (015:110), all of which covary with depositional facies. In the near-basin margin core and near-basin center cores, the PZ-1 interval is characterized by a shallowing-upward facies trend that corresponds to an increase in stoichiometry, dolomite abundance, and cation ordering. In the PZ-1′ interval both the near-basin margin and near-basin center cores exhibit shallowing to deepening facies patterns up core that correspond to an increase and a subsequent decrease in dolomite stoichiometry. Similarly, dolomite abundance in this interval exhibits an increase then a decrease. The PZ-2 interval is also characterized by a shallowing to deepening facies pattern, which corresponds with an increase then a decrease in stoichiometry. Lateral trends between cores indicate that basinward facies have less dolomite, and that the dolomite is less stoichiometric compared to their more landward counterparts. Collectively, these observations argue against a simple model of top-down reflux dolomitization driven by evaporative pumping. Instead, the vertical and lateral relationships between depositional facies and mineralogical properties in the PZ-1′ and PZ-2 intervals suggest that dolomitization may have occurred syndepositionally, and that the observed mineralogical patterns were driven by differences in fluid chemistry associated with lake level fluctuations through time. These findings are broadly consistent with previous studies on peritidal marine carbonates showing that dolomite mineralogy can record temporal and spatial paleoenvironmental changes that can be utilized to evaluate dolomitization mechanisms.

Key Messages: (1) A single petrological dataset (e.g., stable isotopes) can result in multiple interpretations. (2) To avoid erroneous diagenetic interpretation, it is essential to integrate multiple types of petrological data (e.g., mineralogy, stable isotopes, clumped isotopes, trace element, and petrographic relationships).

This study uses high temperature (215°C) dolomitization experiments to explore the effects of sodium (Na) and potassium (K), two common constituents of natural fluids, on dolomite formation rate, stoichiometry and crystallographic characteristics. In these experiments, aragonite ooids were dolomitized in Mg–Ca–Cl solutions with either no additional salt, or in solutions containing NaCl or KCl at different concentrations. Results show that Na solutions, and to a lesser extent K solutions, correlate with faster reaction rates and that Na at hypersaline fluid concentrations produces dolomites with higher stoichiometry, higher micro‐strain and lower cation ordering. Potassium has a different effect on the dolomite than Na at similar concentrations. Unlike Na, K does not cause micro‐strain but leads to dolomite with smaller unit cell parameters. It is proposed that Na and K catalyse dolomite precipitation and increase stoichiometry by weakening Mg hydration bonds which consequently facilitates Mg incorporation into dolomite. The decrease in dolomite cation ordering at higher Na concentrations may stem from the incorporation of Na into dolomite which strains the crystal lattice and reduces cation order. On the basis that high‐temperature experiments are applicable to natural dolomites, several implications pertinent to natural dolomites are drawn from the results. Firstly, the data suggest that the higher concentrations of Na and K in evaporative fluids (i.e. higher salinity) can explain why dolomite is generally more abundant and more stoichiometric in evaporative settings. Secondly, the results challenge a key prediction of the mixing zone dolomitization model by showing that higher Na and K concentrations increase, rather than decrease, both dolomitization rate and stoichiometry. Thirdly, the observed decrease in dolomite cation ordering with increasing Na and K concentrations implies that evaporative fluids would produce less stable dolomite that may be more prone to subsequent recrystallization and thus resetting of primary geochemical signatures.

Dolomite textures are widely interpreted to reflect physical, mineralogical, and geochemical conditions of crystal growth. In particular, nonplanar dolomites, which display non-faceted crystal boundaries and a low percentage of crystals with compromise boundaries with preserved crystal-face junctions, have long been cited as evidence of crystal growth in fluids warmer than a theoretical dolomite critical roughening temperature (CRT) of similar to 50-100 degrees C. No direct experimental evidence across this temperature range exists, however, to confirm the theory that nonplanar dolomite should form exclusively above the CRT. The present study offers new Delta(47) clumped-isotope data from nonplanar dolomites from the Paleocene-Eocene Umm er Radhuma Formation (Qatar) that show that nonplanar dolomite can form below the theoretical CRT. These dolomites are interpreted to have experienced only near-surface to shallow-burial conditions since deposition, and lack common burial features such as two-phase liquid-vapor inclusions, stylolites, compaction-reduced porosity, and burial cements. Scanning electron microscope images reveal that relatively large dolomite crystals (typically > 100 mu m) comprise non-faceted mosaics with indistinct crystal boundaries, indicating a nonplanar texture. Thin-section petrographic measurements confirm the nonplanar texture, as the proportion of dolomite crystals with compromise boundaries with preserved crystal-face junctions ranges from 9% to 20% with an average of 14%, defining these dolomites as nonplanar sensu stricto (<= 30%). The new Delta(47) clumped-isotope data from these nonplanar dolomites reveals average crystallization temperatures ranging from 38.8 to 54.2 degrees C and overall averaging 43.6 degrees C. Calculated uncertainties, however, indicate the nonplanar dolomites could have formed at temperatures as low as 29.1 degrees C or as high as 65.3 degrees C. More than three quarters (similar to 78%) of the samples have mean temperatures that fall below 50 degrees C, and all samples have calculated uncertainties indicating possible temperatures below 50 degrees C, but not all indicate possible temperatures above 50 degrees C. Furthermore, these calculated uncertainties overlap with the crystallization temperatures of planar and mimetic dolomites higher in the section, suggesting that all dolomites formed under similar temperature conditions, and therefore texture is unlikely driven solely by crystallization temperature. Cumulatively, these results indicate that the nonplanar dolomite formed in a shallow-burial setting at temperatures near or below the proposed dolomite CRT. The new Delta(47) data, in conjunction with textural observations from natural dolomites and hundreds of published high-temperature experiments, suggest that nonplanar dolomite cannot be reliably used as an indicator of high-temperature environments of dolomitization.

The clumped isotope paleo-thermometer has become a valuable proxy for the burial history reconstruction of carbonate formations. To maximise the accuracy of these reconstructions, post-depositional alterations, such as recrystallisation and Δ47 isotope exchange reactions, must be understood. In this study, we examine the isotopic behaviour of calcites and early dolomite samples from the same stratigraphic intervals, and thus with similar burial history. This approach provides additional constraints on the kinetics of Δ47 reordering in dolomite during exhumation. Clumped isotope measurements were performed on 19 calcites and 15 early dolomites from the Permian, Jurassic, and Cretaceous periods from four locations in Oman spanning different burial regimes. The calcite and dolomite samples were collected from the rock matrix, based on the assumption that fine material was more susceptible to recrystallisation. Our results show that calcites and dolomites record different Δ47 values despite being subjected to the same thermal history. The maximum Δ47 temperature recorded in dolomites (181 ± 13 °C) corresponds to the oldest and most deeply buried Permian rock. This value is approximately 35 °C higher than those measured in the co-located and coeval calcite matrix (145 ± 14 °C). This discrepancy suggests that calcite and dolomite have different kinetic parameters. Our data confirm (1) that dolomite Δ47 values are more resistant to alteration during burial and exhumation than Δ47 calcite values, and (2) that dolomite has a higher Δ47 closing temperature than calcite during cooling. The presence of two mineral phases with distinct kinetic parameters in the same stratigraphic unit provides additional constraints on models of burial and uplift. In addition, mineralogical data coupled with Δ47 and burial depths suggest that the progressive development of dolomite cation ordering is driven by temperature elevation, as previously suggested.