Redox Change Across the Late Cambrian Steptoean Carbon Isotope Excursion (SPICE) in the Southern Great Basin, USA
Doctor of Philosophy (PhD)
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The late Cambrian witnessed one of the largest positive carbon isotope (δ13C) excursions of the Phanerozoic named as the Steptoean Positive Isotope Carbon Excursion (SPICE; ~497–494 Ma). The SPICE has a magnitude of ≥5‰ and has been interpreted as a result of enhanced organic carbon burial. Because each mole of organic carbon burial would leave one mole of oxygen in Earth’s atmosphere and ocean (CO2 + H2O → CH2O↓ + O2↑), it is predicted that a significant oxygenation event should be associated with the SPICE. Yet most existing geochemical data including sulfur isotopes (δ34S), molybdenum (Mo) concentration and isotopes (δ98Mo), iron speciation, and uranium isotopes (δ238U) from black shales reveal significant oceanic anoxia during the SPICE. To test if ocean oxygenation during the SPICE happened in shallow-water carbonate platforms, rare earth element and yttrium (REE+Y or REY) were used as a redox proxy to examine the temporal redox change across the SPICE in three sections of the southern Great Basin, USA. These sections cover the shelf, shelf margin, and slope environments of the late Cambrian carbonate platform in the west margin of Laurentia. The REYs in all three sections show light REE enrichments and flat REY patterns. Particularly, the Ce anomalies [Ce/Ce* = CeSN/(PrSN2/NdSN)] fall in a narrow range of 0.8–1.2, which are much higher than the average Ce/Ce* value (~0.55) of modern ocean seawater. Trace elemental ratios including U/Th and V/Sc also show very low values (< 2.0) and limited temporal variations. The flat REY patterns, high Ce/Ce* values, and low U/Th and V/Sc ratios suggest that during the SPICE, anoxic environments may have expanded to shallow-water carbonate platforms. The results are consistent with other redox proxy data from black shales of time-equivalent units but challenge the predictions from carbon and sulfur mass balance models. The oxygenation event associated with the SPICE, if it happened, must have come much later in post-SPICE intervals towards the Cambrian-Ordovician transition.
Anoxic widespread; Great Basin (shelf shelf margin and slope environment); High and constant Ce/Ce*; Late Cambrian Great Basin; Rare earth elements and trace element ratios; Redox
Geochemistry | Geology
University of Nevada, Las Vegas
Huan, Chin Chai, "Redox Change Across the Late Cambrian Steptoean Carbon Isotope Excursion (SPICE) in the Southern Great Basin, USA" (2022). UNLV Theses, Dissertations, Professional Papers, and Capstones. 4508.
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