Abstract

The earliest unambiguous evidence for animals is represented by various trace fossils in the latest Ediacaran Period (550–541 Ma), suggesting that the earliest animals lived on or even penetrated into the seafloor. Yet, the O2 fugacity at the sediment-water interface (SWI) for the earliest animal proliferation is poorly defined. The preferential colonization of seafloor as a first step in animal evolution is also unusual. In order to understand the environmental background, we employed a new proxy, carbonate associated ferrous iron (Fecarb), to quantify the seafloor oxygenation. Fecarb of the latest Ediacaran Shibantan limestone in South China, which yields abundant animal traces, ranges from 2.27 to 85.43 ppm, corresponding to the seafloor O2 fugacity of 162 μmol/L to 297 μmol/L. These values are significantly higher than the oxygen saturation in seawater at the contemporary atmospheric pO2 levels. The highly oxygenated seafloor might be attributed to O2 production of the microbial mats. Despite the moderate atmospheric pO2 level, microbial mats possibly provided highly oxygenated niches for the evolution of benthic metazoans. Our model suggests that the O2 barrier could be locally overcome in the mat ground, questioning the long-held belief that atmospheric oxygenation was the key control of animal evolution.

Highlights

  • The last 10 million years of the Ediacaran Period (550–541 Ma), at the eve of Cambrian Explosion, experienced dramatic and enigmatic changes in the biosphere[1,2]

  • Recent Uranium (U) isotope studies indicated there was extensive oceanic anoxia (>21% of the seafloor) during terminal Ediacaran, which was associated with the decline of Ediacaran biota[8]

  • There are previous studies suggesting that, atmospheric pO2 level remained modest, varying between 10–40% Present Atmospheric Level (PAL)[9,10], the deep ocean remained predominantly anoxic[11], and the ocean oxidation was episodic throughout the Ediacaran and Cambrian[12,13]

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Summary

Introduction

The last 10 million years of the Ediacaran Period (550–541 Ma), at the eve of Cambrian Explosion, experienced dramatic and enigmatic changes in the biosphere[1,2]. We developed a new proxy, carbonate associated ferrous iron (Fecarb), to constrain the seafloor O2 fugacity during the emergence of the earliest benthic animals in the latest Ediacaran Dengying Formation in South China.

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