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Abstract / Description of output
The marine carbon isotope record (δ 13C) 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 13C trends that covary with the record from carbonates. We present new carbon and oxygen isotopic data from shale-hosted carbonate cements (herein termed δ 13C carb-sh and δ 18O 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 13C carb and δ 18O carb, n = 1611) and total organic carbon (TOC) concentrations. We show that, in the Nama Group, δ 13C carb-sh compositions in samples of intermediate to high CaCO 3/TOC (>0.4) can approximate contemporaneous δ 13C carb in open marine mixed carbonate-clastic settings. By contrast, δ 13C 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 13C carb. These data suggest that δ 13C 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 13C carb-sh to infill gaps in the existing δ 13C carb record remains uncertain, even when these criteria are fulfilled. Intervals of δ 13C-δ 18O 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 18O 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 13C carb-sh data within the terminal Ediacaran to lower Cambrian (<542.65 Ma to >532 Ma) regional and global δ 13C carb records.
Original language | English |
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Article number | 9 |
Journal | American Journal of Science |
Volume | 323 |
Issue number | 9 |
DOIs | |
Publication status | Published - 8 Nov 2023 |
Keywords / Materials (for Non-textual outputs)
- Ediacaran
- carbon isotope
- chemostratigraphy
- diagenesis
- organic carbon
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Dive into the research topics of 'Carbon isotopes from shales provide insight into the terminal Ediacaran marine carbonate record'. Together they form a unique fingerprint.Projects
- 1 Finished
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Resolving the enigmatic Precambrian-Cambrian boundary event (BACE)
1/10/20 → 30/09/23
Project: Research
Equipment
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Wolfson Mass Spectrometry Facility (WMS)
Ulrike Baranowski (Manager)
School of GeosciencesFacility/equipment: Facility