Abstract The processes governing dolomite [CaMg(CO 3 ) 2 ] formation remain among the most debated topics in sedimentary geology. Although primary dolomite can precipitate at low temperatures in certain modern environments, its scarcity today contrasts sharply with its abundance in ancient rocks—a discrepancy known as the ‘dolomite problem’. Dolomite typically forms through two pathways: (1) primary precipitation during early diagenesis, often influenced by microbial activity and organic matter and (2) secondary replacement of preexisting carbonates during burial at higher temperatures. In this study, we investigate Mg isotope fractionation in a modern sabkha in southern Qatar to evaluate its potential as a tracer of dolomite formation processes. We analysed δ 26 Mg and δ 44 Ca in surface‐ and pore waters, authigenic clays and organic‐ and leached dolomite‐containing fractions. Ca isotopes reveal an ~1‰ fractionation between pore water–organic matter and dolomite, consistent with a two‐step, biologically mediated formation pathway. Contrary, only minor 26 Mg enrichment in the organic fraction relative to pore water suggests that Mg isotopes alone provide limited evidence for such microbial mediation. Dolomite δ 26 Mg values (~−2.15‰) align with predictions for temperature‐dependent inorganic precipitation. Overall, the results indicate that microbial activity probably influences dolomite formation indirectly by altering local water chemistry rather than having a distinct Mg isotopic fractionation. These findings refine the application of Mg isotopes as proxies for dolomite genesis and offer new insights into carbonate diagenesis in saline environments.

Abstract The Permian–Triassic transition is characterised by major environmental changes and the largest known mass extinction event in the Phanerozoic. However, successions with a relatively complete sedimentological and palaeontological record across the Permian–Triassic are limited to a few well‐known sections. The Antalya and Aladağ Nappes in south‐western Türkiye provide extensive outcrops of the Permian–Triassic transition. Some key investigations have revealed the importance and potential for these successions to improve our understanding of the events across the Permian–Triassic. Here, we incorporate 3D open‐access virtual outcrop models with a virtual field guide to introduce the Permian–Triassic transition in south‐western Türkiye, to improve the accessibility, reproducibility and sustainability of fieldwork findings. The fossiliferous Upper Permian to Lower Triassic successions in the studied locations reach over a kilometre thickness. Due to the contemporaneous opening of the Neotethys Ocean, the exposures from the Antalya Nappe (Çürük Dağ in Kemer; Öznurtepe in Gazipaşa; and Demirtaş in Alanya) are considered to be deposited on a southward facing carbonate platform in the Neotethys Ocean, whilst successions from the Aladağ Nappe (Taşkent in Konya) are considered to be deposited on the northern side of the carbonate platform towards the Palaeotethys Ocean. In all sections, the Changhsingian (uppermost Permian) is represented by highly fossiliferous platform carbonates. The Changhsingian successions terminate with a thin oolitic grainstone (‘transitional oolites’), which is identified as the Permian–Triassic mass extinction interval and is characterised by a negative carbon isotope excursion. The transitional oolites are overlain by microbialite‐dominated carbonates, and then oolite‐dominated carbonates deposited in the Griesbachian. This carbonate‐rich deposition was replaced with a mixed carbonate‐siliciclastic succession later in the Early Triassic (Dienerian–Spathian), when marine ecosystems slowly recovered. These environmental and biotic changes are similar to the known record from the tropical palaeolatitudes in the western to eastern Palaeotethys.

