Carbon Isotope Trends of Precambrian-Cambrian Carbonates in Southwestern Laurentia Are Robust to Diagenetic Overprinting

An extreme negative (<-6%) carbon isotope (δ¹³C) excursion is recognized globally in strata recording the Ediacaran-Cambrian boundary. This excursion has been termed the BAsal Cambrian carbon isotope Excursion (BACE). It, like other carbon isotope excursions throughout the geologic record, has been interpreted to record a perturbation to the global carbon cycle based on the assumption that shallow water carbonate sediments and rock preserve an accurate time-series of the δ¹³C composition of the global ocean. However, this assumption has been demonstrated to be inaccurate in some shallow water settings; pervasive early diagenetic alteration of shallow water carbonate δ¹³C records and local carbon cycling within the platform environment can result in the decoupling of global ocean and shallow water carbonate δ¹³C values. Here, we test the extent and isotopic effects of early diagenetic alteration of shallow water carbonate sediments that record the BACE in southwestern Laurentia using carbonate stable isotope (δ¹³C, δ¹⁸O, δ^44/40Ca, δ²⁶Mg) and major and minor element (Mg/Ca, Sr/Ca) geochemistry. The δ^44/40Ca values of the three studied sections are consistent with different modes of diagenetic alteration, ranging from more sediment-buffered to more fluid-buffered early marine diagenesis. Dolostone δ²⁶Mg and δ¹⁸O values suggest that diagenetic alteration and dolomitization occurred prior to significant burial. Because the δ¹³C chemostratigraphic records are reproducible despite the variable diagenetic regimes among the three sites, we argue that the prominent δ¹³C stratigraphic trends were insensitive to diagenetic overprinting and instead reflect changes in the primary δ¹³C composition of dissolved inorganic carbon of platform seawater. Finally, we explore a potential link between the BACE, an increase in early marine fluid-buffered marine diagenesis of platform carbonates, and magmatism associated with the rifting of the southwest margin of Rodinia.