School of Earth and Environment

Animals breathe freely for the first time 520 million-years ago

27.05.2015 - 14:14

A sub-fossil skeleton of a 1.15 metre Nile Perch found in the Bodélé depression. Credit: Charlie Bristow

Haikouella, the earliest fish-like chordate from the Lower Cambrian Maotianshan shales of Chengjiang County in Yunnan Province, China. Credit: Maoyan Zhu, Nanjing Institute of Geology and Palaeontology (NIGPAS)

Life on Earth may have begun more than 3.5 billion-years ago, but the appearance of eukaryotic life about 2 billion-years ago and multi-cellular life about 800-600 million-years ago are both linked to oxygenation events at the Earth's surface. Following this, a burst of animal forms occurred about 540-520 million-years ago in an event known as the 'Cambrian explosion'. Although a causal link between this radiation of animals and a rise in oxygen was suggested half a century ago, redox conditions in the early Cambrian oceans, especially the deep ocean, remain controversial. 

Now, a study led by researchers from Nanjing University (NJU) and ETH Zurich shows that the 'Cambrian explosion' was in step with the expansion of oxygenated bottom waters in the global ocean. The research, published in Nature Communications, also involved researchers from University College London (UCL), Nanjing Institute of Geology and Palaeontology (NIGPAS), the University of Leeds (UoL) and the China University of Geosciences. 

The researchers utilized analyses of the redox-sensitive element molybdenum in black shales in order to investigate the extent of ocean oxygenation during the ‘Cambrian explosion’. Prof. Derek Vance, an ETH researcher who was one of the co-leaders of the study, said: “In the modern world, the deposition and isotope fractionation mechanisms of molybdenum in anoxic basins are quite different from those in the oxic open ocean." 

Prof. Hong-Fei Ling, a researcher based at NJU who was another co-leader of the study, said: "We analysed molybdenum concentrations and isotopic compositions in sediments deposited between 660 and 520 million-years ago on the Yangtze platform, and also compiled published molybdenum data from about 3.5 billion-years ago to the early Cambrian. Using mass balance models, global marine redox conditions can be quantitatively inferred from these molybdenum records." 

"We found that a rise of molybdenum concentrations and molybdenum isotope values in marine sediments coincides with the increasing diversity and ecological importance of both animals and planktonic algae in early Cambrian," said Prof. Maoyan Zhu at NIGPAS. "All of these life forms peaked around 520 million years ago, which according to our data is when the oxygenation level of the ocean reached modern levels for the first time in Earth history." 

Prof. Simon Poulton of UoL, said: "Although surface waters may have been partially oxygenated before the 'Great Oxidation Event', which happened 2.4?2.2 billion-years ago, the deeper ocean remained largely devoid of oxygen until much later in Earth history, but until now we have not known when modern oxygenation levels were first achieved.” 

"Although the exact causal links and feedback mechanisms between ocean oxygen levels and eukaryotic evolution are still not clearly understood, this coincidence between the rise of ocean oxygenation level and the Cambrian radiation of animals strongly implies a close relationship," added Prof. Graham Shields-Zhou at UCL. "In the Precambrian ocean, besides suffocation through lack of oxygen, anoxia may have also caused metal nutrient deficiencies and the frequent invasion of toxic H2S-rich waters into the major habitats of animals ? the shallow oceans. The key features of the Cambrian explosion, such as large size, active locomotion (including carnivory and bioturbation), may have been restricted until the rise to modern levels of ocean oxygenation." 

"The thriving Cambrian animals may also have acted as ecosystem engineers who contributed to the stability and extent of ocean oxygenation by regulating the carbon and nutrient cycles," added Dr. Xi Chen, who was first author of the study based at NJU. 

The research was supported by the National Key Basic Research Program of China (Grant 2013CB835004), National Natural Science Foundation of China (Grants 41230102 and 40872025), German Research Foundation (FOR 736), Chinese Academy of Sciences, and China Scholarship Council. 

For further details see the published paper:

Chen,X., Ling, H.-F., Vance, D., Shields-Zhou, G.A., Zhu, M., Poulton, S.W., Och, L.M., Jiang, S.-Y., Li, D., Cremonese, L. & Archer, C. (2015) Rise to modern levels of ocean oxygenation coincided with the Cambrian radiation of animals. Nature Communications, 6, 7142.