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

An extreme negative (<-6‰) carbon isotope (δ 13 C) 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 δ 13 C 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 δ 13 C records and local carbon cycling within the platform environment can result in the decoupling of global ocean and shallow water carbonate δ 13 C 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 (δ 13 C, δ 18 O, δ 44/40 Ca, δ 26 Mg) and major and minor element (Mg/Ca, Sr/Ca) geochemistry. The δ 44/40 Ca 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 δ 26 Mg and δ 18 O values suggest that diagenetic alteration and dolomitization occurred prior to significant burial. Because the δ 13 C chemostratigraphic records are reproducible despite the variable diagenetic regimes among the three sites, we argue that the prominent δ 13 C stratigraphic trends were insensitive to diagenetic overprinting and instead reflect changes in the primary δ 13 C 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.