Stable isotope and trace element stratigraphy across the Permian–Triassic transition: A redefinition of the boundary in the Velebit Mountain, Croatia

Abstract

Stable isotopes of carbonates (δ 13C carb, δ 18O carb), organic matter (δ 13C org, δ 15N org) and major, trace and rare earth element (REE) compositions of marine carbonate rocks of Late Permian to Early Triassic age were used to establish the position of the Permian–Triassic boundary (PTB) at two continuous sections in the Velebit Mountain, Croatia. The chosen sections — Rizvanuša and Brezimenjača — are composed of two lithostratigraphic units, the Upper Permian Transitional Dolomite and the overlying Sandy Dolomite. The contact between these units, characterized by the erosional features and sudden occurrence of ooids and siliciclastic grains, was previously considered as the chronostratigraphic PTB. The Sandy Dolomite is characterized by high content of non-carbonate material (up to ~ 30 wt.% insoluble residue), originated from erosion of the uplifted hinterland. A relatively rich assemblage of Permian fossils (including Geinitzina, Globivalvulina, Hemigordius, bioclasts of gastropods, ostracods and brachiopods) was found for the first time in Sandy Dolomite, 5 m above the lithologic boundary in the Rizvanuša section. A rather abrupt negative δ 13C carb excursion in both sections appears in rocks showing no recognizable facies change within the Sandy Dolomite, −2‰ at Rizvanuša and −1.2‰ at Brezimenjača, 11 m and 0.2 m above the lithologic contact, respectively. This level within the lower part of the Sandy Dolomite is proposed as the chemostratigraphic PTB. In the Rizvanuša section, the δ 13C org values decline gradually from ~ −25‰ in the Upper Permian to ~ −29‰ in the Lower Triassic. The first negative δ 13C org excursion occurs above the lithologic contact, within the uppermost Permian deposits, and appears to be related to the input of terrigenous material. The release of isotopically light microbial soil-biomass into the shallow-marine water may explain this sudden decrease of δ 13C org values below the PTB. This would support the hypothesis that in the western Tethyan realm the land extinction, triggering a sudden drop of woody vegetation and related land erosion, preceded the marine extinction. The relatively low δ 15N org values at the Permian–Triassic (P–Tr) transition level, close to ≈ 0‰, and a secondary negative δ 13C org excursion of −0.5‰ point to significant terrestrial input and primary contribution of cyanobacteria. The profiles of the concentrations of redox-sensitive elements (Ce, Mn, Fe, V), biogenic or biogenic-scavenged elements (P, Ba, Zn, V), Ce/Ce* values, and normalized trace elements, including Ba/Al, Ba/Fe, Ti/Al, Al/(Al + Fe + Mn) and Mn/Ti show clear excursions at the Transitional Dolomite–Sandy Dolomite lithologic boundary and the chemostratigraphic P–Tr boundary. The stratigraphic variations indicate a major regression phase marking the lithologic boundary, transgressive phases in the latest Permian and a gradual change into shallow/stagnant anoxic marine environment towards the P–Tr boundary level and during the earliest Triassic.