A refined biochemostratigraphic framework for the Induan

The Early Triassic was an interval of protracted marine biotic recovery from a mass extinction, environmental change, and large shifts in carbon isotope records worldwide, yet the present geochronological framework for this interval has been cobbled together over time without formal definitions and placements of most stage and substage boundaries, impeding understanding of carbon-cycle perturbations and environmental changes during the recovery interval after the end-Permian mass extinction (EPME). Here, we generate high-resolution, unified biochemostratigraphic frameworks for two stratigraphically expanded Lower Triassic sections in Iran (Abadeh and Zal), which we compare and correlate with existing conodont and C-isotope records globally. Our new high-resolution carbon-isotope profiles for these sections record two negative shifts in the lower Dienerian (N2a) and mid-Dienerian (N2b) as well as two positive shifts in the upper Dienerian (P2a) and lower Smithian (P2b), clarifying δ¹³C_carb features that have been only intermittently noted in other Lower Triassic sections globally. Given the scarcity of conodonts in some marine sections, a high-resolution standard carbon isotope profile may prove useful in locating the boundaries of Induan substages. In the Iranian sections, the Griesbachian-Dienerian boundary (GDB) is located immediately below the N2a minimum, and the Induan-Olenekian boundary (IOB) is between the P2a and P2b peaks. Since the Iranian sections are among the most complete Induan successions globally, we propose them as auxiliary reference sections for formal definition of Lower Triassic substage boundaries, a task that remains uncompleted at present. We also undertake a review of Lower Triassic sections globally, summarizing key biostratigraphic (i.e., ammonoid and conodont) and chemostratigraphic (i.e., carbon isotope) data for multiple regions (e.g., the Boreal, Panthalassic, and Tethyan realms).