Ostracods (Crustacea) and water oxygenation in the earliest Triassic of South China: implications for oceanic events at the end-Permian mass extinction

Ostracods (Crustacea) are benthic inhabitants well known for their consistent qualities as paleoenvironment markers. In particular, they are reliable indicators of water oxygenation level: filter feeders are more common in poor oxygen conditions, contrasting with deposit feeders, which are abundant in well-oxygenated settings. In the Permian/Triassic (P/Tr) boundary transition in the Great Bank of Guizhou, ostracod species are dominated by deposit feeders, showing well-oxygenated conditions from the latest Permian, through the extinction level into the earliest Triassic. These results are consistent with ostracod faunas from northwest Guangxi Province. However, these two examples are in contrast with coeval ostracods from Sichuan, which show lower-than-normal oxygen levels in the earliest Triassic. The Great Bank of Guizhou forms an isolated platform in the large Nanpanjiang Basin on the south side of the South China Block; northwest Guangxi is nearby, in a marginal setting: both faced the Panthalassa Ocean through the P/Tr boundary times according to several published paleogeographic reconstructions. In contrast, P/Tr boundary transition rocks in Sichuan Province, located ∼600 km north of the Great Bank of Guizhou, lie on the Tethyan side of the South China Block. Both the Great Bank of Guizhou and the Sichuan sites have earliest Triassic microbialites, but these are profoundly different in structure and composition. The difference between the two areas may reflect contrasts in the nature of circulating ocean waters, with reduced levels of oxygenation in the Tethys (Sichuan), associated with modelled slow circulation, in contrast to better circulated Panthalassa ocean waters (Great Bank of Guizhou and northwest Guangxi). This also may be an argument to show that low oxygenated, or even anoxic, waters were not the only reason for the P/Tr boundary crisis.