Seawater-buffered diagenesis, destruction of carbon isotope excursions, and the composition of DIC in Neoproterozoic oceans.
basic_science · Level V
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- Record sourced from PubMed, PMID 31484778.
- Also identified by DOI 10.1073/pnas.1909570116 and PMC identifier 6754552.
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Abstract
Carbonate sediments of nonglacial Cryogenian (659 to 649 Ma) and early Ediacaran (635 to 590 Ma) age exhibit large positive and negative δ<sup>13</sup>C<sub>carb</sub> excursions in a shallow-water marine platform in northern Namibia. The same excursions are recorded in fringing deep-sea fans and in carbonate platforms on other paleocontinents. However, coeval carbonates in the upper foreslope of the Namibian platform, and to a lesser extent in the outermost platform, have relatively uniform δ<sup>13</sup>C<sub>carb</sub> compositions compatible with dissolved inorganic carbon (DIC) in the modern ocean. We attribute the uniform values to fluid-buffered diagenesis that occurred where seawater invaded the sediment in response to geothermal porewater convection. This attribution, which is testable with paired Ca and Mg isotopes, implies that large δ<sup>13</sup>C<sub>carb</sub> excursions observed in Neoproterozoic platforms, while sedimentary in origin, do not reflect the composition of ancient open-ocean DIC.