TERMINAL EDIACARAN MICROBIAL MATS CREATE SEAFLOOR O2 OASES FOR EARLY ANIMAL EVOLUTION FROM THE PERSPECTIVE OF NEW PROXY, FECARB

Abundant horizontal and penetrative trace fossils and oldest trackways produced by bilaterian animals have been discovered in the terminal Ediacaran (550-541Ma), which suggests a relatively high O₂ fugacity at the water-sediment interface (WSI) for these benthic animals. However, the existing proxies such as Fe-S-C systematics and Ce anomaly only indicate dynamic redox conditions of the seawater rather than the seafloor. δ²³⁸U in marine carbonates reflects an extensive global seafloor anoxia, but poorly constrains the redox of the local environment where benthic animals radiated and survived. In addition, atmospheric O₂ level and ocean oxygenation cannot guarantee the redox at seafloor, i.e. modern ocean. Thus, reconstruction of O₂ fugacity at WSI during the earliest evolution of benthic animals is in recognised need.

Here, we employed a new proxy, carbonate associated ferrous iron (Fe_carb), to unravel the local redox of seafloor during the deposition of shallow-marine fossiliferous limestone of the latest Ediacaran Dengying Formation in South China. Fe_carb, controlled by the benthic Fe²⁺ flux within the sediments, has a negatively exponential correlation with the bottom water O₂, which could quantitatively reconstruct the redox condition at the WSI.

Our study shows that the Dengying limestone has extremely low Fe_carb (2.27ppm ~ 85.43ppm). It is comparable to that of the shallow-marine carbonates in late Paleozoic (15.28ppm ~ 166.26ppm), when atmospheric O₂ content reached or even exceeded the modern level. According to modelling results, the O₂ fugacity at WSI of Dengying Formation could reach at least 60% of present atmospheric level (PAL). This result suggests that the Dengying limestone precipitated in oxic seafloor with limited benthic Fe²⁺ flux from sediment porewater, despite the generally low atmospheric pO₂ level (10%-40% PAL) and the extensive anoxia at WSI. Local seafloor oxygenation might be attributed to the development of microbial mats, which produced O₂ and resulted in the seafloor oxygenation and ventilation of the upper layer of sediments, providing oases for the radiation of benthic metazoans. Our new hypothesis implies that the O₂ barrier could be locally overcome in mat ground. Thus, atmospheric oxygenation may not be the essential requirement for animal evolution.