Fig. 6: Photic zone euxinia (PZE) in the Ediacaran ocean and its impact on ocean oxygenation and the Ediacara biota.
From: Recurrent photic zone euxinia limited ocean oxygenation and animal evolution during the Ediacaran
Conceptual shifts in Δ199Hg and δ202Hg under the depicted redox conditions are shown in inserted boxes. The numbers in text boxes represent the sequence of events. During intervals of expanded PZE (panel a), PZE developed on continental margins due to increased marine nutrients and sulfate availability (likely due to enhanced postglacial continental weathering, particularly during E1), and was further promoted by the positive feedback from P regeneration under euxinic water. PZE would have enhanced the photoreduction of Hg(II)-S (“S” represents reduced sulfur ligands) and the oxidation of atmospherically deposited Hg(0), leading to the negative shift in Δ199Hg with concurrent positive shift in δ202Hg. Expanded PZE also promoted anoxygenic photosynthesis and inhibited oxygenic photosynthesis, resulting in relatively rapid termination of the previously proposed ocean oxygenation event. Spatial variations in Hg isotopes indicate that PZE may have migrated upslope toward the end of E1, as depicted by the upper euxinic wedge in panel a. During intervals of contracted PZE (panel b), the postglacial surge of weathering may have waned down, leading to the contraction of PZE and resulting in the negative shift in δ202Hg with concurrent positive shift in Δ199Hg. PZE may have also limited the diversification of complex eukaryotic life. At Weng’an (WA) section that was not affected by PZE, the Weng’an biota was found in the upper phosphorite. At Taoying (TY) section where PZE was likely sporadic, the Wenghui biota was found in the upper Doushantuo black shales. In contrast, at Wuhe (WH) and Yuanjia (YJ) sections where PZE or anoxic water persisted, no benthic fossils have been reported.
