Oceanic environmental changes on a shallow carbonate platform (Yangou, Jiangxi Province, South China) during the Permian-Triassic transition: Evidence from rare earth elements in conodont bioapatite

Abstract

Paleoceanic environmental stresses (e.g., expanded anoxia, elevated siliciclastic fluxes) are thought to have been important factors in the latest Permian mass extinction (LPME). Here, we investigate changes in redox conditions and siliciclastic fluxes during the Permian-Triassic transition in South China through in-situ analysis of the rare earth element (REE) composition of albid crowns of single conodont elements measured using laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS). For comparison, we also analyzed REEs in the sediment matrix enclosing the conodonts (i.e., whole-rock samples). The study locale (Yangou) was located on a shallow carbonate platform on the eastern margin of the Yangtze Platform to minimize the influence of detrital siliciclastics and, thus, enhance the chances of recovering a hydrogenous (seawater-sourced) REE signal.The analyzed conodonts exhibit REE patterns consistent with mixed hydrogenous (seawater) and lithogenous (siliciclastic) sources, with specimens from below the LPME horizon showing dominantly hydrogenous influences (Y/Ho >50) and those from above the LPME horizon showing dominantly lithogenous influences (Y/Ho <50). Locally elevated Ce/Ce* ratios and U concentrations in pre-LPME conodonts suggest an episodic shoaling of the oceanic chemocline, causing incursions of suboxic to anoxic seawaters onto the shallow Yangou carbonate platform. Chemical index of alteration (CIA) values increase sharply at the LPME, suggesting increased rates of continental erosion and a relative increase in chemical over physical weathering, possibly as a consequence of globally elevated temperatures. The present case study confirms that conodonts from relatively clean, low-clay (Al <0.5%) carbonate facies can potentially preserve original seawater REE signatures.