The Permian Chert Event in South China: New geochemical constraints and global implications

Abstract

The Permian Chert Event (PCE) was an unusual global chert accumulation that reflects tectonic–climatic–environmental changes during the late Paleozoic. South China was located near the equator in the eastern Paleo-Tethys Ocean at this time, and this region developed massive chert deposits during the Permian as part of the PCE. However, it is unclear whether upwelling-driven biological processes or tectonically related hydrothermal activity was the dominant origin for these cherts. To address this, we conducted a petrological and geochemical investigation of the chert deposits in the Lower Yangtze region, South China, and compared our results with those for cherts from other regions of South China. Our results show that dissolved silica derived from terrestrial input or silica-rich seawater was the dominant source of silica in the Permian cherts, which locally mixed with hydrothermal or volcanic silica sources. The strong positive correlation between biogenic silica and upwelling intensity, combined with the abundant siliceous organisms in the cherts, further indicates that upwelling-driven biological processes controlled the formation of the Permian cherts in South China. The PCE in South China began in the early Kungurian and peaked in the Guadalupian, which is slightly different from the timing in northwestern Pangea at higher latitudes. However, the PCE ended in all regions at the end of the Lopingian. The beginning of the PCE coincided with the waning of the Late Paleozoic Ice Age, which was caused by coeval tectonic and volcanic activity. With increased silicate chemical weathering during the interglacial period, vigorous upwelling driven by globally enhanced thermohaline circulation transported nutrients and dissolved silica to the surface ocean. This promoted the flourishing of siliceous organisms and the global development of biogenic chert. Coupled oceanic circulation and continental weathering, combined with the gradual lysocline shoaling, had a significant role in the expansion of the PCE. The cessation of chert deposition may have been related to the end-Permian severe environmental crisis and mass extinction.