Oceanic redox conditions across the Cambrian-Ordovician boundary
Trace element and isotope geochemistry is used to evaluate paleoenvironments. Uranium and molybdenum geochemistry in particular have been used as reliable paleoredox indicators, leading to valuable developments. Here, these proxies are applied to the transgressive sedimentary succession spanning the...
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| Format: | Thesis |
| Language: | English |
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Memorial University of Newfoundland
2018
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| Online Access: | https://research.library.mun.ca/13311/ https://research.library.mun.ca/13311/1/thesis.pdf |
| Summary: | Trace element and isotope geochemistry is used to evaluate paleoenvironments. Uranium and molybdenum geochemistry in particular have been used as reliable paleoredox indicators, leading to valuable developments. Here, these proxies are applied to the transgressive sedimentary succession spanning the Cambrian ‒ Ordovician GSSP boundary section of the Cow Head Group exposed at Green Point, western Newfoundland (Canada). In Chapter 2, the distal slope environment of deposition is evaluated based on the green to black shales’ geochemistry. It underwent significant changes during the transgression occurring around the Cambrian-Ordovician boundary. As the platform became increasingly flooded and more carbonate-dominated, the cratonic siliciclastic supply decreased, lowering the proportion of Al (6.2±1.1 wt% to 5.2±1.0 wt%), Cr (64±12 ppm to 55±10 ppm), Sc (9.1±1.8 ppm to 7.5±1.6 ppm), Th (7.4±1.2 ppm to 5.9±1.4 ppm) and Cs (3.7±1.0 ppm to 2.6±0.8 ppm) and increasing the proportion of Ca (3.8±3.0 wt% to 4.8±2.8 wt%) in the shales. Conditions became more reducing as relative sea level rose, becoming more strongly suboxic, with more frequent anoxic intervals. Due to short-term fluctuations (reflected in the highly variable lithology), this is reflected most accurately by the higher proportion of shales demonstrating significant U and Mo authigenic enrichment above the anomaly level, and their relationship to higher TOC values. Bioproductivity was low throughout the section, based on low P (494±141 ppm), Ni (32.8±10.5 ppm) and Cu (36.1±16.8 ppm). Bioproductivity variation across the anomaly level could not be ascertained however. The geochemical patterns observed correlate well with the carbonate carbon isotope stratigraphy, clearly defining trends and systems tract. In Chapter 3, we focus on Mo and U elemental and isotopic geochemistry. We first evaluate the relationship of U/Mo ratio vs U and Mo (both concentrations and EFs) in the Green Point shales and compare the results to several modern and Phanerozoic-aged ... |
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