Paleoenvironmental redox evolution of Ediacaran-Cambrian restricted seas in the core of West Gondwana: Insights from trace-metal geochemistry and stratigraphy of the Bambui Group, east Brazil

Autor(es)

Guacaneme, Cristian ; Caetano-Filho, Sergio ; Paula-Santos, Gustavo M. ; Babinski, Marly ; Fraga-Ferreira, Paula L. ; Bedoya-Rueda, Carolina ; Kuchenbecker, Matheus ; Reis, Humberto L. S. ; Trindade, Ricardo I. F.

• Tipo de documento: Artigo Científico
• Fonte: Journal of South American Earth Sciences; v. 119, p. 19-pg., 2022-09-01.

Resumo

The sedimentary evolution of the Bambuf foreland basin system in the interior of West Gondwana is marked by periods of connection and isolation from the global ocean during the late Neoproterozoic and early Paleozoic. To understand the link between these periods and seawater redox conditions, we present an integrated study of trace-metal geochemistry and stratigraphy of the carbonate-siliciclastic rocks from the two lowermost second order transgressive-regressive sequences of the Bambuf Group, east Brazil. The basal 2nd-order sequence trace -metal pattern shows a progressive decrease of mass fractions of Co, Cr, Ni, Cu, Mo, U, V, Zn, and Cd, concomitant with a progressive decrease of Al and Fe contents and Mo/TOC ratios. Among all these elements, only Cd, Mo and U mass fractions seem to be less or not affected by detrital influence, so they can be used as reliable redox proxies for the paleoenvironmental analysis of the studied Bambuf strata. Moreover, normalization to aluminum shows a progressive increase of trace-metal enrichments for Cd and Mo in the order of 0.1-10 times and for U in the order of 1-100 times, accompanied by a progressive increase of organic carbon content upward section. These changes in sedimentary trace-metal patterns provide evidence for the chemical evolution of basinal deep-waters, whose conditions changed progressively from suboxic-anoxic to anoxic-euxinic at the basal transgressive-regressive sequence. We suggest that the paleomarine system represented by the basal Bambuf Group has probably evolved as an intracontinental silled basin recording changes in seawater chemistry associated with redox variations and restricted hydrographic conditions. Sedimentary trace-metal patterns indicate that Bambuf epeiric sea was initially in communication with open ocean followed by the marine restriction during the transgressive and regressive cycles, respectively. This resulted in a long deepwater residence time and chemical evolution of deep watermass as a response to tectonic pulses and consequent sea-level variations during the restricted stage. Under anoxic-euxinic conditions of seawater, trace metals scarcity and micronutrient fixation limitation would have impacted nitrate bioavailability, preventing the evolution of early benthic metazoans in the Bambuf paleomarine system during the late Ediacaran and early Cambrian. (AU)

• Processo FAPESP: 16/06114-6 - O Sistema Terra e a evolução da vida durante o Neoproterozoico
• Beneficiário: Ricardo Ivan Ferreira da Trindade
• Modalidade de apoio: Auxílio à Pesquisa - Temático
• Processo FAPESP: 19/13228-6 - Ciclo biogeoquímico do nitrogênio em rochas ediacaranas: um estudo nos carbonatos do Grupo Bambuí, Bacia do São Francisco
• Beneficiário: Paula Luiza Fraga Ferreira
• Modalidade de apoio: Bolsas no Brasil - Mestrado
• Processo FAPESP: 17/00399-1 - Intemperismo químico e fluxo de nutrientes para o oceano no Ediacarano superior e suas relações com a diversificação da vida
• Beneficiário: Gustavo Macedo de Paula Santos
• Modalidade de apoio: Bolsas no Brasil - Pós-Doutorado
• Processo FAPESP: 16/11496-5 - Ciclos biogeoquímicos de enxofre e carbono no Ediacarano e seu registro em sucessões sedimentares brasileiras
• Beneficiário: Sergio Caetano Filho
• Modalidade de apoio: Bolsas no Brasil - Doutorado