The fate of a Neoproterozoic intracratonic marine basin : trace elements, TOC and IRON speciation geochemistry of the Bambuí Basin, Brazil.

Abstract

Neoproterozoic marine systems are associated with major paleoecological changes that took place in the Ediacaran and during the Ediacaran-Cambrian transition. During this timespan, the Bambuí basin located on east Brazil held a peculiar paleoenvironmental scenario. Due to its intracratonic evolution, the basin was partially disconnected from neighboring open marine systems. This setting raises a very interesting opportunity to understand how an isolated Neoproterozoic marine system evolved in contrast with typical (globally connected) open marine systems. To understand the paleoenvironmental changes that took place in the Bambuí basin, we investigate the pre-glaciogenic deposits of the Carrancas Fm and the post-glaciogenic mixed (shale-carbonate) successions of the Bambuí Group. Through the analysis of iron speciation, TOC, trace element and C-O isotope systematics, our study suggests a very complex environmental evolution. Firstly, our samples are marked by strong contamination of detrital continental material that can be related to an increased bioproductivity on both the Carrancas Fm. and lower Bambuí group stratigraphic units, and provenance data show that all studied sediments probably shared common source areas. Iron speciation data, Ce anomalies and RSE enrichments shows that lower Bambuí Group stratigraphic units were likely deposited in an open marine scenario featuring high bioproductivity in shallow waters and euxinic incursions in predominant anoxic/ferruginous bottom waters. On the other hand, upper Bambuí stratigraphic units register a marine evolution in a restricted scenario, where anoxic ferruginous conditions probably reached surface waters. Finally, our data show that the lack of oceanic connection prevented the re-supply of marine sulfate, RSE, bionutrients and ultimately of dissolved oxygen which may have decreased biological activity and probably hindered biological evolution, preventing the rise of a typical modern-like Cambrian ecosystem. In this sense, our data suggest that oceanic connectivity and proper re-supply of inorganic marine input were important features in the development of complex life in the EdiacaranCambrian environment.