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

Abstract

The marine carbon isotope record (δ 13 C) 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 δ 13 C trends that covary with the record from carbonates. We present new carbon and oxygen isotopic data from shale-hosted carbonate cements (herein termed δ 13 C carb-sh and δ 18 O carb-sh, n = 107, <16 wt% CaCO 3 ) 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 (δ 13 C carb and δ 18 O carb , n = 1611) and total organic carbon (TOC) concentrations. We show that, in the Nama Group, δ 13 C carb-sh compositions in samples of intermediate to high CaCO 3 /TOC (>0.4) can approximate contemporaneous δ 13 C carb in open marine mixed carbonate-clastic settings. By contrast, δ 13 C carb-sh values in samples with low CaCO 3 /TOC (<0.4) that were deposited in clastic settings distant from the locus of carbonate deposition are more negative than contemporaneous δ 13 C carb . These data suggest that δ 13 C carb-sh may approach seawater composition in samples with low TOC when deposited in settings characterized by high CO 3 2- concentration, where carbonate can rapidly precipitate from seawater during early diagenesis. However, the use of δ 13 C carb-sh to infill gaps in the existing δ 13 C carb record remains uncertain, even when these criteria are fulfilled. Intervals of δ 13 C-δ 18 O 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 δ 18 O 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 δ 13 C carb-sh data within the terminal Ediacaran to lower Cambrian (<542.65 Ma to >532 Ma) regional and global δ 13 C carb records.