Using ancient air trapped in rock salt, geologists have, for the first time, taken direct measurements of oxygen from the Neoproterozoic era – the geologic time period spanning 1,000 to 541 million years ago. Researchers from West Virginia University (WVU) identified that the oxygen content of Earth's atmosphere 813 million years ago was 10.9%, a finding which, they say, pushes back the timeline of oxygenation – the increase of oxygen in the atmosphere – by at least 100 to 200 million years. The development has significant implications for our understanding of Earth's past atmosphere as well the explosion of life on the planet.
Detailing the composition of the atmosphere over the course of the earth's history is an important but difficult task for geologists. Most of the techniques used to determine past atmospheric conditions are based on the 'indirect' geologic analysis of sedimentary rocks – not the 'direct' measurement of ancient atmospheric gas – which are very hard to date because they contain remnants of other rocks which all formed at different times.
Current theories based on these kinds of measurements show that Earth's atmosphere during its very early history contained almost no oxygen until the dawn of two exceptional periods where oxygen levels rose – known as the Great Oxidation Event (GOE), thought to have occurred around two and half billion years ago, and the Neoproterozoic Oxidation Event (NOE), which, according to various models, can be placed anywhere between 800 and 500 million years ago.
The models regarding the NOE are quite uncertain about its onset, development and relationship to the evolution of life. Some models suggest the period was marked by mostly low oxygen levels in the atmosphere preceding a sharp spike, while others suggest there was a more moderate increase in levels over time.
Geologist Kathleen Benison and her team at WVU have been using new direct techniques to measure the Earth's oxygenation, leading them to the conclusion that traditional assumptions about the rise of oxygen in the atmosphere have been wide of the mark.
Using a device known as a quadruple mass spectrometer, the geologists analysed fluid inclusions – microscopic bubbles of liquids in rock salt, which can contain pockets of ancient trapped air – allowing them to understand past surface conditions and oxygen's changing role in the atmosphere over history.
Most of the traditional models regarding the NOE do not resolve the question of whether oxygenation of the atmosphere drove animal evolution, or if animal evolution drove oxygenation – via organisms which produce oxygen as a by-product. The team's findings however, may shine some light on the puzzle.
"Diversity of life emerges right around this time period [800 million years ago]," Benison said. "We used to think that to have diversity of life we needed specific things, including a certain amount of oxygen. [The findings] show that not as much oxygen is required for organisms to develop."
Plant life could have begun 800 million years ago
The team argues that the levels of oxygen in the atmosphere, and commensurate levels in seawater, proposed by all of the traditional models are too low to sustain the emergence of complex life, predicted to have occurred during the late Neoproterozoic era and early Palaeozoic era – between 650 and 500 million years ago.
Instead they propose that their oxygen measurement of 10.9% for the late Tonian period of the Neoproterozoic – around 800 million years ago – is more than sufficient to support an expansion of plant life and the emergence of marine animals, given that is well above the level that all the major models predict for the same time period. Furthermore, they came to the conclusion based on their data that the NOE took place at least 200 to 100 million years prior to the traditional models based on indirect measurements.
The implications of this kind of research could be far-reaching. "Deciphering the oxygenation history of the atmosphere and oceans is critical to understanding weathering processes, sedimentary environments, climate change, mass extinctions, tectonic events, and the evolution of Earth's ecology", the study says.