Permian–Triassic boundary interval in the Middle East (Iran and N. Oman): Progressive environmental change from detailed carbonate carbon isotope marine curve and sedimentary evolution

The end Permian negative δ13C excursion is well known but its cause and chronology are still in question. In an attempt to decipher the timing and reasons for this strong decrease, we established high-resolution sedimentological, biostratigraphical and carbonate carbon isotopic studies in four sections in Iran (Abadeh, Shareeza, Zal and Djulfa) and two in Oman (Wadi Sahtan). The position of the sections on two different margins of the Neotethys has allowed us to distinguish between local and global signals. This high-resolution chemostratigraphy, tested for isochrony on the basis of an updated high-resolution conodont and ammonoid biostratigraphy, allows the discrimination of nine successive isotopic events (IE 0–8) for the Permian–Triassic Boundary Interval (PTBI). The negative excursion of the PTBI has been separated into four distinct parts. A first drop (IE 0) occurs in the Late Wuchiapingian with an amplitude of around 1.3‰. A second decrease begins (IE 1–2) at the base of the Dzhulfites ammonoid beds (Changhsingian) and continues gradually until the extinction event (top jolfensis conodont interval, without important second-order variations. Its amplitude varies between 1.5‰ and 2.8‰ and its duration has been estimated around 2.2 Ma, revealing a non-catastrophic phenomenon. The lower meishanensis–praeparvus conodont Zone records stationary to slightly increasing values (IE 3). The interval of the curve after the extinction event, between the upper meishanensis–praeparvus Zone and the base of the I. isarcica Zone, records numerous second-order variations (IE 4–7). Its amplitude is of 1.2–3.1‰. These second-order variations are correlatable in Iran but with some uncertainty with Oman. Poor correlation of these small peaks with others published sections might be due to a higher sensitivity to local perturbations and a lower buffer capacity of the whole ocean. We assume that the processes leading to the extinction event also caused the isotopic decrease before the extinction level with a maximal amplitude of only 2.8‰. This allows consideration of some possible mechanisms that were rejected as unrealistic to cause a 4–6‰ excursion.