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Redefinition of the Permian-Triassic Boundary in Velebit Mt., Croatia: New geochemical and isotope data

New biogeochemical data redefined the position of the Permian-Triassic boundary (PTB) in the Velebit Mt., Croatia, previously positioned at the lithological boundary, and indicate two stress events. Pronounced enrichment in REE concentrations, negative Ce anomaly, negative shift in d13Cker values and disappearance of microfossils coincide with the occurrence of ooids, marking the first stress event, most probably the Late Permian regression. The second event is the PTB, with the well known negative shift in d13Ccarb from +1.4 to -0.6&#8240; (VPDB ; P: -0.8 to +2&#8240; ; Tr: -1.3 to +0.9&#8240; ) and biota impoverishment. The d13Cker values at PTB are - 27.1&#8240; , P: -27.3 to -24.4&#8240; , and Tr: -29.1 to -26.4&#8240; . The d15Nker values show a preferential marine influence during the Late Permian (~7&#8240; ) before the regression phase, and enhanced terrestrial influence towards the PTB with presence of cyanobacteria (-2 to +4&#8240; ), which seems to be the only surviving taxa. Long chained n-alkanes (C17-C31, max. C26), with even/odd predominance are probably derived from the freshwater green microalga Botryococcus braunii, indicating lagoonal-type environment. The C17/C18 ratios show a stronger algal influence during Early Triassic. Prystane and phytane are present in most samples and they can be derived from chlorophyll in algae and cyanobacteria. The distribution of n-alkylcyclohexanes (C17-C24, max. C21) with even/odd predominance indicate a bacterial biomass contribution. Values of Pr/Ph ratio are mostly <1, probably indicating anoxic conditions. Hopanes, the prokaryota biomarkers, have been identified in almost all samples (C29 to C32), while steranes, biomarkers of eukaryotic organisms, have been found mostly in small abundances, in the range dia-C27 to ste-C29, usually maximizing at ste-C27, indicating marine input.

Stable isotopes; Permian-Triassic boundary; Velebit Mt.

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