Tectonically-induced strontium isotope changes in ancient restricted seas : the case of the Ediacaran-Cambrian Bambuí foreland basin system, east Brazil.
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2021
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Abstract
The Bambuí Group is a marine sedimentary record of an intracratonic foreland basin developed at the terminal
Ediacaran and early Cambrian during the assembly of West Gondwana. Here we present a basin-scale high-
resolution Sr isotope stratigraphy for the basal Bambuí Group, aiming to understand the spatial and temporal var-
iations of the 87Sr/86Sr ratios and to explore the controls over the Sr isotope system in intracontinental marine
environments. Assessment of the stratigraphic evolution of both Sr concentrations and Sr isotopes shows a
major increase in Sr/Ca ratios (up to 0.004) and a decrease in the 87Sr/86Sr ratios from 0.7086 to 0.7076 in the
high stand system tract of the basal 2nd-order sequence. These changes precede a large positive δ13C excursion
typically found across the basin in the middle Bambuí Group. The high variability of both 87Sr/86Sr and Sr/Ca ra-
tios was not caused by globally uniform changes in isotopic compositions of seawater, but rather likely reflect
marine restriction and paleogeographic changes of the depositional environments at basin scale. This would re-
sult from the tectonic uplift of Neoproterozoic orogenic belts around the São Francisco craton, which generated
an isolated foreland marine basin. Compared to the global ocean, such a smaller intracontinental reservoir would
be more sensitive to the Sr isotope composition from the different rock sources. We suggest that changes on the
balance between carbonate production and accommodation associated with tectonically-related flexural subsi-
dence progressively modified the continental drainage patterns, sedimentary sources and the chemical
weathering regimes, altering the strontium influxes and isotopic compositions of the seawater in the early
Bambuí basin cycle. Similar anomalies in the strontium isotope record are also recorded in coeval marine basins
across West Gondwana and suggest that tectonics might have played an important role on seawater chemistry at
the Neoproterozoic-Paleozoic transition.
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São Francisco Basin
Citation
GUACANEME, C. et al. Tectonically-induced strontium isotope changes in ancient restricted seas: the case of the Ediacaran-Cambrian Bambuí foreland basin system, east Brazil. Gondwana Research, v. 93, 2021. Disponível em: <https://www.sciencedirect.com/science/article/pii/S1342937X21000678>. Acesso em: 29 abr. 2022.