Sylvia Riechelmann

Thermodynamic constraints and empirical evidence often suggest that fabric‐retentive replacement dolomites are stable carbonate minerals that preserve ancient seawater properties. This study evaluates this hypothesis by examining the Upper Triassic Dolomia Principale in Northern Italy. Using a combination of petrographic analysis, isotope geochemistry, fluid‐inclusion thermometry and U‐Pb cement dating, a paragenetic sequence of seven diagenetic phases was identified. Only one phase represents marine diagenetic (mesohaline reflux) dolomite, yet it accounts for about 90% of the bulk rock volume. In contrast, late‐stage burial dolomite cements account for 7%, and late‐stage calcite porosity‐filling cement for 3%. The mean oxygen and carbon isotopic values for the bulk Dolomia Principale closely mirror those of the marine replacement dolomite phase. Dolomite recrystallisation and reordering occurred in a marine burial setting, as shown by high average salinity (up to 130 000 ppm) and reconstructed burial temperatures of approximately 50°C, as evidenced by primary fluid inclusions. The data and regional context suggest that the extensive Dolomia Principale platform was dominated by saline, restricted platform‐top waters with only limited exchange with the Norian open ocean. Predominant mesohaline reflux dolomitisation affected all inner platform down to toe‐of‐slope facies, while the impermeable, anoxic intraplatform basinal mudstone deposits were preserved as limestone. The Dolomia Principale dolostone data, when compared to reconstructed Norian seawater values, exhibit ¹³ C depletion and, in some cases, ¹⁸ O enrichment. Similarly, ⁸⁷ Sr/ ⁸⁶ Sr ratios diverge from reconstructed seawater values. Overall, the Dolomia Principale marine replacement dolomites are more likely to preserve records of restricted platform water masses and altered marine pore fluids down to burial depths of several hundred metres, rather than Norian seawater. Although these findings may not be applicable to all dolomitised platforms in the Phanerozoic, they highlight the need for caution when interpreting marine diagenetic dolostones as archives of ancient seawater properties.

Although early marine (replacement) diagenetic dolomitisation is a well-documented process in ancient carbonate rocks, the factors driving its occurrence and timing remain less clearly understood. In the Lombardy Basin (Northern Italy), the Norian Dolomia Principale platform underwent pervasive, mimetic dolomitisation soon after sediment deposition, making it an exemplary case study of early marine dolomitisation. To identify the environmental parameters influencing the occurrence and intensity of early dolomitisation in Triassic shallow marine settings, a comprehensive multi-proxy analysis was conducted. Samples were collected from various depositional environments, including restricted lagoon, inner platform, outer platform/margin, slope, and intraplatform basin. To unravel the diagenetic history and distinguish early diagenetic features from later alterations, a paragenetic sequence was established based on petrographic observations, cathodoluminescence, fluid inclusion microthermometry, U-Pb dating, and stable isotope measurements (δ13C, δ18O, and 87Sr/86Sr). Although the intensity of these events varied according to depositional environment, four main diagenetic events were identified: (i) early marine dolomitisation, replacing precursor sediment and marine cement; (ii) a succession of late dolomite cement precipitations due to burial or local hydrothermalism; (iii) burial calcite cement precipitation; and (iv) late meteoric calcite precipitation. Radiometric dating of the now-stoichiometric "early marine-diagenetic dolomite" indicates that replacement occurred within the first few million years after deposition, likely followed by a prolonged period of "ripening." Fluid inclusion analysis reveals that the dolomitising fluid was a modified seawater mixed with a halite-dissolution brine, supporting a reflux-type dolomitisation model. Geochemical data reveal a progressive depletion in δ13C and δ18O throughout diagenetic evolution, while also highlighting discrepancies among "early dolomites" from different depositional environments. Bulk isotope data of Dolomia Principale dolostones are dominated by the fabric-retentive replacement dolomite phase and have, within limitations, value as archives of past seawater and altered marine porewater data.

