High-resolution carbon, oxygen and magnesium isotope chemostratigraphy of the Kuibis Subgroup, Nama Basin

The Kuibis Subgroup (~551- <547.36 Ma) in the Nama Basin records an important fauna assemblage change during the Ediacaran. Current models support a transition from a more diverse ‘White Sea’ biotic assemblage to a less diverse ‘Nama’ biotic assemblage, for which the drivers remain unresolved. In spite of this decrease in diversity, the Nama assemblage hosts the first appearance of metazoan biomineralization in the geologic record. High resolution chemostratigraphic data are required to globally anchor the timeline of Earth System changes leading to this innovation of life. In order to track these changes at high-resolution, we performed a sampling campaign of Namibian cores from the ICDP project ‘Geological Research through Integrated Neoproterozoic Drilling: The Ediacaran-Cambrian Transition’ (GRIND-ECT), which focuses on terminal Ediacaran stratigraphy of the Witputs and Vioolsdrif sub-basins of southern Namibia. Here we present new Carbon and Oxygen isotope data for carbonate rocks of the Kuibis Subgroup retrieved from drill core 1G. This core records an approximately 200 m-thick succession of mixed carbonate-siliciclastic sedimentary rocks of that unit. Both dolostones and limestones within the lowermost 100 m display negative δ¹³C values between -5 and 0 ‰, consistent with laterally-correlative outcrop data that record the Basal Nama Excursion (BANE). The oxygen isotope profile recorded in core 1G is also very distinctive throughout the BANE. It starts with a negative shift of the δ¹⁸O values from -5 to -15 ‰, which then progressively increase to values around -12 ‰. The uppermost 100 m of core 1G display a gradual increase in δ¹³C values that reach +5 ‰ and are accompanied by increasing δ¹⁸O values that approach -10 ‰. The 0 ‰ crossing point (carbon isotope value) occurs at ~90 m depth, below strata that confidently record the first appearance of fossils of biomineralizing animals. Magnesium isotopic and paleontological data added to our high-resolution carbon isotope profile have the potential to refine the knowledge about the biological and environmental changes of this time period.