Carbon and Oxygen Isotope Stratigraphy of the Ediacaran Jaíba Formation, Upper Bambuí Group, Brazil: Insights into Paleogeography and Sedimentary Environments after a Neoproterozoic Glaciation.

Abstract:

The Neoproterozoic Era was a time of extreme climatic variation as recorded in sedimentary rocks of this age across the globe, leading to a number of controversial hypotheses (e.g. the Snowball Earth glaciations). In eastern Brazil, the Bambuí Gr. is a thick carbonatic-siliciclastic unit that covers the São Francisco Craton and preserves remnants of a Neoproterozoic glaciation and their respective cap carbonate (1). Recent findings of Cloudina in the Januária region (2) suggest that at least part of the sequence might be upper Ediacaran or even Cambrian. Here we present the first carbon-oxygen isotope data for the Jaíba Fm., a ca. 50 m thick carbonate unit that occurs in the topmost portion of the Bambuí Gr. in this same region. The Jaíba Fm. post-dates the cap carbonate sequence and the fossil-bearing layers, and thus was probably deposited in the Ediacaran-Cambrian transition. Three stratigraphic columns were analyzed, and yielded similar ratios. Values of δ¹³C_VPDB are between 0.8 and 3.4 ‰, while δ¹⁸O_VPDB values are mostly around -8 ‰. These values contrasts with the negative δ¹³C values found for the base of the Bambuí Gr., followed by highly positive δ¹³C (up to +14‰) on its middle portion. The unusually high δ¹³C values are commonly interpreted as evidence for deposition on a restricted basin, such as in a foreland setting. The return to values which are close to the PDB standard in the uppermost Bambuí Gr. might thus indicate a change in the paleogeography and tectonic environment of the basin, suggesting an open, ventilated environment along with a recovery of the biological and hydrological cycle after a Late Neoproterozoic glaciation. Ongoing detailed sedimentological, geochemical and isotopic work might help to further clarify these issues and to provide new clues for unraveling Late Neoproterozoic paleoclimate, paleogeography and ocean chemistry. We thank FAPEMIG (Brazil) for finnacial support through grants n. APQ-00914-14 and PPM-00539-15.

(1) Caxito, F.A. et al., 2012. Precambrian Research, 200:38-58.

(2) Warren, L. et al., 2014. Geology, 42:391-394.