Abstract Early marine diagenesis can alter the δ 13 C values of carbonate sediments in the marine environment, hindering interpretations of changes in global carbon cycling through geological time. In this study, the influence of sediment accumulation rate on the localisation and intensity of diagenetic alteration in the marine burial environment is documented. New measurements of δ 18 O and δ 13 C values of bulk carbonate sediments as well as siliciclastic and total organic carbon content were conducted on Holocene–Pleistocene aged sediments collected by Ocean Drilling Program Leg 133 on a proximal–distal transect on the slope adjacent to the Great Barrier Reef. Significantly lower bulk carbonate δ 13 C values and total organic carbon content were found to occur during periods of reduced sediment accumulation, but sediments in these intervals lack obvious sedimentological evidence of alteration, like the development of hardgrounds. New findings suggest that changes in sediment accumulation rate may explain large, asynchronous changes towards lower stable carbon isotope values of marine carbonates deposited in oxygenated slope settings in the geological record. Based on these results, periods of reduced sediment accumulation rate are proposed to enhance diagenetic reactions by providing continued access to oxidants. Alteration during periods of low sediment accumulation rate is likely to be exacerbated in settings containing mixed siliciclastic‐carbonate sediments. Finally, while this new observation indicates that subtle early marine burial diagenesis can obscure records of the global carbon cycle preserved in periplatform sediments, new insights may prove useful in constraining the dynamic nature of sedimentation patterns in slope environments through Earth history.

Abstract The Permian succession of the Paraná Basin records the progressive disconnection from the Panthalassic Ocean that bathed the southwestern Gondwana Supercontinent from the Ordovician to the early Permian. The development of the Gondwanides Belt in the southwestern part of the continent acted as an orographic barrier, restricting marine connections and trapping marine waters in a megalake. In this study, we present a detailed analysis of the transition from marine to continental environments in the Serra Alta and Corumbataí formations, documenting high‐resolution stratigraphic sequences with significant hydrological and salinity changes. Our data illustrate how sedimentation and accommodation dynamics shaped the evolution of this continental‐scale lake system, which was greatly influenced by meteorological phenomena, including storms and seiches. The megalake experienced transitions between overfilled, brackish to freshwater balanced‐fill and saline underfilled stages, associated with distinct fourth‐order transgressive and regressive stratigraphic sequences. These changes in water balance and salinity fostered the development of a unique, endemic bivalve‐dominated fauna derived from marine ancestors, highlighting the basin's response to changing environmental conditions. Notably, this study identifies tectonic events and climate shifts as primary allogenic forces controlling deposition. Meanwhile, local sediment dynamics and episodic events such as storms and seiches originated key autogenic changes in the resulting stacking patterns. Such meteorological phenomena generated an intriguing heterolithic pattern in the fine‐grained lake deposits, which would otherwise be confused with astronomical tides. Our results provide insights into the understanding of sedimentological processes in large lacustrine systems, with implications for paleoclimatic and paleoenvironmental reconstructions in megalakes from the geological record.

Abstract The Dohat Faishakh sabkha in Qatar was among the first modern environments studied to understand low‐temperature dolomite formation in association with gypsum and other evaporites. Since the 1960s, research conducted in this sabkha has significantly influenced geological models that remain widely used today, helping in the interpretation of sedimentary sequences that dominated certain periods of Earth's history. Here, we present results of an investigation of the dolomite occurring in this sabkha using techniques more advanced than those available during the initial pioneering studies. By integrating our new results with previously published data, we establish an ‘identity card’ for this sabkha dolomite and the environment it forms. The dolomite exhibits a rhombohedral morphology, contains 50.8 mol% Mg, and has an ordering degree of 0.25 (poorly ordered). Isotopic values are approximately: δ 13 C = 5.0‰, δ 18 O = 4.1‰ and δ 26 Mg = −2.6‰ to −1.5‰ and Δ 47 = 0.611‰. Annual temperature data indicate an average of 32.2°C in the subsurface intervals with the highest dolomite content. The associated pore water has an Mg/Ca ratio of 156, a salinity roughly nine times that of sea water and a pH of 6.9. Sediment total organic carbon is ~2%. Microbial diversity in the dolomite‐bearing layers is dominated by Euryarchaeota—an extremophilic, opportunistic and metabolically versatile archaeal phylum. Together, these data provide a reference for identifying sabkha‐type dolomites in the geological record, calibrating paleoclimatic proxies and interpreting biomarker signals that may be recorded in ancient dolomites.