218-1 Defining Shallow-Water Redox Conditions Across the SPICE in the Central Appalachian Mountains

Abstract:

The Steptoean Positive Isotopic Carbon Excursion, or SPICE event, was a global +3–6‰ shift in δ¹³C values during the late Cambrian (~493 Ma). It has been hypothesized that this event represents widespread marine deoxygenation prompting biological turnover and increased burial of organic carbon, driving a positive carbon isotope excursion. However, more work is needed to assess the timing and magnitude of redox changes in shallow water tropical environments. Here, we present two proxies for local redox conditions, the cerium anomaly (Ce/Ce*) and iodine to calcium ratios, along with complementary major and trace element concentration data, to constrain redox conditions in two shallow carbonate sections from Clear Spring, Maryland and Lebanon, Virginia, USA that record the SPICE. Both sections are composed of dominantly peritidal with minor subtidal carbonate shoal facies, but the Clear Spring section exhibits greater facies variability compared to the Lebanon section.

At the Lebanon section, Ce/Ce* results record anoxic conditions through the SPICE interval (mean = 1.18) and a shift towards more oxic conditions (mean = 0.87) concurrent with a return to background carbon isotope values. The Clear Spring section records persistently anoxic conditions, with relatively invariant Ce/Ce* values through the Pre-SPICE, SPICE, and Post-SPICE interval (mean = 1.1). In this section, inferred anoxic conditions are supported by I/Ca ratios, which are consistently low (<1 μmol/mol) throughout the section. These preliminary results indicate that the deeper-water Lebanon section records a redox shift, with more anoxic seawater corresponding to the SPICE and a shift towards more oxic conditions in the falling limb and post-SPICE interval. The contrasting persistently anoxic conditions that are recorded in the Clear Spring section suggest a highly heterogenous redox landscape across the shelf in the Appalachian Basin during the SPICE.

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