An astronomical timescale for the Permian-Triassic mass extinction reveals a two-step, million-year-long terrestrial crisis in South China

The Permian-Triassic Mass Extinction (PTME) is the greatest biotic crisis of the Phanerozoic. In terrestrial settings, the PTME appears to have been diachronous and it has been suggested that losses initiated before the marine crisis. We examine organic carbon-isotope (δ13Corg) and geochemical proxies for environmental change in a palaeotropical wetland succession from southwest China. A newly constructed astronomical timescale provides a temporal framework for constraining the timing of the terrestrial PTME. Two major, negative carbon isotope excursions (CIEs) of 5.3 and 3.9 are observed between the top of the (Permian) Xuanwei Formation and the middle of the (Permian-Triassic) Kayitou Formation respectively. Our cyclostratigraphic model suggests that carbon cycle destabilization lasted ∼0.6 ±0.1 Myr. We calculate total erosion rates for basaltic landscapes as a proxy for volumes of bare soil resulting from deforestation. Two phases of accelerated erosion saw denudation rates rise over a ∼1 Myr period from ∼150 t/km2/yr in the upper Xuanwei Formation (Permian) to >2000 t/km2/yr at the base of the Dongchuan Formation (Triassic). Calibrating the collapse of terrestrial ecosystems indicates that although the equatorial terrestrial PTME initiated before the marine crisis, it was a protracted process with the final coup-de-grâce not until ∼700ky later. This has a bearing on extinction scenarios in which the terrestrial PTME is a causal factor in marine losses via enhanced nutrient runoff, because the final devastation on land post-dates the much more abrupt marine PTME.