DID PERSISTENTLY LOW OXYGEN CONDITIONS SLOW DIVERSIFICATION DURING THE LATTER HALF OF THE CAMBRIAN?

Following the Cambrian explosion, repeated mass extinctions in the latter half of the Cambrian (509 to 485 Ma) caused a diversity plateau. Elevated extinction rates may have been triggered or amplified by persistently low oxygen conditions hypothesized for Cambrian oceans. Several carbon isotope excursions interpreted to indicate the spread of low oxygen conditions, occurred during the Middle and Late Cambrian. The Drumian Carbon Isotope Excursion (DICE) records a negative 2 to 4‰ shift in δ¹³C globally and has been interpreted as recording the upwelling of anoxic ¹²C-rich water onto the shelf during transgression. The relationship between this event and paleoecological change is not well defined. If the DICE records an interval of expanding anoxic conditions that influenced extinction patterns, then marine invertebrates with high O₂ requirement should show lower abundances and diversity through this interval.

The DICE excursion was identified in two sections of the Wolsey, Meagher, and Park Formations in SW Montana based on preliminary C isotope analysis. These sections were evaluated for macrofossil content and bioturbation intensity.

Preliminary results, based on a comparison of similar offshore facies between the Wolsey Shale and the Park Shale, show a disappearance of burrowing organisms and an increase in flat pebble conglomerate facies (indicative of a lack of bioturbation) in the Park Shale Formation, suggesting a possible local decrease in oxygen concentrations during the DICE.

Future work will include counts of skeletal clasts and sedimentological constituents via petrographic analysis to further characterize faunal change and interpret paleoenvironments. Rare Earth Element analysis from these sections will provide a direct redox proxy (cerium anomaly, Ce/Ce*) to establish local oxygen levels. Using both paleontological and geochemical data, this research aims to characterize how fauna were impacted by fluctuating oxygen levels in Cambrian oceans.