The Relationship between Bacterial Sulfur Cycling and Ca/Mg Carbonate Precipitation—Old Tales and New Insights from Lagoa Vermelha and Brejo do Espinho, Brazil

Over the few past decades, the concept of microbial sulfur cycling catalyzing the precipitation of CaMg (CO₃)₂ at low temperatures (<40 °C) has been studied intensely. In this respect, two hypersaline lagoons, Lagoa Vermelha and Brejo do Espinho, in Brazil, have been the subject of numerous studies investigating sedimentary Ca/Mg carbonate formation. Here, we present the sulfur and oxygen isotopic compositions of dissolved sulfate from surface water, as well as sulfate and sulfide from pore-water (δ³⁴S_SO4, δ¹⁸O_SO4, and δ³⁴S_H2S), the sulfur isotopic composition of sedimentary pyrite (δ³⁴S_CRS), and sulfur and oxygen isotopic compositions of carbonate-associated sulfate (CAS, δ³⁴S_CAS and δ¹⁸O_CAS). The pore-water profiles at Lagoa Vermelha indicate ongoing bacterial sulfate reduction by increasing δ³⁴S_SO4, δ¹⁸O_SO4 and δ³⁴S_CRS values downcore. At Brejo do Espinho, the pore-water profiles displayed no depth-dependent isotope trends; the Ca/Mg ratio was, on average, lower, and the δ¹⁸O_SO4 values in both surface and pore-water were strongly enriched in ¹⁸O. There was an overall mismatch between δ³⁴S_SO4 and the significantly higher δ³⁴S_CAS values. A negative correlation was observed between the Ca/Mg ratio and higher δ³⁴S_CAS values. The results show that the size difference between the two lagoons induces differences in the intensity of evaporation, which leads to the increased secretion of extrapolymeric substances (EPSs) by microbes in the smaller Brejo do Espinho. EPS provides the microenvironment where Ca/Mg carbonate can nucleate and preserve increased δ³⁴S_CAS values. Apart from EPS, increased sulfur oxidation is proposed to be a second factor causing relative enrichment of Ca/Mg carbonates at Brejo do Espinho. Our results emphasize the role of evaporative processes on Ca/Mg carbonate formation, and indicate that the respective δ³⁴S_CAS values reflect microenvironments rather than preserving an open marine δ³⁴S_SO4 signature.