Recording crisis and recovery: carbonate concretions as potential archives of Permo–Triassic change in the Karoo Basin

The end-Permian mass extinction (~252 Ma) represents the most profound biotic turnover in Earth’s history. While marine successions are well dated and chemically characterised, the terrestrial record has only recently begun to achieve comparable chronological resolution. The Karoo Basin in South Africa preserves one of the most continuous continental records spanning the extinction and early Triassic recovery, yet few sequences have direct radiometric ages. Recent progress has come from U–Pb dating of detrital zircons, although differences in maximum depositional ages, uneven stratigraphic sampling (particularly across the Triassic), and the potential for the youngest grains to be undersampled in detrital populations limit the development of a fully resolved continental timescale.

Pedogenic carbonate concretions are widespread within Karoo successions and offer an alternative chronometer. Their ubiquity and potential to preserve primary geochemical signatures make them promising but still underutilised targets for both U–Pb dating and palaeoenvironmental reconstruction. Here we evaluate their suitability through an integrated petrographic, geochemical, and geochronological approach. More than 100 carbonate concretions were collected from known end-Permian localities, representing diverse concretion morphologies, facies settings, and stratigraphic positions. This dataset enables assessment of which concretion types and growth histories best retain datable U–Pb systematics and palaeoenvironmental signals.

LA–ICP–MS U–Pb dating was carried out using a Nu Instruments Attom coupled to a Resolution SE 193 nm laser ablation system. Trace-element mapping via Q–ICP–MS (e.g. Mg, Sr, Ba, U, Th, Pb) was used as a screening tool to identify zones with favourable chemistries (e.g. U > 1 ppm and U > Pb). Initial results show that several concretion types preserve coherent U–Pb systematics and yield resolvable ages, while trace element and calcite fabrics reveal a spectrum from primary preservation through early diagenesis to complete recrystallisation. These relationships allow us to establish criteria for selecting the most reliable microdomains and concretion types for dating and to inform future field work.

This study adds to a rapidly growing body of research using LA-ICP-MS-U-Pb geochronology on pedogenic carbonates globally, and provides the first systematic attempt to geochemically characterise and directly date carbonate concretions in the Karoo Basin. It demonstrates the potential for pedogenic carbonates to refine terrestrial timescales across the end-Permian transition and contributes a framework that can be applied to continental settings globally where conventional chronometers remain limited.