Investigating the Drumian Carbon Isotope Excursion (DICE) in the Cambrian Bonanza King Formation of the Western Great Basin.

Abstract:

The Drumian Carbon Isotope Excursion (DICE) is an ~ –2.5‰ magnitude carbon isotope excursion that occurred around 506.5 Ma. It has been observed in the central and eastern Great Basin USA, Scandinavia, and China and is associated with the appearance of agnostoid trilobites. Inorganic carbon, organic carbon, and pyrite sulfur isotope data were collected from stratigraphic sections of the Bonanza King Formation in eastern California (near Shoshone, CA) and western Nevada (in the Bare Mountains, near Beatty, NV) to investigate the sulfur and carbon isotope geochemical landscape of the westernmost Great Basin and explore potential spatial variation in DICE-aged units. Carbonate-carbon isotope compositions capture the DICE at both localities. In Contrast, organic carbon isotope compositions average –27.5‰ at Shoshone and -16.3‰ at Beatty and do not show the excursion observed in the carbonate data. Petrological analyses indicate widespread recrystallization of Bare Mountains carbonate. This recrystallization may have coincided with ¹²C loss and produced the higher organic d¹³C values seen here. However, retention of the DICE suggests that this recrystallization did not erase the carbonate d¹³C signal. The consistency of the carbonate-carbon isotope data both between our study sites and with the broader record suggest a regional and perhaps global driver of the DICE. Through a simple mass balance model, we calculate an ~8-12% decrease in the fraction of carbon leaving the ocean as organic matter during the DICE. Pyrite sulfur isotope data range from +9.8 to +24.7‰ in Shoshone and -6.3 to +24.7‰ in Beatty, with no obvious correlation with carbonate d¹³C values. The high variability in sulfur isotope values and occurrence of ³⁴S-enriched values that approach probable seawater compositions may suggest low concentrations of sulfate in the ocean throughout deposition of the Bonanza King, including 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.