The Permian–Triassic boundary in Western Slovenia (Idrijca Valley section): magnetostratigraphy, stable isotopes, and elemental variations

Abstract

Stable isotope analyses of carbonate ( δ 13C carb) and total organic carbon (TOC; δ 13C org), together with geochemical analyses of 54 major, minor, and trace elements and magnetic susceptibility measurements were carried out on whole rock samples of the undisturbed Permian–Triassic (P/Tr) limestone boundary sequence in the Idrijca Valley (W. Slovenia). At the P/Tr boundary, there is a 0.8-cm thick clayey marl layer (PTB) showing a characteristic magnetic susceptibility pulse and considerable enrichment in most major, minor, and trace elements. The P/Tr transition is characterized by (1) an abrupt decrease in sedimentation rate from 32 to 5 cm/100 ka, (2) a well known prominent negative excursion of δ 13C carb and δ 13C org, reflecting global perturbations in the carbon cycle, and (3) the drastic disappearance of typically Upper Permian marine fauna. Although the shape of the δ 13C carb curve indicates gradual long-term change across the P/Tr boundary, the sharp negative δ 13C org anomaly within an interval from 30 cm below to 6 cm above the boundary suggests an interruption of those gradual processes. Our observations for selected redox sensitive elements (Mo, U, V and Zn), abundances for organic carbon and sulphur and the rare earth elements (REE) distribution in the boundary sequence, as well as the shape of the Ce/Ce* curve suggest that oceanic anoxia was typical of the Upper Permian, and that the transition to oxygenated conditions occurred at the P/Tr boundary. Oxygenation at the P/Tr transition is coincidental with the terminal phase of the Upper Permian marine regression. The shape of the Ce/Ce* curve indicates that the redox environmental conditions changed again in the earliest Triassic, resulting once more in oxygen deficient conditions.