Features and dolomitizing mechanisms in inner platform facies across the Permian-Triassic boundary (External Dinarides, Croatia)

The strata encompassing the Permian-Triassic boundary interval capture a pivotal period in Earth's history, with significant changes in Phanerozoic Earth system dynamics, culminating in a severe mass extinction. In carbonate platforms, this boundary is marked by a shift from skeletal to microbial carbonate production. Whereas extensive research has focused on the End-Permian Mass Extinction in open-marine shelf environments, the transition within inner platform facies remains underexplored due to limited dating options and pervasive dolomitization. This study examines the Permian-Triassic boundary interval at the continuous dolostone, Bru & scaron;ane-Sy section, in the External Dinarides (Croatia), that retains much of its original fabric. High-resolution petrography, biostratigraphy and chemostratigraphy (delta 13Ccarb and delta 13Corg) were utilized to detail sedimentary responses across the boundary. The Upper Permian fine-crystalline dolostone features well-preserved cryptomicrobial/bioclastic, peritidal microfacies with calcareous algae and foraminifera. In contrast, the Lower Triassic dolostone, shows a transition to a medium-crystalline, fabric-destructive dolostone texture. The transition from fabric-retentive Permian to fabric-destructive Triassic dolostone is attributed to two dolomitization processes: (i) Late Permian transgression facilitating aragonite/high Mg-calcite deposition, later transforming neomorphically into fabric-retentive dolostone texture due to abundant precursor dolomite nuclei; and (ii) dispersed Early Triassic primary dolomite precipitation later stabilized during shallow burial with decaying microbial mats serving as loci for crystal growth but decreased nucleation. This shift is recorded by a minimal negative delta 13Ccarb excursion (<= 0.7 parts per thousand) and a more pronounced shift in Delta 13C (delta 13Ccarb - delta 13Corg; ca 4.6 parts per thousand). Contrasting with typical open-marine Permian-Triassic boundary excursions, such isotopic features reflect the localized shift in primary production to photoautotrophy (algae and cyanobacteria) and early dolomitization in the presence of seawater-derived dissolved inorganic carbon. Understanding these sedimentary and diagenetic dynamics provides crucial insights into environmental changes and biogeochemical cycles affecting Permian-Triassic boundary dolomitization, offering a comprehensive view of the End-Permian Mass Extinction across a wider range of shallow marine carbonate dominated depositional environments.