Early Cambrian ocean anoxia in South China

Arising from: M. Wille, T. F. Nägler, B. Lehmann, S. Schröder & J. D. Kramers Nature 453, 767–769 (2008)10.1038/nature07072 ; Wille et al. reply The cause of the most marked changes in the evolution of life, which define the first-order stratigraphic boundary between the Precambrian and the Phanerozoic eon, remains enigmatic and a highly topical subject of debate. A global ocean anoxic event, triggered by large-scale hydrogen sulphide (H2S) release to surface waters, has been suggested by Wille et al.1, on the basis of two data sets from South China and Oman, to explain the fundamental biological changes across the Precambrian/Cambrian (PC/C) boundary. Here we report a new precise SHRIMP U–Pb zircon age of 532.3 ± 0.7 million years (Myr) ago (Fig. 1) for a volcanic ash bed in the critical unit that reflects the ocean anoxic event, the lowermost black shale sequence of the Niutitang Formation in the Guizhou Province, South China. This age is significantly younger than the precise PC/C boundary age of 542.0 ± 0.3 Myr ago2, approximately 10 Myr younger than the extinction of the Ediacaran fauna, and thus challenging the view of a major ocean anoxic event having been responsible for the major changes in the direction of evolution at the PC/C boundary. Figure 1 U–Pb concordia plot for zircon grains from a volcanic ash bed in the lowermost black shale sequence of the Niutitang Formation in Guizhou Province, South China. The analysed spots shown in yellow circles and the zircons display a clear oscillatory magmatic zonation (see inserted cathodoluminescence images) without inherited cores and were therefore chosen for constraining the age of crystallization (that is, 532.3 ± 0.7 Myr ago; with 1 SE error and MSWD (mean square of weighted deviates) = 0.24). PowerPoint slide