Resumen
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 (d34SSO4, d18OSO4, and d34SH2S), the sulfur isotopic composition of sedimentary pyrite (d34SCRS), and sulfur and oxygen isotopic compositions of carbonate-associated sulfate (CAS, d34SCAS and d18OCAS). The pore-water profiles at Lagoa Vermelha indicate ongoing bacterial sulfate reduction by increasing d34SSO4, d18OSO4 and d34SCRS 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 d18OSO4 values in both surface and pore-water were strongly enriched in 18O. There was an overall mismatch between d34SSO4 and the significantly higher d34SCAS values. A negative correlation was observed between the Ca/Mg ratio and higher d34SCAS 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 d34SCAS 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 d34SCAS values reflect microenvironments rather than preserving an open marine d34SSO4 signature.