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Molecular and Isotopic Records of the Permian-Triassic Transition at the Velebit Mt., Croatia

During the Middle/Late Permian and Early Triassic a continuous shallow water deposition took place at the eastern shelf of the Pangaea. Part of these carbonate rocks outcrop today in the central part of the Mt. Velebit, Croatia. Prevailing rock type is dolomite (70 to 99 wt% of CaMg(CO3)2), which is represented by the two lithological units: Transitional Dolomite and Sandy Dolomite, with narrow intercalation of oolithic dolomite in between. A detailed paleontological, isotopic, organic and inorganic geochemical studies have been carried out for this section, in order to investigate the environmental changes. Fossil communities show the evidence of two biotic crises, one which coincides with lithological change in the Upper Permian, and the other accompanied with the pronounced negative shift in  13Ccarb by up to 3‰ (VPDB), which is probably the consequence of global extinction at the Permian Triassic transition. Upper Permian values for  13Ccarb vary between – 0.8 to +2‰ , with average of +1.2&plusmn ; ; 0.5‰ , while Lower Triassic values range from – 1.3 to +0.9‰ , with an average of 0.0&plusmn ; ; 0.5‰ . At the transition level,  13Ccarb is +1.4 to  0.6‰ , average 0.4&plusmn ; ; 1.4‰ .  18Ocarb values in the Upper Permian range from – 3.2 to – 1.3‰ , with average – 2.6&plusmn ; ; 0.4‰ , and in the Lower Triassic from – 3.2 to – 2.2‰ , average – 2.7&plusmn ; ; 0.3‰ . This uniform slow decrease in  18Ocarb values is probably due to an enhanced terrigenous input towards the P/Tr transition and in the Lower Triassic. Upper Permian  13C for kerogens vary from – 27.3 to – 24.4‰ , average – 25.8&plusmn ; ; 0.9‰ . The values at the transition level are – 27.0 to – 27.1‰ , and the Lower Triassic values range from – 29.1 to – 26.4‰ , average – 27.5&plusmn ; ; 0.4‰ . The range of ~4.7‰ is probably a result of the variations in the primary composition of the organic matter. A constant decrease in the values of  13C ker is accompanied with the negative trend of  13C for carbonates. The value of  13Ckarb-ker shows the same trend as the  13Ccarb and  13Cker, indicating lower productivity at the Permian Triassic transition, and a primary source for inorganic and organic signals, with no secondary change.  15N values for the Upper Permian samples are – 0.7 to +8.2‰ , with an average value of +3.7&plusmn ; ; 2.8‰ , for the transition samples – 0.7 to +0.8‰ , average 0.0&plusmn ; ; 1.0‰ , and for the Lower Triassic samples +0.3 to +6.9‰ , average +3.8&plusmn ; ; 2.0‰ . All together, values for the  15Nker vary between – 2.4 and +8.2 ‰ which point to the mixed contribution of 15N-rich marine (~7‰ ), mostly in the Upper Permian, 15N-depleted terrestrial (~0 ‰ ) organic materials towards the transition, and cyanobacteria (– 2 to +4 ‰ ) at the transition level and in the Lower Triassic. The distribution of n-alkanes (C13 to C34), acyclic isoprenoids (C21 to C28), hopanes and steranes indicate input of bacterial and algal biomass, however there is no pronounced change between the Late Permian and Early Triassic samples. Odd long-chain n-alkanes (maximazing at C26) and C39 steranes confirm the important contribution of continental debris. Results of compound specific C isotope analyses of alkanes indicate an increase in 13C values after the transition, showing gradual recovery of biota after the extinction event.

Permian-Triassic transition; stable isotopes; Velebit Mt.

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