Transient shallow-ocean oxidation associated with the late Ediacaran Nama skeletal fauna: Evidence from iodine contents of the Lower Nama Group, southern Namibia

Abstract

The terminal Ediacaran interval from ~ 550 Ma to 541 Ma witnessed the first appearance of skeletal metazoans and complex ecosystems in Earth history. This biotic innovation event is thought to be related to increase of oxygen in Earth’s surface environments, but many studies suggested that pervasive oceanic anoxia continued through the late Ediacaran and early Cambrian. To further evaluate the redox conditions of the terminal Ediacaran interval and their potential relationship with biotic changes, we analyzed the iodine contents in the Lower Nama Group (~550–547 Ma) from the Driedoornvlagte and Zebra River sections, southern Namibia. The I/[Ca + Mg] values fluctuate from 0.08 to 6.20 μmol/mol, with an average of 0.94 μmol/mol in the Driedoornvlagte section and 0.70 μmol/mol in the Zebra River section. High I/[Ca + Mg] values (>2.6 µmol/mol) occur immediately above the boundary between the lower and upper Omkyk Members, with a peak up to 6.20 μmol/mol indicative of well-oxygenated surface waters comparable with those of the modern marine environments. Abundant skeletal fossils including Cloudina, Namacalathus, Namapoikia and trace fossils are observed only in the intervals after the high I/[Ca + Mg] peak. The results are consistent with the interpretation that the Nama skeletal communities grew in oxic (O2 > 20–70 μM) and dysoxic (O2 ≥ 10 μM) waters above the chemocline. The I/[Ca + Mg] ratios show significant tempo-spatial variations, which is also consistent with previous studies that suggested redox-stratified terminal Ediacaran sedimentary basins and highly heterogeneous oceanic redox conditions.