The mineral dolomite (CaMg(CO₃)₂) and the uncertainties surrounding its origin have captivated geoscientists for over two centuries. Déodat de Dolomieu published his seminal paper on the subject in 1791, and today, the Web of Science lists 1,029 papers under the term "dolomite problem" and 27,469 under "dolomite". The essence of the “dolomite problem” lies in the paradox of this mineral's relative scarcity in modern near-surface diagenetic environments compared to its abundance in Earth’s rock record. Each year, numerous new studies are submitted that claim to have resolved the dolomite problem or, at least, to have made significant contributions toward its resolution. I disagree and argue that this controversy arises from an oversimplified understanding of calcium/magnesium carbonates, particularly regarding their formation and subsequent diagenetic pathway. Dolomite and related magnesian carbonates, including their amorphous and unordered precursors, belong to a surprisingly complex group of minerals. These minerals may be secreted from bodily fluids, induced by microbial activity, replace pre-existing carbonate minerals, or precipitate (cement) from different aqueous solutions with varying hydrogeochemical properties and temperatures, ranging from Earth's surface to low-grade metamorphic conditions at 300 °C. This diversity leads to confusion. Ancient dolomite minerals, such as those that form the regionally extensive, stratigraphically thick dolostone bodies of the Precambrian and Phanerozoic eras, often have complex petrographic histories. Consequently, the dolomite that builds these rock bodies should not be compared with the much rarer calcium/magnesium carbonates, whether they represent direct precipitates or replacement phases, collectively referred to as “dolomite” in contemporary marine diagenetic environments. Arguably, one of the most important domains for the formation and stabilisation of (replacement) dolomites is the marine pore-water realm, which can extend to burial depths of several hundred meters. Dolomite formation and stabilisation, however, continue through prograde diagenetic and metamorphic pathways over geological timescales. This raises a critical question: Is the fabric-retentive dolostone that builds ancient carbonate platforms genuinely formed nearly synchronously with sediment deposition, or is it rather a product of the (shallow to deep) burial domain, the most prolonged and arguably the least well-understood diagenetic environment? To understand the discrepancy between the scarcity of early marine diagenetic Mg/Ca carbonates and the vast dolostone rock bodies of the geological past, we must approach this question using empirical data, petrographic and crystallographic analysis, geochemical evidence, and theoretical frameworks. It is essential to avoid distorting these findings into a simplified model that creates the illusion of a problem, which might be nothing more than a myth or a marketing construct that never truly existed.

How to cite: Immenhauser, A.: The Dolomite Problem: Facts, Myths and Marketing , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-3029, https://doi.org/10.5194/egusphere-egu26-3029, 2026.

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.

How to cite: Folliot, V., Della Porta, G., Mueller, M., Berra, F., Walter, B., Beranoaguirre, A., Riechelmann, S., Schramm, O., and Immenhauser, A.: Environmental influences and timing constraints on early marine dolomitisation of a giant carbonate platform (Dolomia Principale, Northern Italy), EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-4214, https://doi.org/10.5194/egusphere-egu26-4214, 2026.

Despite prolonged research, the formation environments of dolomite remain debated. Previous studies have associated the apparent decrease in dolomite abundance in the geological record with a major transition in marine depositional environments from warm, saline shallow platforms, to deeper and cooler environments in which dolomite formation was largely inhibited. Others suggested that large volumes of pre-Cenozoic dolomites reflect dolomitization at elevated burial temperatures of these rocks, whereas Cenozoic carbonate platforms mostly never reached sufficient thermal maturity. A third, hybrid possibility is that Mg-rich dolomite precursor precipitated in shallow environments and later underwent cation ordering during burial diagenesis.

