Redox conditions across the Upper Cambrian in Western Newfoundland, Canada

Okafor, Miracle-Mary Chisom (2023) Redox conditions across the Upper Cambrian in Western Newfoundland, Canada. Masters thesis, Memorial University of Newfoundland.

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Abstract

The lower part of the Martin Point section (~ 110 m - rhythmites) spans the uppermost Shallow Bay and lowermost Green Point formations (uppermost Franconian‒ lowermost Trempealeauan) of the Cow Head Group (western Newfoundland, Canada). It records two post-SPICE events (SPICE – Steptoean Positive Carbon Isotope Excursion) marked by negative δ¹³Ccarb excursions (NL1 and NL2) that were also documented in South China. The petrographic and chemical screening of the investigated carbonate samples proves the preservation of at least near-primary geochemical signatures. The proxies of weathering (e.g., Mn, Fe, Al, and ΣREE), bioproductivity (e.g., δ¹³Corg, TOC, P, Ni, Zn, and Cu), and paleoredox (e.g., Cr, Th/U, Ce/Ce*) have been utilized to reconstruct the paleoenvironmental conditions during the post-SPICE events. The TOC, P, Ni, Cu, Cr, ΣREE, Mn, Fe, and U enrichment-factor profiles exhibit a positive pulse at the base of NL2 (upper negative δ¹³Ccarb excursion), followed by a drop. This might have resulted from shoaling organic-rich anoxic waters into an oxygenated shallow environment during transgression, which led to a negative shift in the δ¹³Ccarb and a decrease in TOC contents as well as an increase in the δ¹³Corg. The correlated enrichment in Zn and δ¹⁵Norg values and the Ce/Ce* around unity are consistent with the shoaling scenario. A similar scenario seems to be consistent with NL1 (lower negative δ¹³Ccarb excursion) although the lower resolution of correlated proxy variations makes the interpretations based on a few points and to be therefore taken with caution. The upper part of the Martin Point section spans the upper Martin Point Member (upper Furongian, ~90 m) of the Green Point Formation of the Cow Head Group. It records the HERB event (HERB - Hellnmaria-Red Tops Boundary) marked by a negative δ¹³Ccarb excursion at the base of the Econodontus notchpakensis conodont Zone. The HERB paleoenvironmental proxy enrichment profiles exhibit comparable patterns to those of the post-SPICE, which suggests that the HERB event started with a similar scenario of transgression and shoaling of organic-rich anoxic water into the water settings. However, the TOC profile shows a small gradual enrichment (~1 %), above the peak of the excursion, correlated with a broad negative δ¹³Corg shift (~2 ‰), and a positive Al/Ti shift. This may suggest possible minor interruptions by sealevel drop and/or an episode/pulse of enhanced wind-blown inputs of weathered terrigenous material.

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