Search

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. [ABSTRACT FROM AUTHOR]

• REY variations from carbonate rocks from Bambuí Group follow sequence stratigraphy. • Increase in seawater alkalinity is recorded in Cloudina sp bearing strata. • Evidence for high alkalinity in seawater at terminal Ediacaran across West Gondwana. • Link between high alkalinity of seawater and biomineralization reinforced. Rare earth elements plus yttrium (REY) mass fractions of ancient carbonate rocks are used to track changes in chemistry of past seawater. Here we investigate REY patterns in two carbonate sections from the Ediacaran Bambuí Group, São Francisco Basin (Brazil), which comprise its two lowermost transgressive-regressive second-order sedimentary sequences. Shale normalised distributions vary with the sequence stratigraphy framework. In the basal 2nd-order sequence, carbonate samples from the basal sequence transgressive systems tract display light REY (LREY) distributions slightly depleted to enriched that reflect input of freshwater, possibly in a post glacial episode. Upwards, carbonate rocks from the early highstand systems tract (EHST) yielded LREY enriched distributions, which progressively turns into LREY shale normalized depleted distributions on samples from the late highstand systems tract (LHST). This portion of the sequence also displays Y positive anomaly in some cases. Carbonate samples from the upper second-order sequence do not display coherent patterns. Ce/Ce* values > 1 in most samples throughout the two sections suggest permanent anoxia of seawater. The REY change from the EHST to LHST in the basal sequence marks an important paleoenvironmental overturn in the basin, with increasing alkalinity in seawater driving REY fractionation and LREY depletion. Confinement of the basin in the inner areas of West Gondwana due the uplift of marginal neoproterozoic orogens probably changed the weathering style of source areas to more congruent, thus delivering a higher ionic influx to a restricted setting, increasing alkalinity during LHST. Cloudina sp. fragments were reported in this stage of the Bambuí Group and in carbonate rocks with high Sr mass fractions in other West Gondwana basins, supporting the hypothesis that the high alkalinity of seawater during late Ediacaran may have driven the appearance of the first biomineralizing organisms. [ABSTRACT FROM AUTHOR]