Cyanobacterial blooms tied to volcanism during the 5 m.y. Permo-Triassic biotic crisis

In a recent paper, Xie et al. (2010) elegantly demonstrate that cyanobacterial blooms recorded in Permian–Triassic (P–Tr) rocks are closely associated with local volcanic activity in South China, and with the development of large negative global carbon isotope excursions. Using a compiled data set of published U/Pb and 40Ar/39Ar ages obtained on volcanic ash layers from South China, and volcanic rocks of the Siberian Traps (ST), respectively, Xie et al. argue that the volcanism associated with the ST is predominantly younger than the P–Tr boundary age. Xie et al. note that the majority of ST 40Ar/39Ar ages (e.g., Reichow et al., 2009) are similar to U/Pb zircon ages for two Triassic boundaries, and consequently that ST volcanism was likely responsible for the prolonged stress in the Early Triassic ecosystems. However, the suggested age correlation is flawed, and the purpose of this Comment is to challenge the comparison based on ages obtained by different methodologies, and demonstrate that one of the conclusions drawn by Xie et al. is invalid.