99-12 Redox Conditions Remained Stable Through the Cambrian Drumian Carbon Isotope Excursion (DICE) in SW Montana

Session: Evolution of Life in the Cambrian Seas: Biotic, Biogeochemical, and Sedimentological Contexts, Part I

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Episodes of expanded anoxia during carbon isotope excursions have been interpreted as an important driver of Cambrian faunal turnover. However, whereas turnover is typically measured globally, the impact of geochemical perturbations is heterogenous over large geographic areas. To understand the influence of expanded anoxia on Cambrian faunal turnover, which reflects the sum of many local extinction events, it is critical to link habitat specific redox proxies with in-situ fossil data. The Drumian Carbon Isotope Excursion (DICE) is a -2 to -4 ‰ δ¹³C excursion that has been identified globally in middle Cambrian strata. Previous studies using biomarkers and local redox proxies have indicated persistently anoxic conditions in water column and deep water environments through the DICE. However, conditions in shallow water environments, which represent an important habitat for marine benthos, have not yet been defined. Here, we present stratigraphic records of carbon isotopes, cerium anomalies (a local, shallow water redox proxy) and quantitative paleontological data from the Camp Creek locality in Southwestern Montana that contains the uppermost Meagher Limestone (Wuliuan) and Park Shale (Drumian) formations. Using these records, we reconstruct local shallow water redox conditions through the DICE, describe faunal diversity and abundance changes through time, and discuss the relationship between the two datasets. We interpret a -2‰ δ¹³C excursion between the Meagher Limestone and Park Shale as the DICE. Within the Park Shale, two faunal assemblages were identified: a low diversity inarticulate brachiopod-dominated fauna in the shales, and a higher diversity trilobite-dominated fauna in the carbonates. This shift in diversity corresponds to a local deepening trend in the section, independent of shifts in carbon isotopes. Cerium anomaly (Ce/Ce*) data shows that most samples maintain values < 1, which are representative of oxic conditions. Additionally, similar Ce/Ce* values were observed during and after the DICE, which suggest that redox conditions were stable and oxic in SW Montana throughout this interval. The absence of shifts in local redox conditions and facies-specific fossil assemblages indicate that shallow water faunal patterns in this region were controlled by water depth and not redox changes associated with the DICE. This illustrates that global geochemical events are geographically heterogeneous and the effects on faunal assemblages are highly localized, underscoring the need for regional studies that pair geochemical and paleontological data.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-10974

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