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

Abstract

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 δ34Spy excursions matching regressive trends (δ34Spy up to 51 ‰), with a drop in δ34Spy 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, δ34Spy, Δ33Spy and Δ36Spy 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 1st-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.