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

Abstract

Abstract. Life on Earth underwent a protracted recovery following the Permian–Triassic mass extinction. The slowness of the recovery process 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, South China, using a combination of carbonate carbon (δ13Ccarb) and carbonate-associated sulfate sulfur isotopes (δ34SCAS), rare earth elements, and elemental palaeoredox and palaeoproductivity proxies. Unlike the positive δ13Ccarb-δ34SCAS covariation that characterizes most of the Early Triassic, the SSB at Shitouzhai exhibits negative covariation between δ13Ccarb (+4‰) and δ34SCAS (−14‰). This relationship may reflect an increase in organic carbon burial (e.g., due to elevated marine productivity) concurrently with oxidation of isotopically light H2S, a pattern that we attribute to enhanced vertical advection of nutrient- and sulfide-rich deepwaters to the ocean-surface layer. Enhanced upwelling was a likely response to the well-documented climatic cooling event at the SSB that terminated the extreme hothouse conditions of the Griesbachian–Smithian, a cooling that we infer to have transiently invigorated the global-ocean overturning circulation. Evidence at Shitouzhai for concurrent decreases in chemical weathering intensity and detrital sediment influx are also consistent with climatic cooling. A penecontemporaneous decline in marine biodiversity was probably associated with the late Smithian thermal maximum rather than the SSB event itself, and the affected marine faunas did not recover immediately in response to climatic and environmental amelioration at the SSB but, rather, underwent a stepwise recovery during the early to middle Spathian. The ultimate cause of the SSB event is uncertain but may have been related to reduced intrusive magmatic activity in the Siberian Traps Large Igneous Province.