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The fossil record shows a marked increase in animal and algae fossils roughly 635 million years ago. Researchers believe that oceanic oxygen levels spiked suddenly at this time, in the wake of a severe glaciation, reaching the level necessary to allow animals to flourish. The new evidence pre-dates previous estimates of a life-sustaining oxygenation event by more than 50 million years.
“For more than three quarters of the Earth’s history, the oxygen level in the atmosphere and ocean was insufficient to support animal life,” said Swapan Sahoo, lead author and University of Nevada Las Vegas (UNLV) doctoral student. “Our findings support a link between glaciation, oxygenation of surface environments and the diversification of animals. Knowing the environment where the first animals lived is critical for understanding the evolutionary stress of ecosystems.”
Sahoo visited Arizona State University to carry out trace metal analyses in professor Ariel Anbar’s lab, under the supervision of Brian Kendall, who was a faculty research associate at ASU and is now a professor at Waterloo. Both are co-authors on the paper.
An analysis of iron and trace metal concentrations in organic-rich rocks collected from the Doushantuo Formation in South China revealed spikes in metals that denote higher levels of seawater oxygen. These elevated levels of molybdenum, vanadium and uranium slightly predated the earliest oxygen-demanding animal fossils, supporting the link between ocean oxygenation and animal evolution.
“This is the latest study using changes in the amount of the rare element molybdenum in ancient rocks to learn how Earth’s atmosphere has changed with time – and how those changes may have shaped the evolution of our distant ancestors,” says Anbar, a professor in the School of Earth and Space Exploration and Department of Chemistry and Biochemistry, which are in ASU's College of Liberal Arts and Sciences.
High element concentrations found in the South China rocks are comparable to modern ocean sediments and point to a substantial oxygen increase in the ocean-atmosphere system. Researchers say the oxygen rise is likely due to increased organic carbon burial. Nutrient supplies tend to go up because of weathering during glacial retreat, which leads to a positive feedback between organic carbon burial and oxygen release.
“Photosynthesis is the most efficient process to generate oxygen,” said UNLV’s Ganqing Jiang, principal investigator of the study. “Fast burial of a large quantity of photosynthetic organic carbon in sediments would leave free oxygen in the ocean-atmosphere system, leading to significant oxygen rise.”
The large variability of iron content and trace metal concentrations in the South China rocks may cause scientists to rethink existing geological interpretations about ancient oceans and could lead to accompanying investigations of similar-aged rocks in other continents.
The joint research was supported by grants from the National Science Foundation, the NASA Exobiology Program and National Natural Science Foundation of China.