AN EPISODE OF WIDESPREAD OCEAN ANOXIA DURING THE LATEST EDIACARAN PERIOD REVEALED BY LIGHT U ISOTOPE COMPOSITIONS IN CARBONATES

Reconstruction of ocean redox chemistry during the Ediacaran Period is important for understanding the causal relationship between environmental oxygen levels and early metazoan evolution. Geochemical data (e.g., high Mo and U concentrations and/or heavy Mo and U isotope compositions from sedimentary rocks) provide evidence of extensive ocean oxygenation shortly after the Marinoan glaciation at ca. 632 Ma [1], during the late Ediacaran Period at ca. 560-551 Ma [2], and multiple times during the early Cambrian Period [3, 4]. These episodes of oxygenation may have been separated by intervals of less oxygenated conditions [1, 2]. However, the global redox state of the ocean during the terminal Ediacaran period (ca. 551-541 Ma) is poorly constrained. We address this knowledge gap by measuring carbonate U isotope compositions (δ²³⁸U) – a novel global ocean redox proxy – of the Gaojiashan Member of the late Ediacaran Dengying Formation (ca. 551-541 Ma) in South China. An abrupt negative shift in δ²³⁸U from values scattering around -0.45‰ to values averaging -0.95‰ (±0.20‰, 2s) was observed in the middle Gaojiashan Member, suggesting a globally widespread expansion of ocean anoxia during the terminal Ediacaran Period. The negative δ²³⁸U shift coincides with the onset of a pronounced positive carbon isotope excursion (from 0‰ to +6‰), suggesting that ocean anoxia is the major driving force behind enhanced organic carbon burial that led to the carbon isotope excursion. The widespread anoxia recorded by the Gaojiashan Member is bracketed by known intervals of extensive ocean oxygenation, thus indicating that the Precambrian-Phanerozoic transition was characterized by oscillating ocean redox conditions. The Ediacara biota (ca. 541 Ma) [5] disappeared shortly after the widespread ocean anoxia, suggesting that an expansion of ocean anoxia may have triggerred the onset of a mass extinction in the latest Ediacaran time.

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