INVESTIGATING THE DRUMIAN CARBON ISOTOPE EXCURSION (DICE) IN THE CAMBRIAN BONANZA KING FORMATION OF THE WESTERN GREAT BASIN, U.S.A.

The Drumian Carbon Isotope Excursion (DICE) is a -2 to -3‰ magnitude negative carbon isotope excursion that occurred around 506.5 Ma and has been observed in the central and eastern Great Basin USA, Scandinavia, southern China, and South Korea. Samples were collected from the Bonanza King Formation from near Shoshone and the Bare Mountains of the westernmost Great Basin to investigate spatial variations in DICE-related excursions. Inorganic carbon, organic carbon, and pyrite sulfur isotope analyses are used to reconstruct the chemical characteristics of the depositional environment. Carbonate carbon isotope compositions in the Late Cambrian Bonanza King Formation range from -2.20 to +1.34‰ (VPDB) and capture the DICE at both localities. The organic carbon isotope data suggest that the Bonanza King Formation may represent a shallow platform carbonate deposit, or that the organic matter was thermally degraded during burial. Petrological analyses indicate widespread recrystallization of Bare Mountains carbonate, consistent with burial diagenesis and organic matter degradation. The consistency of the carbonate-carbon isotope data among local and distant sites suggests a global driver of the DICE. A simple mass balance model reveals a 49% decrease in the proportional flux of carbon leaving the ocean as organic matter during the DICE and implies a significant shift in redox conditions during the DICE. A paucity of carbonate-bound sulfate and pyrite sulfur isotope data suggests low concentrations of sulfate in the ocean throughout the deposition of the Bonanza King Formation, including during the DICE interval. These findings add to a growing body of evidence that suggests that Cambrian marine environments retain many characteristics of the preceding Neoproterozoic.