Abstract
The Ediacaran-Cambrian transition is marked by the diversification of metazoans in the marine realm. However, this is not recorded by the Ediacaran-Cambrian Bambuí Group of the São Francisco basin, Brazil. Containing the sedimentary record of a partially confined foreland basin system, the Bambuí strata bear rare metazoan remnants and a major carbon isotope positive excursion decoupled from the global record. This has been explained by changes in the paleogeography of the basin, which became a restricted epicontinental sea in the core of the Gondwana supercontinent, promoting episodes of shallow water anoxia. Here, we report new δ15Nbulk data from the two lowermost second-order transgressive-regressive sequences of the Bambuí Group. The results show a rise of δ15N values from +2 to +5‰ in the transgressive system tract of the basal sequence, which was deposited when the basin was connected to other marginal seas. Such excursion is interpreted as an oxygenation event in the Bambuí sea. Above, in the regressive systems tract, δ15N values vary from +2 to +5‰, pointing to instabilities in the N-cyle that are concomitant with the onset of basin restrictions, higher sedimentary supply/accommodation ratios, and the episodic anoxia. In the transgressive systems tract, the δ15N values stabilise at ∼+3.5‰, pointing to the establishment of an appreciable nitrate pool in shallow waters in spite of the basin full restriction as marked by the onset of a positive carbon isotope excursion. In sum, our data show that the N-cycle and its fluctuations were associated with variations in sedimentary supply/accommodation ratios induced by tectonically-related paleogeographic changes. The instability of the N-cycle and redox conditions plus the scarcity of nitrate along regression episodes might have hindered the development of early benthic metazoans within the Bambuí seawater and probably within other epicontinental seas during the late Ediacaran-Cambrian transition.
Highlights
Nitrogen (N) has two stable isotopes, 14N and 15N, and they occur in a proportion of 99.633 and 0.337%, respectively (Meija et al, 2016)
This sequence ends at maximum flooding surface (MFS), which in the Sete Lagoas High is marked by a succession of shale and mudstones, whilst in Januaria bindstones lacking structures related to flows were chosen as the limiting surface, this interval is associated to the chemostratigraphic intervals (CIs)-1 of Paula-Santos et al (2017)
Three stratigraphic sections from the Bambuí Group were studied in this work: Januaria (∼130 m-thick), an assemblage of three minor sections in the central-east portion of the São Francisco Basin in Januaria High domain, Arcos (180 m-thick) and Well 1 (430 m-thick), the latter two sections corresponding to drill cores, at the South of the Basin, in the Sete Lagoas High domain (Figure 2)
Summary
Nitrogen (N) has two stable isotopes, 14N and 15N, and they occur in a proportion of 99.633 and 0.337%, respectively (Meija et al, 2016). The TST corresponds to a retrogradational pattern that marks a transgression over the forebulge of the Bambuí domain and its connection to the global ocean, and it comprises deposits of diamictites from Carrancas and carbonates from the basal Sete Lagoas Formation This sequence ends at MFS, which in the Sete Lagoas High is marked by a succession of shale and mudstones, whilst in Januaria bindstones lacking structures related to flows were chosen as the limiting surface, this interval is associated to the CI-1 of Paula-Santos et al (2017). The fossil Treptichnus pedum was reported in the upper Três Marias formation, which places the deposition age of this unit in the early Paleozoic (Sanchez et al, 2020)
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