Proliferation of shallow-water radiolarians coinciding with enhanced oceanic productivity in reducing conditions during the Middle Permian, South China: evidence from the Gufeng Formation of western Hubei Province
The Middle Permian is a critical period when a possible cooling regime prevailed and massive deep-water cherty sediments were deposited in the South China Craton. An integration of radiolarian paleoecologic and geochemical studies from the Middle Permian Gufeng cherty succession of western Hubei Province, South China tests the relationships among oceanic productivity, redox conditions, and radiolarian responses as well as origin of the cherts. A total of 21 species and six undetermined species in 10 radiolarian genera are identified from the Gufeng Formation in the northern margin basin of the Yangtze Platform of the South China Craton. Three radiolarian Pseudoalbaillella globosa, Follicucullus monacanthus, and Follicucullus scholasticus zones were established and constrained the Gufeng Formation as Roadian to Middle Capitanian in age. The Gufeng cherts embrace a biologic and hydrothermal origin in the northern marginal basin and southern margin basin (the Nanpanjiang Basin) of the South China Craton, respectively. The high oceanic primary productivity in surface oceans resulted in the widespread reducing conditions in the bottom water column. Such high oceanic productivity event slightly predated the Middle Permian Kamura cooling event of the Panthalassian Ocean. Both the elevated continent weathering due to pre-eruptive crustal uplift of the Emeishan flood volcanism and active oceanic upwelling may have stimulated high oceanic productivity and facilitated the deposition of the Gufeng cherts. All radiolarians bloomed in relatively oxic conditions. Only shallow-water radiolarians, mainly spherical forms, proliferated in the reducing conditions, which also coincided with an enhanced oceanic productivity. The abundance of spherical radiolarians therefore is an ideal proxy indicating oceanic primary productivity.