Ediacaran ocean redox evolution

The relative role that environmental versus intrinsic biological factors played in shaping the history of life on Earth is a fundamental but unanswered question in the natural sciences. Most famously, it has been heavily debated if the emergence and diversification of early animals during the Ediacaran Period (ca. 635--541 Ma) was tied to a rise in atmospheric-oceanic oxygen levels. Temporally discontinuous geochemical data and patchy fossil record in literature are inadequate to chart the Ediacaran redox history and its causal relationship with the biotic evolution. My PhD study aims for a multiproxy geochemical record within a well-established stratigraphic framework of the Ediacaran (ca. 635--541 Ma) succession in South China. This succession is particularly suited for obtaining a high-resolution redox record based on redox-sensitive trace elements (RSEs) and pyrite sulfur isotopes due to the availability of organic-rich black shales. Modern-level RSE enrichments in black shales immediately overlying the Marinoan-age glacial diamictites reveal, for the first time, a pervasive ocean oxygenation event in the aftermath of the Earth's extreme cold. This oxygenation event may have triggered the first appearance of primitive animals in Earth history. Following this oxygenation, RSE concentrations returned back to crustal and near-crustal values until the middle Ediacaran (ca. 580 Ma), implying significant ocean anoxia. Low RSE values appear again after spikes at ca. 580 Ma and ca. 550 Ma. The rises and falls of RSEs and pyrite sulfur isotopes document a dynamic Ediacaran (and likely including Cambrian) ocean redox state: multiple oxygenation events in the overall anoxic Ediacaran ocean. The brief (<5--10 Ma) oxygenation events may have triggered staged biotic innovations separated by long-term evolutionary stasis.