The nitrogen biogeochemical cycle on Ediacaran rocks: A study on carbonates from the Bambuí Group, São Francisco Basin

• Autor(a) principal: Ferreira, Paula Luiza Fraga
• Data de Publicação: 2021
• Tipo de documento: Dissertação
• 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-09092021-075332/
• topic: Bambuí Group; Biogeochemical cycle; Chemostratigraphy; Ciclo biogeoquímico; Ediacaran-Cambrian; Edicarano-Cambriano; Grupo Bambuí; Isótopos de nitrogênio; Nitrogen isotopes; Paleoambiente; Paleoenvironments; Quimioestratigrafia

The Ediacaran-Cambrian transition is marked by the diversification of metazoans. However, this is not true for the Bambuí Group, South-East Brazil, in which, despite its age, is almost fossil-barren. The reasons for such inhospitable conditions for the development of complex life forms in this progressively restricted foreland basin in the core of the West Gondwana continent are still being investigated. Thus, to enhance the comprehension of environmental conditions during the deposition of this basin, this work brings \'delta POT.15\'Nbulk data from rocks of its two lowermost 2nd-order sequences. Stable nitrogen isotopes are the only tool that allows the rebuilding of past N-cycles, and not only this is element is an essential part of life, they are a useful proxy to investigate the redox structure of the water column as the stability of N species are conditioned by the availability of oxygen in the environment. Here, a positive \'delta POT.15\'N excursion in the first transgressive system tract of the basin is reported, suggesting that the Bambuí basin was still connected to the global ocean. This excursion is interpreted as an event of increasing oxygenation of the Bambuí sea, as the increment of \'delta POT.15\'N values indicates that the dominant path of the N-cycle went from N2 direct fixation to NO3- assimilation. At the Early Highstand System Tract \'delta POT.15\'N values show that the nitrate pool that was established during the transgression persisted, however, as the restriction of the basin progressed, nitrate became scarce and N2 fixation became the main form of acquisition of N by microorganisms. Finally, \'delta POT.15\'N data from the second 2nd-order sequence points again to the existence of a nitrate pool during this interval, however, not as big or stable as the one from the first transgression. Hence, the data show that during the deposition of the basal Bambuí Group rocks, N2 fixation and NO3- assimilation alternated as the dominant path of the N cycle. Importantly, the instability of the N-cycle and redox conditions plus the scarcity of nitrate, especially during the deposition of the late highstand system tract, might have affected the development of complex life forms.