A large epeiric methanogenic Bambuí sea in the core of Gondwana supercontinent?
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2021
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Carbon isotope compositions of both sedimentary carbonate and organic matter can be used as key proxies of the
global carbon cycle and of its evolution through time, as long as they are acquired from waters where the dis-
solved inorganic carbon (DIC) is in isotope equilibrium with the atmospheric CO2. However, in shallow water
platforms and epeiric settings, the influence of local to regional parameters on carbon cycling may lead to DIC
isotope variations unrelated to the global carbon cycle. This may be especially true for the terminal Neo-
proterozoic, when Gondwana assembly isolated waters masses from the global ocean, and extreme positive and
negative carbon isotope excursions are recorded, potentially decoupled from global signals. To improve our
understanding on the type of information recorded by these excursions, we investigate the paired δ13Ccarb and
δ13Corg evolution for an increasingly restricted late Ediacaran-Cambrian foreland system in the West Gondwana
interior: the basal Bambuí Group. This succession represents a 1st-order sedimentary sequence and records two
major δ13Ccarb excursions in its two lowermost lower-rank sequences. The basal cap carbonate interval at the base
of the first sequence, deposited when the basin was connected to the ocean, hosts antithetical negative and
positive excursions for δ13Ccarb and δ13Corg, respectively, resulting in Δ13C values lower than 25‰. From the top of
the basal sequence upwards, an extremely positive δ13Ccarb excursion is coupled to δ13Corg, reaching values of
þ14‰ and 14‰, respectively. This positive excursion represents a remarkable basin-wide carbon isotope
feature of the Bambuí Group that occurs with only minor changes in Δ13C values, suggesting change in the DIC
isotope composition. We argue that this regional isotopic excursion is related to a disconnection between the
intrabasinal and the global carbon cycles. This extreme carbon isotope excursion may have been a product of a
disequilibria between the basin DIC and atmospheric CO2 induced by an active methanogenesis, favored by the
basin restriction. The drawdown of sulfate reservoir by microbial sulfate reduction in a poorly ventilated and
dominantly anoxic basin would have triggered methanogenesis and ultimately methane escape to the atmosphere,
resulting in a13C-enriched DIC influenced by methanogenic CO2. Isolated basins in the interior of the Gondwana
supercontinent may have represented a significant source of methane inputs to the atmosphere, potentially
affecting both the global carbon cycle and the climate.
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Methanogenesis, Ediacaran, Cambrian, Carbon isotopes, São Francisco Basin, Bambuí Group
Citação
CAETAMO FILHO, S. et al. A large epeiric methanogenic Bambuí sea in the core of Gondwana supercontinent?. Geoscience Frontiers, v. 12, p. 203-218, 2021. Disponível em: <https://www.sciencedirect.com/science/article/pii/S1674987120300955>. Acesso em: 29 abr. 2022.