To test these models, we measured cation ordering, oxygen, and clumped isotopes (δ¹⁸O and TΔ₄₇, respectively) in Triassic dolomite from the Mohila Formation in Makhtesh Ramon. This dolomite is thought to have formed in evaporative settings. It was compared to a modern analogue of dolomite from Dohat Faishakh Sabkha, Qatar. We incorporate these data into a global compilation of dolomite records. δ¹⁸O and TΔ₄₇ data from Qatar indicate formation at Earth surface temperature and evaporated (2.5-4.8 ‰ VSMOW) seawater. In contrast, δ¹⁸O and TΔ₄₇ of dolomites from the Mohila Fm. are consistent with cation ordering in deep burial environments flushed with normal seawater (0 to -1‰ VSMOW). Together, these data support a two-step dolomite formation process, in which carbonates were initially enriched in Mg²⁺ in the lagoon and later recrystallized in burial-diagenetic environments. For most dolomite in our dataset, cation ordering is positively correlated with TΔ₄₇, suggesting that a burial recrystallization step is coupled with and possibly driven by ordering of proto-dolomite. However, dolomites from one study in the data set diverge from this trend and show a high degree of ordering despite low clumped isotope temperatures, suggesting a secondary – early dolomite formation pathway. Dolomites associated by stratigraphy with surface environments show a negative correlation between δ¹⁸O and TΔ₄₇ values, similar to massive dolomites that lack such context. Considered together, our results suggest that burial recrystallization is a common process in the formation of dolomite records that are typically interpreted as forming under surface conditions.

How to cite: Cooper-Frumkin, S., Affek, H. P., Shalev, N., Bontognali, T. R. R., Zaarur, S., and Ryb, U.: Dolomite recrystallization associated with thermally activated cation-ordering , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-21877, https://doi.org/10.5194/egusphere-egu26-21877, 2026.

The origin and evolution of ancient, deeply buried dolostone reservoirs remain elusive, largely due to their complex diagenetic overprinting. This study presents a multidisciplinary investigation using clumped isotope thermometry (Δ₄₇), in-situ U-Pb geochronology, and isotopic and elemental geochemistry to quantitatively reconstruct the diagenetic history of the Guadalupian dolostones, which are important reservoir rocks for hydrocarbon exploration in the Sichuan Basin (southwest China). Initial replacive dolomitization event (Rd1/Rd2) occurred during mid‑late Permian (U-Pb ages = 265–257 Ma) at relatively low temperatures (restored Δ₄₇ temperatures = 7–41 °C), with dolomitizingc fluids being dominantly sourced from Guadalupian and/or slightly younger seawater. During burial, these early-formed, marine dolostones have experienced diagenetic alteration from at least two episodes of hydrothermal fluid flow that are documented in the void‑filling saddle dolomite (SD; Δ₄₇ temperatures = 67–135 °C) and blocky calcite (BC; Δ₄₇ temperatures = 182–189 °C) cements. U‑Pb dating further constrains the timing of the hydrothermal events to 264–234 Ma (SD) and 239–235 Ma (BC), which we interprete to be closely related to two regional tectono-hydrothermal events, i.e., the Emeishan Large Igneous Province and the Indo‑Sinian orogeny. The δ¹⁸O values of parent fluids evolved progressively from near‑marine signatures (Rd1/Rd2) to strongly ¹⁸O‑enriched compositions (up to +17‰ VSMOW for BC), suggesting that the hydrothermal fluids were largely sourced from the deep basin and have been experienced intensive water‑rock interaction with the surrounding rocks. Overall, tectono‑hydrothermal processes, changing these dolostones structurally and geochemically, have improved reservoir quality through: 1) recrystllization of the early marine, tightly-packed dolomites into high-temperature Rd3 dolomites that host considerable intercrystalline porosity, 2) generation of open fractures, and 3) formation of dissolution‑enlarged vugs . This study highlights the critical role of tectonically driven hydrothermal fluid flows in the development of deep-burial dolostone reservoirs. Furthermore, by integrating a robust thermochronological diagenetic framework with well‑defined sedimentary facies and fracture characterization, this approach offers an applicable strategy for predicting reservoir quality in deeply buried carbonate successions within tectonically active sedimentary basins.

