Multiple sulfur isotopes and stratigraphic constraints for the 34S enrichments in the late Ediacaran-Cambrian pyrite-sulfur record: A product of supercontinent assembly and restricted seas.

The late Ediacaran record encompasses frequent occurrences of ³⁴S-enriched pyrites, which can surpass the coexisting sulfate isotope values. Deciphering what controls this marine isotope record is important to understand its biogeochemical implications, especially considering the early metazoan evolution. We present here multiple sulfur isotope compositions from pyrites of two distinct geotectonic settings within the late Ediacaran-early Cambrian West Gondwana, allowing to constraint spatial and temporal distribution of ³⁴S-enrichments. The Tamengo Formation represents a carbonate-siliciclastic ramp at Gondwana’s continental margin, whereas the Bambuí Group records an epeiric sea within the core of this supercontinent. Both units display positive δ³⁴S_py excursions matching regressive trends (δ³⁴S_py up to 51 ‰), with a drop in δ³⁴S_py values associated with transgressions. A scenario of sulfate-distillation events throughout regressions is supported by the multiple sulfur isotope data, after transgressive events that episodically recharged sulfate in these environments. However, in the epicontinental Bambuí sea, δ³⁴S_py, Δ³³S_py and Δ³⁶S_py values suggest almost quantitative conversion of the initial sulfate into sulfide at the end of regressive stage, forced by enhanced stratification and inefficient circulation within continental areas. In this case, widespread toxic euxinic conditions were developed in inland seas, accompanied by methanogenesis, which resulted in highly ¹³C-enriched DIC due to methanogenic CO₂, as recorded in the Bambuí Group. In the Tamengo ramp, on the other hand, sulfate levels would have not been completely consumed due to more frequent and efficient mixing with open waters, preventing perturbations in the carbon cycling and representing more favorable conditions to metazoan colonization, considering the abundant metazoan fossil record in this unit. The studied basins illustrate how 1^st-order tectonic cycles may have been the major player over remarkable ³⁴S-enrichments in pyrite for specific time intervals of the sedimentary record, through supercontinent assembly and expansion of restricted settings more prone to sulfate distillation.