Speleothems are an important archive for reconstructing past climate variability. The Magnesium isotope proxy tested so far in limestone-hosted caves provides the possibility of reconstructing climate conditions from changes in the silicate-to-carbonate weathering ratio. Other caves, however, are situated in dolostone host rock. Consequently, the Mg content of the host rock is much higher than that of limestone. Dripwater monitoring in a set of dolostone-dominated caves in Germany and Morocco, as well as the collection of soil (silicate minerals), host rock (carbonate), and speleothem samples, aims to apply the Mg isotope proxy in dolostone-hosted caves. The time-series analyses of the Mg isotope composition of dripwaters revealed, for most dripwater sites, significant variations in δ26Mg values, which can be related to changes in the silicate-to-carbonate weathering ratio. Silicate weathering is enhanced under dry, warm conditions, whereas cold, wet conditions favour carbonate weathering. Due to significant differences in the Mg isotope composition of silicate (soil) and carbonate (host rock) minerals, changes in the weathering regime are detectable in drip-water Mg isotope ratios in both climate regions. In German caves, where changes in temperature are more pronounced than changes in rainfall amount, the weathering ratio is driven by temperature variations. In Morocco, however, both temperature and rainfall amount complement each other and drive changes in the silicate-to-carbonate weathering ratio. Furthermore, the different transfer times at each drip site ranged from a few months to at least a year. Some drip water sites show no variation in Mg isotope composition. In these cases, the signal of the weathering ratio is strongly buffered by longer water transfer times/residence times and mixing of waters in the aquifer. Although possible, no dependence of Mg isotope variations in the dripwaters on prior calcite precipitation was observed. Corresponding speleothems from the monitored dripwater sites exhibit varying Mg isotope compositions of calcite and aragonite. There is no overprint of other factors during carbonate precipitation; thus, these variations are solely due to changes in the silicate-to-carbonate weathering ratio and, consequently, changes in temperature and rainfall amount. Furthermore, observations on the Mg isotope fractionation factor of aragonite-dominated samples revealed a smaller Δ26Mg than for calcite speleothem samples. Generally, the Mg isotope proxy is a valuable tool for reconstructing past climate conditions in both limestone- and dolostone-dominated caves.

Geothermal energy exploitation has emerged as a critical solution to combat global climate crises, such as reducing CO₂ emissions and climate warming. Scaling is the process of mineral precipitation in fluid pathways and geothermal equipment. It is known to significantly hamper geothermal energy production by decreasing the rates of heat extraction. Numerous research efforts are dedicated to characterising dissolution and precipitation processes, not only to provide know-how for further and safer developments in geothermal energy, but also to adapt such findings to the ever emerging field of geothermal energy recovery. This paper presents an overview of experiments—performed under variable pressure and temperature conditions—with a focus on scaling. We assess the different factors that influence disequilibrium reactions in carbonate rocks, the different experimental setups, and their application to the field. The influence of experimental variables (such as temperature and pressure) on mineral dissolution and precipitation is discussed, and the main learning points from experiments are compared and contrasted. We address techniques for preventing and controlling scaling in geothermal wells based on a comprehensive analysis of experimental studies in carbonate rocks. We propose that the intelligent combination of fieldwork, numerical approaches, and laboratory experience provides a foundation for the success of future work in this field.

