Oceanic redox conditions across the Cambrian-Ordovician boundary

Bisnaire, Alexandre Charest (2018) Oceanic redox conditions across the Cambrian-Ordovician boundary. Masters thesis, Memorial University of Newfoundland.

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Abstract

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 siliciclastic environments to evaluate repeatability and predictability. Secondly, this method is applied to the Green Point carbonates and modern Bahamas carbonates. In the third part of this chapter, we focus on the δ⁹⁸Mo ratios from the Green Point carbonates and their reliability as a predictive paleoredox tool. Uranium and Molybdenum were found to have divergent relationships to the U/Mo ratios. In the Green Point shales, the relationship between U/Moauth vs MoEF or UEF enrichment factors were found to be reliable redox indicators, particularly U/Moauth vs MoEF. When applied to other siliciclastic datasets, plotting U/Moauth to MoEF or UEF yielded a different yet consistent pattern relative to the UEF vs MoEF plot of Algeo and Tribovillard (2009), differentiating oxygen levels, most notably within the suboxic range, and marine basin restriction. Within the carbonates, the main factor controlling the low U concentrations was calcite being the carbonate species present. The decrease across the anomaly is likely due to the TOC decrease; enhanced by low dissolved porewater sulfide levels and the presence of carbonate material. The positive U/Mo vs U trend supports the conclusion that authigenic enrichment was not tot the main control on observed variations. The Mo values were within the range of previously carbonates, indicating pH and carbonate species present may not affect Mo enrichment. The U/Mo vs Mo plot behaved like in the shales, showing potential as a redox indicator. When applied to the Bahamas carbonates, both relationships consistently differentiated low and high pore-water sulfide levels, and shows potential to differentiate paleoenvironments. The Mo isotope ratios were interpreted as being a mixed detrital-authigenic signature, and so are minimum values. The increase across the anomaly level suggests increasingly reducing conditions during the deposition of the Green Point carbonates. This agrees with previous interpretations, indicating δ⁹⁸Mo ratios are reliable at Green Point.

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