The first actual record of deep open-ocean conditions in the Ediacaran: Fe speciation in pelagic deep-sea sediments in accretionary complexes in Wales, UK

Abstract:

The first oxidation of a deep ocean in Earth history is considered to have occurred in the Neoproterozoic, coincident with the metazoan diversification; however, the Neoproterozoic geological record has so far been limited to only continental shelves, slopes, or basins at the deepest. Here, we document Neoproterozoic pelagic deep-sea sediments in reconstructed oceanic plate stratigraphy (OPS) in accretionary complexes (ACs) in Anglesey and Lleyn, Wales, UK. The OPS mostly consists of mid-ocean ridge basalts, pelagic red-bedded cherts, hemipelagic siliceous mudstones and turbidite sandstones, in ascending order. Only at Porth Felen in Lleyn Peninsula does the OPS contain black mudstones (ca. 10 m-thick) instead of pelagic red-bedded cherts. Based on the tectonic reconstruction of these ACs, the OPS at Porth Felen has the oldest depositional age. Our new U-Pb date of detrital zircons separated from the turbidite sandstones at Porth Felen has the youngest age of 580±13 Ma. These results suggest that the black mudstones at Porth Felen were deposited no later than the early Ediacaran. We have analyzed these black mudstones by ⁵⁷Fe Mössbauer spectroscopy, and found that about a quarter of their iron content is contained in pyrite, while the other components are paramagnetic Fe²⁺ or occasionally paramagnetic Fe³⁺ in clay minerals. The red cherts in the younger OPS contain hematite as the main iron mineral, paramagnetic Fe³⁺, and paramagnetic Fe²⁺. The occurrence of hematite in a deep-sea chert essentially indicates a primary oxidizing depositional condition, whereas pyrite is indicative of a reducing environment. The present data confirm that a reducing deep-sea existed in the early Ediacaran during the black mudstone deposition, and that an oxidizing deep-sea had been established by the late Ediacaran. In conclusion, our results provide the first direct evidence of an actual deep open-ocean in the Ediacaran to clarify the timing and extent of the Neoproterozoic Oxygenation Event.