How to cite: Pan, L., Hao, Y., Li, W., and Liang, F.: From cold seawater to hydrothermal fluid flow: thermochronological and geochemical elucidation on the diagenesis of a Guadalupian dolostone reservoir from Sichuan Basin (China), EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-4819, https://doi.org/10.5194/egusphere-egu26-4819, 2026.

 Laser ablation U-Pb dating technology for carbonate rocks has matured and plays a significant role in studies of carbonate reservoirs, tectonic activities, and related research. However, the lack of effective dating methods for siliceous rocks, which are extremely abundant in the Earth's crust, has constrained research into the diagenetic processes associated with them. Consequently, building upon the foundation of carbonate laser U-Pb dating, a laser U-Pb dating technique for siliceous rocks was successfully developed through a series of methodological improvements. Both carbonate and siliceous rock laser U-Pb dating were applied to the recently explored but less-studied reservoirs of the Upper Cambrian Lower Qiulitage Formation in the Tabei area of the Tarim Basin. Integrated with analyses including petrology, sedimentology, cathodoluminescence, trace and rare earth elements, in-situ carbon and oxygen isotopes, and elemental mapping, the following conclusions were reached: ① The dolomites of the Lower Qiulitage Formation in the Tabei area underwent multiple diagenetic stages. The first stage was a widely developed penecontemporaneous to shallow burial dolomitization. The second stage was a burial dolomitization, locally distributed along faults and induced by tectonic compression during the Early-Middle Devonian. The third stage was a hydrothermal dolomitization, also localized along faults, caused by Permian volcanic activity. Both the second and third stages accompanied by recrystallization, silicification, and mineral precipitation. ② The dolomite reservoirs of the Lower Qiulitage Formation are primarily developed within grain shoal sedimentary facies. Reservoir space primarily originated from meteoric water dissolution. Multiple episodes of burial fluids increased reservoir heterogeneity and partially destroyed pore spaces. Therefore, sedimentary facies of paleo-highland grain shoals, located away from fault activity zones of the Devonian and Permian periods, are favorable areas for reservoir development. The research outcomes provide effective theoretical support for the exploration of dolomite reservoirs in the Lower Qiulitage Formation. Furthermore, the successful application of laser dating techniques to both carbonate and siliceous rocks in this dolomite reservoir study offers new perspectives and methodologies for related research.

Keywords: Laser U-Pb dating of siliceous rocks; Laser U-Pb dating of carbonate rocks; Diagenesis; Reservoir genesis; Tabei area

How to cite: Junmao, D.: Geochronology, Geochemical Characteristics, and Main Controlling Factors of Reservoirs in Dolomite and Siliceous Rocks of the Upper Cambrian Lower Qiulitage Formation, Tarim Basin, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-4832, https://doi.org/10.5194/egusphere-egu26-4832, 2026.

The mechanisms controlling incompleteness and time averaging of fossil assemblages include (1) sediment accumulation, (2) mixing, and (3) skeletal disintegration. Although sediment accumulation is a major factor controlling these attributes of the fossil record, predicting the effects of mixing and disintegration depends on their interaction and can be counterintuitive. Stochastic models of fossil preservation show that even when disintegration is fast in the taphonomically-active zone (surface well-mixed layer, SML), its effect on time averaging and incompleteness in the historical layer can be minimized when shells can be sequestered by burrowers into the lower parts of the incompletely-mixed layer. To assess the prevalence of this sequestration effect, we estimate the covariation between incompleteness on one hand and disintegration, burial and exhumation on the other hand, fitting age-frequency distributions from 18 Holocene sediment cores to stochastic transition-rate matrices. The model has five parameters, including burial below the SML (a function of both sediment accumulation and downward mixing), exhumation into the SML, disintegration in the SML and below it, and disintegration of diagenetically-stabilized shells that were exhumed to the SML. We find that the majority of cores show a major decline in disintegration within the upper decimeters and exhibit the persistence of very old shells in the surface seabed, indicating some role of their diagenetic stabilization below the SML. Both burial and exhumation positively covary with disintegration in the SML. However, the incompleteness is negatively related to burial (varying between 90-99% at sites with slow sediment accumulation and between 50-90% at sites with fast sediment accumulation) but does not correlate with disintegration in the SML. These results indicate that although bioturbation positively covaries with disintegration in the SML, it can also increase preservation potential of some shells by transferring them below the SML.

