THE PERMIAN-TRIASSIC BOUNDARY AT NHI TAO, VIETNAM: EVIDENCE FOR RECURRENT INFLUX OF SULFIDIC WATERMASSES TO A SHALLOW-MARINE CARBONATE PLATFORM

The Permian-Triassic boundary (PTB) at Nhi Tao, Vietnam* occurs within a nearly pure carbonate succession, facilitating recognition of changes in the hydrogenous (seawater-derived) fraction that may represent contemporaneous changes in paleocean chemistry. The extinction horizon (EH) at which most Late Permian faunal elements disappear coincides with a 12-cm-thick oolitic grainstone layer that also marks the appearance of several geochemical anomalies that continue into the Lower Triassic part of the section. Eight pyrite [S] peaks within a 5-m-thick interval above the EH exhibit a strong negative relationship to both carbonate δ¹³C and pyrite δ³⁴S (i.e., higher S concentrations associated with isotopically more depleted values). These relationships can be accounted for through episodic upwelling of sulfidic, ¹²C- and ³²S-enriched deep-ocean waters onto the shallow Jinxi Platform, located on the southern margin of the South China Craton. Quasi-cyclic episodes of upwelling may represent multiple false starts at renewed global-ocean overturn modulated by Earth's 20-kyr orbital precession cycle following prolonged Late Permian stagnation of the deep ocean. Large-scale upwelling of deepwaters suggests that the end-Permian marine mass extinction was caused by anoxia and/or sulfide toxicity, and that recurrent episodes may have been a factor in delaying significant recovery of marine ecosystems until the start of the Middle Triassic. The magnitude (-3‰) and rate of the carbonate δ¹³C shift across the PTB are consistent with a “Strangelove ocean” model with a sustained ~50% decline in primary productivity. A major increase in magnetic susceptibility values 12 cm above the EH may record the influx of fine soil-derived particles following destruction of terrestrial ecosystems. The lag is consistent with the interval required for transport of fine particles across the Nanpanjiang Basin to the relatively distal site of the Jinxi Platform. Within existing limits of uncertainty, the end-Permian marine and terrestrial crises recorded at Nhi Tao appear to have occurred synchronously. *Algeo, T.J., Ellwood, B.B., Nguyen, K.T., Rowe, H., Maynard, J.B., in press, The Permian-Triassic boundary at Nhi Tao, Vietnam: Evidence for recurrent influx of sulfidic watermasses to a shallow-marine carbonate platform: Palaeogeography Palaeoclimatology Palaeoecology.