Insights into the terminal Ediacaran marine carbonate record from shale-hosted carbonate carbon isotopes

The marine carbon isotope record (δ¹³C) used for chemostratigraphy and reconstruction of carbon cycle dynamics is constructed using carbonate rocks, but there is evidence that carbonate cements hosted within fine-grained clastics (shales and mudstones) in some settings may also express δ¹³C trends that covary with the record from carbonates. We present new carbon and oxygen isotopic data from shale-hosted carbonate cements (δ¹³C_carb-sh and δ¹⁸O_carb-sh, n = 107, <16 wt% CaCO₃) of the terminal Ediacaran Nama Group, Namibia (≥550.5 to <539.6 Million years ago; Ma). These data are compared with the published carbon and oxygen isotopic record from coeval carbonates (δ¹³C_carb and δ¹⁸O_carb, n = 1611) and total organic carbon (TOC) concentrations. We show that δ¹³C_carb-sh compositions in samples of intermediate to high CaCO₃/TOC can approximate contemporaneous δ¹³C_carb in open marine mixed carbonate-clastic settings. By contrast, δ¹³C_carb-sh values in samples with low CaCO₃/TOC that were deposited in clastic settings distant from the locus of carbonate deposition are more negative than contemporaneous δ¹³C_carb. These data suggest that δ¹³C_carb-sh may approach seawater composition in samples of low TOC when deposited in high dissolved inorganic carbon (DIC) settings, where carbonate can rapidly precipitate from seawater during early diagenesis. However, the use of δ¹³C_carb-sh to infill gaps in the existing δ¹³C_carb record remains uncertain, even when these criteria are fulfilled. Intervals of δ¹³C-δ¹⁸O co-variability in the Nama Group succession appear to correlate with units where seawater mixing with meteoric fluids was more likely during early diagenesis, such as clastic-dominated settings, which also show significant decreasing δ¹⁸O through time with gradual sub-basin infill.We further consider uncertainties in lithostratigraphic correlation of the upper Urusis Formation of the Nama Group that enable three new possible correlations to be proposed for δ¹³C_carb-sh data within the terminal Ediacaran to lower Cambrian (<542.65 Ma to >535 Ma) regional and global δ¹³C_carb records.