Chemostratigraphy of Early Ediacaran Carbonate Rocks of the Cachoeirinha Group (northeastern Brazil): Implications for Paleonvironmental Conditions and Atmospheric Oxygenation

The Cachoeirinha Group, situated in northeastern Brazil, primarily consists of meta-pelites, metagraywackes, meta-conglomerates, banded iron formations, carbonate deposits, metavolcanics, and metatuffs. This group comprises two formations: Serra Olho d'Água and Santana dos Garrotes. Within the Serra Olho d'Água Formation, polymictic meta-conglomerates and associated carbonate rocks have been dated to a maximum depositional U–Pb age of 880 ± 8 Ma. The presence of negative δ13C values ranging from −2.83‰ to −0.71‰ is associated with glaciomarine depositional environments, while low Y/Ho ratios (≤26) indicate a nearshore/restricted basin environment. The δ53Crauth values in carbonate samples range from 0.0605‰ to 0.3248‰, and the absence of Ce/Ce* and Eu/Eu* anomalies suggest deposition under low oxygenation conditions. The 87Sr/86Sr ratios ranging from 0.70776 to 0.70862 indicate a possible deposition during the Cryogenian-Ediacaran period. On the other hand, the Santana dos Garrotes Formation predominantly consists of pelitic rocks with interbedded felsic metavolcanics, banded iron formations, and associated carbonate rocks. Detrital zircons from this formation were dated (U–Pb in zircon) and indicated an age of 630–610 Ma. High Y/Ho ratios (>45) and positive δ13C values between +3.1‰ and +5.5‰, associated with organic matter (OM) buried in marine sediments, together with positive δ53Crauth values ranging from 0.058‰ to 0.487‰, suggest the existence of a seawater pool with high 53Cr/52Cr ratios, related with a well-established marine environment. The 87Sr/86Sr ratios ranging from 0.70799 to 0.71226 are consistent with an early Ediacaran interglacial period. The analysis of isotopic and elemental systems in Cachoeirinha Group carbonates provides evidence of long-lasting seawater stratification and an isotopically stratified water column during climatic changes associated with the “Snowball Earth” scenarios linked to the Neoproterozoic Oxidation Event (NOE).