ANOMALOUS d34S SIGNATURES IN TRACE SULFATE FROM A POTENTIAL CAP CARBONATE IN THE NEOPROTEROZOIC BAMBUI GROUP, BRAZIL

An unusually textured unit in the Sete Lagoas Formation, Bambuí Group may be a cap carbonate atop a second as yet unrecognized Neoproterozoic glacial interval in Brazil. Textures, insoluble residues, and carbon, oxygen, sulfur, and strontium isotopic compositions of high-resolution samples from two nearby quarries outside of Belo Horizonte, Brazil were investigated. At the Sambra Quarry, carbonates record a positive d¹³C excursion from the base at -4 to near 0‰ some 8 meters higher; d¹³C values remain relatively constant above this transition zone. Over the same interval, d¹⁸O values appear to co-vary with carbonate ¹³C abundance. Organic matter is anomalously enriched in ¹³C near the base, with remarkably constant values of ca. -18 to -20‰, resulting in remarkably reduced carbon isotope fractionations (Dd) between inorganic and organic phases. Variable, but high Sr and low Mn abundances characterize the seafloor cements at the base of the exposure. These well-preserved limestones record ⁸⁷Sr/⁸⁶Sr values of 0.7074. Abundances of carbonate associated trace sulfate range between 28 and 174 ppm. The d³⁴S values are strongly positive, up to +38‰ relative to CDT. Nearby carbonates collected from the Paraíso Quarry record carbonate d¹³C values around 0‰. Trace sulfate concentrations range from 6 to 86 ppm. The d³⁴S values of samples from the Paraíso Quarry are as high as +47.5‰. The extreme enrichment in ³⁴S of carbonate-associated sulfate is the expected consequence of high rates of bacterial sulfate reduction in anoxic waters during Neoproterozoic ice ages. During post-glacial transgression the photosynthetic production of oxygen would have likely driven sulfate reducers back into sediments. Due to the diffusive limitation of sulfate into sediments, SRB would have been cut off from oceanic sulfate and d³⁴S of newly formed sulfides and carbonate-associated sulfate could have risen to values higher than contemporaneous seawater. As modern river inputs to seawater have d³⁴S values in the range of 6-10‰, this sulfur isotope anomaly most likely reflects oceanographic processes, rather than the sudden and intense post-glacial weathering of exposed continents under a CO₂ charged atmosphere.