Environmental and biotic turnover across the Permian–Triassic boundary on a shallow carbonate platform in western Zhejiang, South China

Abstract

The Huangzhishan section, 40 km from the well-known Meishan section, Zhejiang Province, South China, records a carbonate–siliciclastic Permian/Triassic (P/Tr) boundary succession. The P/Tr boundary sequences in both sections correlate well with one another, although the succession is much thicker at Huangzhishan than its equivalent at Meishan. The P/Tr transition is constrained by the Neogondolella yini, N. meishanensis and Hindeodus parvus conodont zones. Of these, the boundary between the first two zones marks the end-Permian mass extinction horizon. The base of H. parvus Zone defines the P/Tr boundary. The P/Tr mass extinction event is also indicated by a dramatic negative shift of the δ13Ccarb profile. This ecological crisis was facilitated by an abrupt environmental devastation indicated by an anoxic event. At Huangzhishan benthic habitats varied from open, oxygenated, to anoxic, to dysoxic-oxygenated and to dysoxic-anoxic conditions throughout the latest Changhsingian to early Griesbachian. Biotic turnover across the P/Tr boundary suggests the P/Tr mass extinction may have included two episodes in Huangzhishan. The first, perhaps major, episode occurs at the base of N. meishanensis Zone, corresponding to the event recorded at the base of Bed 25 in Meishan. The second coincides with the boundary between the H. parvus Zone and overlying Claraia–Ophiceras Assemblage zone, equivalent to the event recorded at the base of Bed 28 in Meishan. The first event did not see a significant decline in biodiversity, but is marked by an apparent loss of high-classification groups such as corals, sponges, macroalgae and most foraminifers. The Huangzhishan section records the most abundant and diverse surviving faunas among the P/Tr boundary sections around the world, with 54 species in 36 genera. However, they are severely affected by the Lilliput effect over the P/Tr extinction. Several fossil groups (bryozoans, blastoids and crinoids) temporarily survived the first episode of the P/Tr extinction and persisted into the earliest Triassic in Huangzhishan. Most survivors became extinct in the second episode of the P/Tr crisis. The Eumorphotis–Towapteria–Pteria bivalve fauna, widespread in shallow-water facies conodont-free P/Tr boundary sections in South China, occurs within the H. parvus Zone in Huangzhishan. This bivalve fauna therefore marks the P/Tr boundary in the conodont-barren P/Tr boundary sections. The top of this bivalve assemblage zone marks the second episode of the P/Tr crisis. Another slightly older bivalve fauna dominated by primitive forms of Claraia (e.g. C. bioni) marks the first/or main episode of the P/Tr extinction in the conodont-barren P/Tr boundary sections.