On the reliability of Sr isotope chemostratigraphy of late Ediacaran carbonates from the Bambuí Group.

A marked co-variation of δ13C and 87Sr/86Sr is observed in Ediacaran to early Cambrian Bambui Group carbonates at the southern part of the São Francisco epicontinental basin (Brazil). Three chemostratigraphic intervals were defined related to different evolution stages of the basin: a restricted shallow basin influenced by freshwater (δ 13C = −5 to 0‰; 87Sr/86Sr = 0.7074-0.7082), then a basin connected to the Ediacaran seas (δ 13C = ~0‰; 87Sr/86Sr = ~0.7082), and finally a restricted basin (δ 13C up to +16‰; 87Sr/86Sr = ~0.7075). The transition from open marine to restricted marine environment exhibit regular changes in 87Sr/86Sr ratios. However, when the Sr isotope data of the entire basin is considered, some inconsistencies are observed. While δ13C values follow the previously stablished chemostratigraphy, starting at 5‰ at the base and increasing progressively to +16‰ at the top, 87Sr/86Sr ratios for equivalent strata at different sectors of the basin may vary significantly, between 0.7072 and 0.7144. A dominantly restrict environment could explain these large variations of 87Sr/86Sr ratios due to the mixture of variably radiogenic continental and marine sources. In addition, restrict environments may also induce Sr enrichments and reduce strontium residence time in seawater. Our study implies that great caution must be used in applying Sr isotope chemostratigraphy in the São Francisco basin and by analogy in other epicontinental basi