Insights Into the Terminal Ediacaran Marine Carbonate Record From Shale-Hosted Carbonate Carbon Isotopes

Abstract

The marine carbon isotope record (δ13C) used for chemostratigraphy and reconstruction of carbon cycle dynamics is commonly assembled using carbonate rocks. There is, however, 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 (herein termed δ13Ccarb-sh and δ18Ocarb-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 (δ13Ccarb and δ18Ocarb, n = 1611) and total organic carbon (TOC) concentrations. We show that, in the Nama Group, δ13Ccarb-sh compositions in samples of intermediate to high CaCO3/TOC (>0.4) can approximate contemporaneous δ13Ccarb in open marine mixed carbonate-clastic settings. By contrast, δ13Ccarb-sh values in samples with low CaCO3/TOC (<0.4) that were deposited in clastic settings distant from the locus of carbonate deposition are more negative than contemporaneous δ13Ccarb. These data suggest that δ13Ccarb-sh may approach seawater composition in samples with low TOC when deposited in settings characterized by high CO32- concentration, where carbonate can rapidly precipitate from seawater during early diagenesis. However, the use of δ13Ccarb-sh to infill gaps in the existing δ13Ccarb 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 δ13Ccarb-sh data within the terminal Ediacaran to lower Cambrian (<542.65 Ma to >532 Ma) regional and global δ13Ccarb records.