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

Abstract

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 δ 13C 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 δ 13C 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‰ δ 13C 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 δ 13C 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.