Carbon-isotope stratigraphy across the Permian–Triassic boundary: A review

The Palaeozoic–Mesozoic transition is marked by distinct perturbations in the global carbon cycle resulting in a prominent negative carbon-isotope excursion at the Permian–Triassic (P–T) boundary, well known from a plethora of marine and continental sediments. Potential causes for this negative δ13C trend (and their links to the latest Permian mass extinction) have been intensively debated in the literature. In order to draw conclusions regarding causation, a general δ13C curve was defined after consideration of all available datasets and with due reference to the biostratigraphic background. The most important features of the P–T carbon-isotope trend are the following: the 4–7‰ δ13C decline (lasting ∼500,000 years) is gradual and began in the Changhsingian at the stratigraphic level of the C. bachmanni Zone. The decreasing trend is interrupted by a short-term positive event that starts at about the latest Permian low-latitude marine main extinction event horizon (=EH), indicating that the extinction itself cannot have caused the negative carbon-isotope excursion. After this short-term positive excursion, the δ13C decline continues to a first minimum at about the P–T boundary. A subsequent slight increase is followed by a second (occasionally two-peaked) minimum in the lower (and middle) I. isarcica Zone. The negative carbon-isotope excursion was most likely a consequence of a combination of different causes that may include: (1) direct and indirect effects of the Siberian Trap and contemporaneous volcanism and (2) anoxic deep waters occasionally reaching very shallow sea levels. A sudden release of isotopically light methane from oceanic sediment piles or permafrost soils as a source for the negative carbon-isotope trend is questionable at least for the time span a little below the EH and somewhat above the P–T boundary.