Organic geochemistry across the Permian–Triassic transition at the Idrijca Valley, Western Slovenia

Abstract

Bulk and molecular stable C isotopic compositions and biomarker distributions provide evidence for a diverse community of algal and bacterial organisms in the sedimentary organic matter of a carbonate section throughout the Permian–Triassic (P/Tr) transition at the Idrijca Valley, Western Slovenia. The input of algae and bacteria in all the Upper Permian and Lower Scythian samples is represented by the predominance of C 15–C 22 n-alkanes, odd C-number alkylcyclohexanes, C 27 steranes and substantial contents of C 21–C 30 acyclic isoprenoids. The occurrence of odd long-chain n-alkanes (C 22–C 30) and C 29 steranes in all the samples indicate a contribution of continental material. The decrease of C org and C carb contents, increase of Rock-Eval oxygen indices, and 13C-enrichment of the kerogen suggest a decrease in anoxia of the uppermost Permian bottom water. The predominance of odd C-number alkylcycloalkanes, C 27 steranes, and C 17 n-alkanes with δ 13C values ∼−30‰, and 13C-enrichment of the kerogens in the lowermost Scythian samples are evidence of greater algal productivity. This increased productivity was probably sustained by a high nutrient availability and changes of dissolved CO 2 speciation associated to the earliest Triassic transgression. A decrease of C org content in the uppermost Scythian samples, associated to a 13C-depletetion in the carbonates (up to 4‰) and individual n-alkanes (up to 3.4‰) compared to the Upper Permian samples, indicate lowering of the primary productivity (algae, cyanobacteria) and/or higher degradation of the organic matter.