Carbon and sulfur biogeochemical cycles in the Brazilian Ediacaran record

• Autor(a) principal: Caetano Filho, Sergio
• Data de Publicação: 2020
• Tipo de documento: Tese
• Idioma: eng
• Título da fonte: Biblioteca Digital de Teses e Dissertações da USP
• Texto Completo: https://www.teses.usp.br/teses/disponiveis/44/44141/tde-23122020-125734/
• topic: Carbono; Ediacarano; Enxofre; Geoquímica isotópica; not available; Quimioestratigrafia

Biological innovations associated with geochemical disturbances characterize the Ediacaran geological record, at the end of Neoproterozoic Era, which are attributed to the oxygenation of Earth\'s surface environments - the so-called Neoproterozoic Oxygenation Event. Carbon and sulfur stable isotopes studies are key tools to examine major oxidation events in the marine environment. Despite the existence of Ediacaran intervals that record coupled carbon and sulfur isotope anomalies worldwide, an increasing number of studies suggest that variation may also reflect local paleoenvironmental/ paleogeographic controls that differ basin to basin. These results justify detailed isotope geochemistry studies integrated through a proper sedimentological/stratigraphic assessment to decipher marine geochemistry dynamics acting when first animals were evolving. For this purpose, this thesis investigated two different Ediacaran settings from Brazil, aiming to compare the carbon and multiple sulfur isotope evolution and distinguish local and global processes. In the central-east Brazil, the Bambuí Group, São Francisco Basin, corresponds to an Ediacaran-Cambrian epicontinental basin with a very poor fossil record and well-known major carbon isotope disturbances probably associated with basin restriction. On the other hand, the Tamengo Formation, Corumbá Group, South Paraguay Belt (central-west Brazil), rocords a rich paleontological record within an Ediacaran carbonate platform located at the continental margin. Paired carbon isotopes for these two units allow reconstructions of carbon cycling within paleogeographic context, in which the epicontinental Bambuí sea experienced a greater degree of variability resulting from decoupling of basinal and global carbon cycles, whereas the Tamengo ramp records isotope signals apparently linked to enhanced oxygenation in environments inhabited by early biomineralizing metazoans. The sulfur isotope record in these basins points to a major forcing presiding over the sulfur system in both basins. However, local controls that depend on the tectonic context of these basins seem to have resulted in different responses. A dramatic event of sulfate exhaustion in the epicontinental Bambuí sea could have resulted in enhanced euxinia and toxic environments, challenging the colonization of the continental interior by early metazoans. On the other hand, sulfate depletion under sulfate-limited conditions may have favored a biogeochemical turnover in the basin for a methanogenic dominated environment, which could represent large methane fluxes to the atmosphere with potential impacts over global climate. Methanogenic basins in the continent interior might have contributed to drive Earth out from Neoproterozoic worldwide glaciations to a more warm planet, favorable to the full diversification of life in the Cambrian.