How to cite: Tomašových, A., Kidwell, S. M., Kosnik, M., Kowalewski, M., Nawrot, R., Scarponi, D., and Zuschin, M.: Effects of disintegration on the incompleteness of the Holocene fossil record, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-13548, https://doi.org/10.5194/egusphere-egu26-13548, 2026.

Reconstructions of ocean temperature through deep time frequently rely on isotopic or elemental compositions biotic carbonates (e.g. foraminifera, coccolithophores, molluscs, brachiopods). But what can be done when fossil carbonate is absent? Several high-latitude early Cenozoic and Mesozoic sites (e.g. Svalbard, Denmark) are characterized by an early dissolution of such biotic carbonates, with rapid reprecipitation of the CaCO₃ as diagenetic carbonates, such as ikaite, calcite concretions, and interstitial calcite. Can these be of use in reconstructing palaeotemperature or paleoenvironmental conditions? What exactly do these carbonates represent? We investigate their age relative to their host sediment, and whether the pore waters they formed in could be considered to reflect ocean bottom-waters. We also look at possible kinetic biases and elemental fractionation during their formation, and if we can apply temperature proxies such as Mg/Ca ratios, stable oxygen isotopes, and/or clumped isotope thermometry.

How to cite: Vickers, M. L., Bernasconi, S. M., Fiebig, J., Bajnai, D., Looser, N., Evans, D., Rickaby, R. E., Longman, J., Frieling, J., Harper, D. T., and Jones, M. T.: Diagenetic carbonates as a tool for reconstructing water temperatures in fossil-poor sections, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-18601, https://doi.org/10.5194/egusphere-egu26-18601, 2026.

Earthworm-produced calcite granules are a distinct component of pedogenic carbonate that can be preserved in palaeosols, yet the processes that generate their carbonate chemistry and microstructure, as well as the extent to which these attributes are modified after excretion and during early diagenesis, remain poorly constrained. This limits confident identification in the rock record and interpretation of granule-derived carbonate signals within palaeosols and mixed siliciclastic–carbonate successions.

This project uses a staged experimental design to reconstruct the early diagenetic changes of Lumbricus terrestris calcite granules under controlled conditions. In Phase 1, granules are generated in replicated soil microcosms under independently varied organic carbon inputs (C₃ versus C₄ litter) and hydrological regimes (constant moisture versus dry–rewet cycling), producing material with known formation histories. In Phase 2, harvested granules are transferred to continuous-flow reactors supplied with defined Ca–HCO₃ solutions, with and without soil, to isolate post-excretion pedogenic modification processes including carbonate overgrowth, dissolution–reprecipitation, and trace-element redistribution. In Phase 3, granules are heated at 25–60 °C in controlled pore-fluid compositions to simulate shallow burial and low-grade early diagenetic conditions.

A consistent, multi-scale analytical framework is applied to granules recovered from each experimental stage, integrating isotopic, elemental, and microstructural information to assess equilibrium versus disequilibrium precipitation and progressive signal modification. Results obtained to date will be presented and evaluated alongside fossil granules from Miocene palaeosols, providing process-based constraints on the biogenic, pedogenic, and diagenetic contributions to soil-derived carbonates and their implications for carbonate-based paleoenvironmental proxies.

How to cite: Kerr, L., Brasier, A., Rogerson, M., and Hallet, P.: From formation to burial: experimental reconstruction of earthworm calcite granule modification, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-20873, https://doi.org/10.5194/egusphere-egu26-20873, 2026.

