Articles | Volume 12, issue 5
Research article
12 Mar 2015
Research article |  | 12 Mar 2015

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

L. Zhang, L. Zhao, Z.-Q. Chen, T. J. Algeo, Y. Li, and L. Cao

Abstract. 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 (δ13Ccarb) and carbonate-associated sulfate sulfur isotopes (δ34SCAS), rare earth elements, and elemental paleoredox and paleoproductivity proxies. The SSB at Shitouzhai is characterized by a +4‰ shift in δ13Ccarb and a −10 to −15‰ shift in δ34SCAS, recording negative covariation that diverges from the positive δ13Ccarb−δ34SCAS 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 nutrient- and sulfide-rich deep waters to the ocean-surface layer. Enhanced upwelling was likely a response to climatic cooling and the reinvigoration of global-ocean 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.

Short summary
The Smithian--Spathian boundary was a key event in the recovery of marine environments and ecosystems following the end-Permian mass extinction ~1.5 million years earlier. Our analysis of the Shitouzhai section in South China reveals major changes in oceanographic conditions at the SSB intensification of oceanic circulation leading to enhanced upwelling of nutrient- and sulfide-rich deep waters and coinciding with an abrupt cooling that terminated the Early Triassic hothouse climate.
Final-revised paper