Microbially induced chromium isotope fractionation and trace elements behavior in lower Cambrian microbialites from the Jaíba Member, Bambuí Basin, Brazil

et al. 2021

Sci Rep

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We combine U–Pb in-situ carbonate dating, elemental and isotope constraints to calibrate the synergy of integrated mountain-basin evolution in western Gondwana. We show that deposition of the Bambuí Group coincides with closure of the Goiás-Pharusian (630–600 Ma) and Adamastor (585–530 Ma) oceans. Metazoans thrived for a brief moment of balanced redox and nutrient conditions. This was followed, however, by closure of the Clymene ocean (540–500 Ma), eventually landlocking the basin. This hindered seawater renewal and led to uncontrolled nutrient input, shallowing of the redoxcline and anoxic incursions, fueling positive productivity feedbacks and preventing the development of typical Ediacaran–Cambrian ecosystems. Thus, mountains provide the conditions, such as oxygen and nutrients, but may also preclude life development if basins become too restricted, characterizing a Goldilocks or optimal level effect. During the late Neoproterozoic-Cambrian fan-like transition from Rodinia to Gondwana, the newborn marginal basins of Laurentia, Baltica and Siberia remained open to the global sea, while intracontinental basins of Gondwana became progressively landlocked. The extent to which basin restriction might have affected the global carbon cycle and climate, e.g. through the input of gases such as methane that could eventually have collaborated to an early Cambrian greenhouse world, needs to be further considered.

Section: Discussionmentioning

confidence: 52%

Goldilocks at the dawn of complex life: mountains might have damaged Ediacaran–Cambrian ecosystems and prompted an early Cambrian greenhouse world

Caxito

Lana

et al. 2021

Sci Rep

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“…Positive δ 13 C values are described in the upper Sete Lagoas Formation (Santos et al 2004, Vieira 2007 and Lagoa do Jacaré Formation (Misi et al 2007) and are used as stratigraphic markers for the Bambuí Group (Paula- Santos et al 2017, Uhlein et al 2019). In the Jaíba Member, uppermost Serra da Saudade Formation (North of Minas Gerais), microbialites and grainstones show δ 13 C ranging from +0.8 to +3.4‰ and δ 18 O happen mainly around -8‰, with lighter oxygen isotopes in the lower interval (Uhlein et al 2019(Uhlein et al , 2021.…”

Section: Carbon and Oxygen Isotopic Variationsmentioning

confidence: 99%

Ediacaran/Early Cambrian Serra da Saudade Formation, Bambuí Group: the sedimentary record of a foreland basin in Southeastern Brazil

Moreira

et al. 2021

Braz. J. Geol.

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The Serra da Saudade Formation, Ediacaran/Cambrian Bambuí Group, cropping out along the ridge of the same name in Central-Western Minas Gerais, Brazil, contains important deposits of glauconitic siltstones associated with phosphatic rhythmite as well as other siliciclastic, volcaniclastic and carbonate lithofacies. Lithofacies indicate deposition by proximal and distal storm waves on a continental marine shelf. The basal gray siltstone and part of the green siltstone represent a transgressive systems tract. The glauconitic siltstone indicates sea level rise, with the maximum flooding surface located approximately at the interval of highest K 2 O content. Biogenic carbonate and an upward increase in sandstone beds are interpreted as a highstand systems tract. Gradual lateral facies changes reflect the transition from foredeep (to the west) to forebulge (to the east) deposition in a foreland basin as the final stage of a shallowing-upward second-order sequence. Black organic-rich limestone has δ 13 C values between -8.78 and 9.79‰, negative δ 18 O values from -11.50 to -9.35‰, and 87 Sr/ 86 Sr around 0.7075. This isotopic signature, similar to that of other Bambuí carbonates, indicates a probably-restricted depositional environment with organic sedimentation, methanogenesis and high evaporation rates from the Ediacaran through the early Cambrian period.

“…Recently, an occurrence of putative trace fossils identified as Treptichnus pedum in association with MISS has been described in the Três Marias Formation, the uppermost unit of the Bambuí Group (Sanchez et al 2021). Microbial structures are less conspicuous and generally poorly preserved in predominantly siliciclastic units of the Bambuí Group (Sanchez et al 2018; Santos et al 2018; Uhlein et al 2020), hampering their identification and interpretation.…”

Section: Introductionmentioning

confidence: 99%

Simple Structures and Complex Stories: Potential Microbially Induced Sedimentary Structures in the Ediacaran Serra De Santa Helena Formation, Bambuí Group, Eastern Brazil

Okubo

Inglez

et al. 2023

Palaios

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Microbially induced sedimentary structures (MISS) are abundant in Ediacaran and lower Cambrian successions. However, the relationship between MISS distribution and facies has not been thoroughly explored in Ediacaran–Cambrian successions in South America. This study documents the occurrence of MISS and other potential biogenic structures from the late Ediacaran Serra de Santa Helena Formation in the Bambuí Group of eastern Brazil. This unit overlies the Cloudina-bearing Sete Lagoas Formation and is a mixed carbonate-siliciclastic succession devoid of macroscopic body fossils. Potential microbial structures include wrinkled structures such as “elephant-skin” and Kinneyia-like textures, as well as pustular structures and abundant positive epirelief discoidal structures. Another putative biogenic structure is a mm-wide meandering groove resembling a simple locomotion trail of a small vagile benthic metazoan. Microbial surface textures (i.e., “elephant skin” and Kinneyia-type wrinkles) were mainly observed in heterolithic deposits, usually at the interface between sandstone and siltstone/shale. On the other hand, discs show a facies-independent distribution, observed in heterolithic as well as carbonate and marl deposits. Petrographic analyses of these discs suggest that they have complex origins and some of them may be diagenetic structures. Thus, while facies may have strongly controlled the preservation of MISS-related structures and textures in the Serra de Santa Helena Formation, their abundance and diversity in tidal flat deposits indicate the wide distribution of matgrounds in these shallow marine paleoenvironments. Also, we demonstrate how detailed description and classification of simple features, such as discoidal structures, is an important task for paleoenvironmental reconstruction of marine ecosystems at the Ediacaran–Cambrian transition when the microbially bounded substrates played important roles in the dynamics of coastal environments.