A CARBON AND NITROGEN ISOTOPE PERSPECTIVE ON 13C EXCURSIONS AND EXTINCTIONS AT THE EDIACARAN/CAMBRIAN BOUNDARY

The South Oman Salt Basin (SOSB) hosts one of the most complete and thermally best preserved Ediacaran sedimentary sequences that have recorded not only the globally-occurring Shuram ¹³C-isotope anomaly, but also a negative excursion in stable carbon isotopes that marks the Precambrian–Cambrian boundary and the extinction of the sessile benthic metazoans Cloudina and Namacalathus. We here report on bulk stable carbon isotopes and organic matter ‘quality’—as determined by elemental ratios of C, H and O—in synchronously deposited basin and platform carbonates of the SOSB, as well as on maleimide biomarkers that represent molecular fragments of porphyrins derived from photosynthetic pigments. While classic biomarkers indirectly indicative for euxinia are absent, carbon-isotopically heavy acyclic isoprenoids and higher alkylated maleimides point towards a strongly stratified and reducing system, where anoxygenic phototrophs thrived at a chemocline. Compound-specific nitrogen isotope analyses point to nitrogen assimilation from different pools in surface waters versus at the chemocline and suggest an indirect, nutrient-control on the terminal Ediacaran carbon cycle. We will discuss potentially causal connections between our observations, as well as the role of environmental redox by controlling nitrogen nutrient availability.