The sulfur isotopic consequence of seawater sulfate distillation preserved in the Neoproterozoic Sete Lagoas post-glacial carbonate, eastern Brazil

Abstract

Since the report of tubular structures interpreted as Cloudina from the Neoproterozoic Sete Lagoas Formation (SLF) in the Bambuí basin of eastern Brazil, this stratigraphic unit has become a focus of numerous geochemical, palaeomagnetic, geochronological and sequence stratigraphic studies. Geochemical data from the SLF have been used to infer palaeoenvironmental conditions in the Bambuí basin and to study the 13 C enrichment reported in this and overlying units. To further address depositional environments of the SLF, we present high-resolution δ 13 C and δ 34 S data from the SLF in the Januária and Sete Lagoas regions. We report the presence of superheavy pyrite with δ 34 S value greater than carbonate-associated sulfate (CAS) and observe a decrease in CAS abundance coupled with a δ 34 S change from c. +20 to near +50‰. These data are interpreted as evidence for distillation of sulfate through CAS and pyrite removal from the epeiric Bambuí basin. These changes are qualitatively consistent with quantitative simulations of isotopic distillation over a period of c. 10 myr of a seawater sulfate reservoir comparable with modern concentration. Considering that Neoproterozoic seawater sulfate concentrations were probably much lower than in modern oceans, the distillation time interval was probably much shorter than 10 myr. Supplementary material : MATLAB/Octave code to replicate Figs 11, 12 and 13 is available at https://doi.org/10.6084/m9.figshare.c.5845089 Thematic collection: This article is part of the Sulfur in the Earth system collection available at: https://www.lyellcollection.org/cc/sulfur-in-the-earth-system