Paleoenvironmental variations during the Late Cambrian: implications from Zn isotopes and I/(Ca+Mg) ratios

Gonzalez Robacio, Luisa (2023) Paleoenvironmental variations during the Late Cambrian: implications from Zn isotopes and I/(Ca+Mg) ratios. Masters thesis, Memorial University of Newfoundland.

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Abstract

The Upper Cambrian at the Martin Point section partially spans the coeval Tuckers Cove (Shallow Bay Formation) and the Martin Point (Green Point Formation) members of the Cow Head Group of western Newfoundland (eastern Laurentia), Canada. These units comprise alternating shale, minor conglomerate beds and limestone rhythmites of a toe-of-slope apron. Earlier studies of the C-isotope profile, based on the lime mudstone interbeds, documented global negative δ¹³ Ccarb shifts (from old to young: NL1, NL2, HERB) correlated with the lowermost Proconodontus posterocostatus, Proconodontus muelleri and base of Eoconodontus notchpeakensis conodont zones, respectively. Samples were extracted from the most micritic spots, where the micritic to near-micritic grain size (≤ 4 μm−10 μm) and fabric retention confirm petrographic preservation. Insignificant correlations of diagenetic proxies with their environmental counterparts support high degree of geochemical preservation. The zinc-isotope (δ⁶⁶Zn) signatures (0.09 – 0.73 ‰ JMC Lyon) and iodine-to-calcium-plus-magnesium (I/(Ca+Mg)) ratios (0.02 – 0.48 μmol/mol) vary consistently throughout the studied events and were employed to shed light on paleoenviromental conditions. The decrease in δ⁶⁶Zn values seems to denote inhibition of bioproductivity in a dysoxic water column, consistent with warm and humid climate during general transgressive settings and shoaling of organic matter into the shallow environment. On the contrary, slight increases in δ⁶⁶Zn (<0.8 ‰ JMC Lyon) might reflect weak bioproductivity and possible influence of carbonate weathering input during minor fluctuations (episodic falls) in sea level during the main course of the transgression. The low I/(Ca+Mg) ratios (< 0.5 μmol/mol) reinforce this interpretation, suggesting general dysoxic settings along the entire section.

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