Variations of depositional redox conditions across the Cambrian-Ordovician boundary (GSSP, Green Point Formation) in western Newfoundland: implications from Mo-, U-, and S-isotope signatures and I/Ca ratios

Li, Jie (2024) Variations of depositional redox conditions across the Cambrian-Ordovician boundary (GSSP, Green Point Formation) in western Newfoundland: implications from Mo-, U-, and S-isotope signatures and I/Ca ratios. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

The Green Point Formation in western Newfoundland, Canada, includes the Global Boundary Stratotype Section and Point (GSSP) of the Cambrian-Ordovician (Є-O) boundary. This formation consists mainly of slope deposits of alternating lime mudstone and shale interbeds (rhythmites). In this study, we conducted an analysis of the isotopic and elemental signals of Mo and U in the shale and limestone. Additionally, we examined the textures and bulk δ³⁴Spy signals of biological pyrite in the shale and investigated the I/Ca ratios of the limestone. These analyses aim to provide a more comprehensive understanding of the depositional environments and marine redox conditions during the Є-O transition. The analysis revealed broad ranges of the shale authigenic δ⁹⁸ (−0.40 to +3.16‰) and the limestone δ⁹⁸Mocarb (−0.30 to +2.34‰) values. These observations, combined with significantly low Mo concentrations and the lack of fossils and bioturbations, suggest that the shale and limestone were both deposited under weakly oxygenated (dysoxic to anoxic) bottom conditions. Furthermore, an inverse correlation between the shale δ⁹⁸Moauth and δ²³⁸Uauth values, similar to those observed in modern semi-restricted basins, indicates that the local continental margin seawater was probably partially isolated from the open ocean. In accordance with this inference, significant inverse relationships are observed between the δ³⁴Spy values and the contents of terrigenous elements (Al, Th, and ΣREE), indicating that terrestrial inputs might have influenced the regional seawater sulfate level. Interestingly, parallel changes are noted between the bulk δ³⁴Spy values and abundances of pyrite precipitated in sedimentary porewaters, suggesting that the δ³⁴Spy variability could also be influenced by varying amounts of pyrite formed at different diagenetic stages within the samples. As a result, the fluctuation of bulk δ³⁴Spy signals alone may not necessarily indicate perturbations of the global marine sulfur cycle. Despite the evidence for partial basin isolation, the average shallow seawater δ²³⁸U signals (−0.70 ± 0.24‰, 1σ), inferred from the limestone δ²³⁸Ucarb values, were probably close to coeval open oceans. With this estimated mean oceanic δ²³⁸U value, our three-sink U-isotope mass balance model predicts that the Є-O transition was a time of expanded marine anoxia with anoxic to euxinic seawater covering 1.0 to 21.1% of the ocean floor. This prediction aligns with the observation of substantially low I/Ca ratios ranging from 0.02 to 0.33 μmol/mol in the limestone. These ratios fall well below the Proterozoic Eon background I/Ca values of ~0.5 to 1 μmol/mol, indicating the presence of a shallow marine oxycline along the regional continental margin. Similar observations have been reported in several other age-equivalent sections deposited along the shelf and slope of the ancient Iapetan and Laurentian continental margins. Consequently, it is plausible to suggest that seawater surrounding the Iapetan and Laurentian continental margins during the Є-O transition might have been poorly oxygenated, with shallow oxyclines or expanded oxygen minimum zones in the shelf and/or slope areas. These findings agree with earlier viewpoints that the middle-late Cambrian to the Early Ordovician marine biodiversity plateau was linked to widespread oceanic anoxia.

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