Carbon isotope record of the P/T boundary and the Lower Triassic in the Southern Alps: Evidence for rapid changes in storage of organic carbon

The carbonate carbon isotope record of Lower Triassic shelf sediments was investigated in the Pufels and L´Om Picol (Uomo) sections of the western central Dolomites to derive a complete record from the Permian/Triassic transition to the uppermost Lower Triassic from the western Tethys. The carbon isotope record shows the well-studied decrease at the Permian/Triassic boundary (PTB), followed by an increase to about 1.5‰. At the Dienerian/Smithian boundary the δ13C values increase to a pronounced + 6‰ maximum. A steady rapid drop until the upper Smithian below − 2‰ follows this δ13C peak. At the Smithian/Spathian transition, a steep rise to positive values is documented. The positive carbon isotope peak is interpreted to reflect an interval of enhanced burial of organic carbon, whereas the isotopic minima provide evidence of periods of oxidation of organic matter and/or release of isotopically light carbon. This may be ascribed to quickly changing environmental conditions, which have already been postulated by paleontologists due to the delayed biotic recovery from the Permian–Triassic extinction event. Because of the short-time interval these positive isotope excursions represent, their generation by burial of organic carbon in periods of high productivity and/or anoxia appears plausible. Concurrent carbon isotopic excursions are reported from Iran, Pakistan and Oman as well as from Southern China, which indicate an at least Tethys-wide event. If the excursion proves to be of global significance, dark shale deposited in the pelagic Panthalassa Ocean may have acted as sink for organic carbon, supporting the hypothesis of a periodically stratified ocean in the Lower Triassic.