WIDESPREAD LATE CAMBRIAN OCEAN ANOXIA IMPLIED BY CONSTANT CERIUM ANOMALY ACROSS THE STEPTOEAN POSITIVE CARBON ISOTOPE EXCURSION IN THE GREAT BASIN

The late Cambrian strata host one of the most prominent Phanerozoic carbon isotope excursions known as the Steptoean Positive Carbon Isotope Excursion (SPICE). The SPICE has been interpreted as resulting from enhanced organic carbon/pyrite burial that may have released at least 19×10¹⁸ mole of O₂ to the atmosphere and ocean. To test the ocean redox change in response to this oxygen release event, we have analyzed the rare earth element (REE+Y) concentrations of carbonate rocks across the SPICE in three sections across the shelf to slope environments of the late Cambrian carbonate platform in western Utah and Nevada. The results show that cerium anomaly [Ce/Ce* = Ce_sn/(Pr_sn²/Nd_sn)] values across the SPICE are unusually stable, with values between 0.8 and 1.2 (mostly 0.9–1.1) in all three sections including the shelf margin section where carbonates were mostly deposited above the fair-weather wave base. The lack of Ce/Ce* variations across the SPICE raises two alternative interpretations: (1) the carbonate rocks are diagenetically altered and the Ce/Ce* values do not record ancient seawater signature; and (2) the Cambrian ocean during the SPICE were largely dysoxic/anoxic and ocean oxygenation happened long after the SPICE. The first interpretation seems less likely because even the best-preserved (petrographically and geochemically) samples have similar high Ce/Ce* values. The second interpretation is consistent with previous sulfur and redox-sensitive trace element data from black shales of the correlative interval indicative of widespread euxinia in the late Cambrian ocean. Future work on the post-SPICE intervals would clarify the potentially delayed ocean oxygenation towards the Ordovician.

Key words: Rare earth element, late Cambrian, carbon isotope excursion, western United States