Latest Permian to Middle Triassic redox condition variations in ramp settings, South China: Pyrite framboid evidence

摘要

A detailed, 10 m.y. redox history of Changhsingian to Anisian (latest Permian to Middle Triassic) oceans in ramp settings is reconstructed based on framboidal pyrite analysis from South China. The result shows that the well-established phenomenon of intense ocean euxinia-anoxia is faithfully recorded in pyrite framboid data. Three major euxinia-anoxia episodes, namely, the end-Changhsingian to end-Smithian, middle to late Spathian, and early to middle Anisian, have been recognized from the ramp successions. The first reducing episode is subdivided into four subepisodes: Permian-Triassic boundary, Griesbachian-Dienerian boundary, earliest Smithian, and end-Smithian. Redox variations broadly track other oceanographic proxies. Euxinia-anoxia episodes coincide with positive excursions of conodont ΩCe anomalies, negative excursions of δ34Scas (carbonate-associated sulfate), increases in sea-surface temperature, and negative excursions of δ13C in most cases. However, euxinia-anoxia near the Dienerian-Smithian boundary coincided with positive excursions of δ13C and a general cooling period. This exception may be the result of locally developed water-column anoxia. The Permian-Triassic boundary subepisode witnessed two ephemeral euxinia-anoxia events separated by a dysoxic to oxic period. The former, together with a rapid increase in sea-surface temperature (up to 8 °C), may have been responsible for the biodiversity crisis, while the latter anoxic event destroyed ecosystem trophic structures. In addition to the Permian-Triassic boundary euxinia-anoxia event, which spread over habitats in all oceans, the Spathian and Anisian euxinia-anoxia episodes also prevailed in global oceans. Variation of the oxygen minimum zone are suggested as the driving mechanism that facilitated the movement of oxygen-poor water columns in various paleogeographic settings over this critical period.