Amelioration of marine environments at the Smithian–Spathian boundary, Early Triassic

The protracted recovery of marine ecosystems following the Permian–Triassic mass extinction may have been caused, in part, by episodic environmental and climatic crises during the Early Triassic, among which the Smithian– Spathian boundary (SSB) event is conspicuous. Here, we investigate the SSB event in the Shitouzhai section, Guizhou Province, South China, using a combination of carbonate carbon (δCcarb) and carbonate-associated sulfate sulfur isotopes (δSCAS), rare earth elements, and elemental paleoredox and paleoproductivity proxies. The SSB at Shitouzhai is characterized by a +4 ‰ shift in δCcarb and a −10 to −15 ‰ shift in δSCAS, recording negative covariation that diverges from the positive δCcarb-δ SCAS covariation that characterizes most of the Early Triassic. This pattern is inferred to reflect an increase in organic carbon burial (e.g., due to elevated marine productivity) concurrently with the oxidation of isotopically light H2S, as the result of enhanced vertical advection of nutrientand sulfide-rich deep waters to the ocean-surface layer. Enhanced upwelling was likely a response to climatic cooling and the reinvigoration of globalocean overturning circulation at the SSB. Coeval decreases in chemical weathering intensity and detrital sediment flux at Shitouzhai are also consistent with climatic cooling. A decline in marine biodiversity was probably associated with the late Smithian thermal maximum (LSTM) rather than with the SSB per se. The SSB thus marked the termination of the extreme hothouse conditions of the Griesbachian– Smithian substages of the Early Triassic and is significant as a record of accompanying climatic, environmental, and biotic changes. The ultimate cause of the SSB event is uncertain but may have been related to a reduction in intrusive magmatic activity in the Siberian Traps large igneous province.