The link between metazoan diversity and paleo-oxygenation in the early Cambrian: An integrated palaeontological and geochemical record from the eastern Three Gorges Region of South China

Abstract

The link between environmental perturbations, especially oceanic redox conditions, and evolution during the “Cambrian Biodiversification” event is still highly controversial, although this issue has attracted considerable attention for decades. We here report on the fossil record of the early Cambrian biota discovered in the uppermost Yanjiahe and Shuijingtuo formations that crop out in the Luojiacun and Qiaojiaping sections, eastern Three Gorges area, South China. The uppermost member of the Yanjiahe Formation contains abundant benthic algae. Based on the fossil record, four distinct intervals were recognized in the Shuijingtuo Formation: i) few sessile suspension feeders, such as sponges, were recovered from Interval I; ii) Interval II is dominated by bivalved arthropods living in the water column; in this interval are also found a few sponges and eodiscoid trilobites, iii) no fossils other than few sponge spicules were recovered from Interval III; iv) diverse benthic communities composed of sponges, cnidarians, brachiopods, as well as bradoriids dominate Interval IV. Comprehensive research on framboidal pyrite morphology, whole rock geochemistry, as well as in-situ LA-ICP-MS analyses were carried out on samples from the Luojiacun section in order to assess paleo-redox conditions. The results show that the overall evolution of the paleo-redox conditions was characterized by a double episode of sharp anoxic events (AE) observed at the base of intervals I and III, followed by a gradual oxygenation event (OE). Evidence of oxygenation fluctuations were detected, especially within Interval IV. A case study is presented herein, which brings together early Cambrian changes in metazoan diversity and changes in environmental conditions. It is suggested that periodical oxygenation of the water column increased the biological diversity in the restricted basins of the eastern Three Gorges. Thus, four intervals of habitat succession can be tentatively reconstructed in the studied area for the Cambrian Age 2–Age 3 interval indicating a transition from uninhabited sediments to well-populated habitats. Finally, both the temporal and spatial characteristics of the Cambrian AEs/OEs and the fossil record in South China are compared and links between them are discussed in the context of their specific paleo-geographical setting.