This study investigated the mineralogical and chemical characteristics of ochreous precipitates and mine water samples from abandoned Upper Carboniferous hard coal mines in an extensive former mining area in western Germany. Mine water characteristics have been monitored and assessed using a multi-methodological approach. Thirteen mine water discharge locations were sampled for hydrochemical analysis, with a total of 46 water samples seasonally collected in the whole study area for stable isotopic analyses. Mineralogical composition of 13 ochreous precipitates was identified by a combination of powder X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), and field emission scanning electron microscopy (FE-SEM/EDS). Results showed that abandoned mine drainage was characterized by circumneutral pH, Eh values ranging from 163 to 269 mV, relatively low concentrations of Fe and Mn, and was dominated by HCO₃⁻ > SO₄²⁻ > Cl⁻ > NO₃⁻and Na⁺ > Ca²⁺ > Mg²⁺ > K⁺. Goethite and ferrihydrite were the dominant precipitated Fe minerals, with traces of quartz, dolomite, and clay minerals. Some metal and metalloid elements (Mn, Al, Si, and Ti) were found in the ochreous sediments. The role of bacteria in the formation of secondary minerals was assessed with the detection ofLeptothrix ochracea. The δ¹⁸O and δ²H values of mine water plotted on and close to the GMWL and LMWLs indicated local derivation from meteoric water and represented the annual mean precipitation isotopic composition. Results might help to develop strategies for the management of water resources, contaminated mine water, and public health.

The Late Ordovician marks an epoch of substantial change during Earth’s geologic history. It documents the first mass extinction event of the Phanerozoic, which wiped out a plethora of marine genera. Though signatures of the extinction event are quite evident on the species level, there are many underlying uncertainties as to which factors drove the near destruction of Ordovician marine life. The exact causes and contributing factors of the Late Ordovician Mass Extinction (LOME) are highly debated. Previous studies propose processes such as redox shifts, volcanism, deep-water anoxia, low atmospheric oxygen, or excess CO₂ as significant influencers. In this study, we utilized a multi-archive/proxy approach in reconstructing paleoenvironmental, stratigraphic, chronologic, and diagenetic signatures of halite and carbonate sequences from two localities containing the Ordovician–Silurian boundary. Our preliminary findings suggest that an abrupt, multi-pulsed glaciation was probably the main driver of the LOME. However, the exact duration, intensity, and number of cooling pulses are still up to interpretation and will require additional geochemical work to paint the entire picture. Samples were obtained from two localities of similar low paleo-latitudes. Marine halite and carbonate were obtained from the upper Ordovician Red Head Rapids Formation, Hudson Bay Basin, Canada. Another suite of halite was collected from the Ordovician/Silurian Mallowa Salt Formation, Canning Basin, Western Australia. The diagenetic integrity of Red Head Rapids halite and carbonate were assessed through strontium isotope analysis. Since the strontium isotope remains in equilibrium with seawater, we can cross-analyse the preservation potentials of both marine archives. All sample results lay within the ± 0.00006 ‰ natural variation of ⁸⁷Sr/⁸⁶Sr measured in modern marine counterparts, providing evidence of primary material and accurate paleo-interpretations. Trace element chemistry study was conducted on halite samples from both localities. Previous studies suggested that the spikes of Hg, Mo, and U concentrations signify the aftermath of a greenhouse event that triggered the expansion of deep-water anoxia. Furthermore, heightened volcanic activity led to significant greenhouse gas emissions and produced a period of warming. Instead, our halite Hg, Mo and U concentrations are extremely low, many below detection limits, which correspond to glacial signatures of an icehouse. Interpretation of the sedimentary cerium anomaly of the Red Head Rapids and Mallowa salts also revealed characteristics of an oxygenated marine environment, with all calculated values below the threshold, typical of anoxia. Rigorous halite fluid inclusion analysis was conducted to qualitatively distinguish between primary and secondary material based on size, shape, orientation, and multiple growth pat­terns. Microthermometry homogenization temperatures derived quantitative distinctions between primary and secondary fluid inclusions of preserved and altered material, respectively. Paleotemperatures of primary fluid inclusions exhibited oscillations that were evident of daily/seasonal variation of inter- and/or glacial times, averaging approximately 24.2°C ± 0.5°C for the latest-Ordovician–earliest-Silurian. Apart from minor fluctuations, we noticed up to four major cool­ing pulses associated with the major glaciation characterizing the latest-Ordovician–earliest-Silurian time.