The Cradle of Humankind (locally referred to as the Cradle) is associated with the recovery of almost a third of the world’s early pre-human (hominin) fossils from a series of now heavily eroded cave sites.  Dating of these hominin and associated faunal fossils has been conducted through U-Pb based chronology of speleothems bounding fossiliferous clastic cave sediments. The dolomitic bedrock of the Cradle prompts the precipitation of aragonite within speleothems. Through varying levels of diagenesis, these aragonite crystals are observed as remnants within calcite. These layers within the speleothems are also rich in U, up to two orders of magnitude more concentrated than surrounding layers, even on a cm scale, and as such  have been specifically targeted for U-Pb dating (). However, the suitability of these diagenetic layers and quality of resulting age data has been questioned. Using 44 speleothem samples (flowstone and stalagmite) from 3 caves  across the Cradle, we investigate the integrity of the U-Pb chronometer  and applicability for future dating and palaeoclimatic and palaeoenvironmental reconstructions. Through petrography and trace element profiles, we identify conservative diagenesis and elevated uranium concentrations in most diagenetic layers with aragonite remnants. This petrographic-geochemical method acts as a screening and selecting tool for speleothems undergoing further analysis. Retention of anti-phase, step-like patterns of trace elements such as Sr and Mg, and high (>1 ug/g) U concentrations  support early conservative diagenesis for the majority of the these speleothems  and further enables identifying unsuitable samples based on their petrographic fabrics and noisy trace element profiles. Overall, our results suggest that these speleothems have remained geochemically ‘closed systems’, within minimal, likely early, diagenesis, which does not compromise the validity of the U-Pb ages.

How to cite: Luti, G., Edwards, T., Weij, R., and Pickering, R.: Aragonite remnants in calcite speleothems - to date or not to date?: a case study of speleothems from the Cradle of Humankind, South Africa, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-410, https://doi.org/10.5194/egusphere-egu26-410, 2026.

Karst features develop due to dissolution of carbonate and evaporitic rocks, producing geomorphologically complex and hydrologically sensitive landscapes. While karst development in humid and temperate regions is well documented, its occurrence in arid environments is less documented and understood. In Qatar, major unconformity occurs between Dammam (Eocene) and Dam formations (Miocene). Karsts occurs primarily within the Dammam Formation (Eocene) and are manifested as depressions and caves generated by dissolution, subsidence, and collapse processes. Field observations along with petrographic, cathodoluminescence, strontium isotope, and X-ray diffraction analyses were performed to determine the origin and timing of karstification affecting the Dammam Formation. Textural, geochemical and isotopic affinities between karstic infills and sediments of the overlying Dam Formation - particularly the Al-Kharrara and Al-Nakash members - indicate that material from the Dam Formation was trapped within Dammam cavities. Thus, Dam age sediment fills are found in karst features where the Dam is now absent by erosion suggesting a broader areal extent of the Dam Formation during the Miocene. Diagenetic and lithological relationships suggest that karst sediment infilling—and thus karst formation—postdates Oligocene dolomitization and silicification, with major speleogenesis occurring during the Oligocene. This event coincides with regional eustatic regression and tectonic uplift associated with the reactivation of the Qatar–South Fars Arch during the Zagros Orogeny. This study identifies a previously unrecognized karstification phase predating Pleistocene features, refining current models of Qatar’s landscape evolution and related geohazards. This suggests that geohazards associated with karsts are relatively stable due to the age of the major karst event.

How to cite: Ortiz, A., Allanic, C., Couëffé, R., Perrin, J., Noury, G., Dezes, M., Vilmus, T., Daranlot, J., Lemaire, B., Higgins, O., Matti, B., Lerevenu, C., Schönrock, A., Pillai, R., Al-Yafei, S., Ahmed, E., Samad, U., and Bukhari, S.: Oligocene karstification of the Dammam Formation in Qatar, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-7487, https://doi.org/10.5194/egusphere-egu26-7487, 2026.