Rare earth elements of carbonate rocks from the Bambuí Group, southern São Francisco Basin, Brazil, and their significance as paleoenvironmental proxies

Abstract

Rare Earth Elements plus Y (REY) data obtained for carbonate rocks of the Bambuí Group in the southern portion of the São Francisco basin match the C and Sr isotope variations that subdivide the unit in three Chemostratigraphic Intervals (CI-1 to CI-3). The lower CI-1 samples exhibit flat REY shale normalised distributions that record a high freshwater input in the basin after a glacial period. The subsequent marine transgression diminished the continental influence and initiated the development of La, Gd, and Y positive anomalies in the CI-2 samples. Interestingly, a “seawater” like REY pattern is present in the CI-3 samples, which correspond to the closure of the São Francisco Basin. Rather than open marine conditions, the CI-3 records a decrease in the silicate chemical weathering of sources located on the marginal belts of the São Francisco craton due to higher denudation rates and a change in the dissolved influx towards carbonate rocks. The resultant high concentration of carbonate ions in the restricted sea induced a REY fractionation process similarly to that of the modern ocean. Constant positive Ce/Ce∗ values throughout the shallow and deep sections suggest anoxic early diagenetic conditions, inefficient circulation and stratified water column, which could have worked as a barrier for life diversification in the basin at the Ediacaran-Cambrian limit. Finally, our data show that the REY concentrations of ancient biochemical environments are not an unequivocal tool either to track open and restricted marine settings or to distinguish proximal and distal environments.