Researchers pinpoint beginning of the Earth's atmosphere
Breakthrough research at the University of Queensland has pinpointed the beginning of the Earth's present oxygen-rich atmosphere.
The change from a hostile atmosphere devoid of free oxygen to an oxygen-rich atmosphere sustaining an explosion of new life forms happened about two billion years ago.
The study's findings, published in the March edition of the prestigious journal Science, dispel theories placing the time of the evolution of the present atmosphere at either 570 million years ago (the first appearance of multi-cellular animals in the fossil record) or approximately 3.7 billion years ago (the age of the Earth's oldest traces of life).
The key to the discovery by Earth Sciences Department head Professor Ken Collerson and visiting Swiss postdoctoral research fellow Dr Balz Kamber lies in documenting and understanding the reason for changes in the ratio of the element Niobium (Nb) to Uranium (U) in the Earth's mantle over geological time.
The Earth's mantle extends from just under the continental and oceanic crusts to a depth of around 2800km where it meets the Earth's core. Professor Collerson and Dr Kamber discovered that the Nb/U ratio in the mantle steadily increased from between 3.8 billion years ago to 2 billion years ago.
According to their study, this increase in the Nb/U ratio reflects the growth of continental crust from the mantle. More Uranium is incorporated into the crust than Niobium.
However, from about 2 billion years ago, the Nb/U ratio steadily fell, in spite of continents continuing to grow, the researchers found.
Professor Collerson said the solution to this apparent paradox was that Uranium became soluble during weathering under an atmosphere containing appreciable amounts of oxygen.
"In this way continental Uranium is transported back into the mantle via rivers and oceans. Therefore, the kink in the Nb/U ratio in the Earth's mantle dates the time at which the modern oxygen-rich atmosphere evolved," he said.
The researchers also found acceleration of continental erosion once the modern atmosphere was established. Dr Kamber said this hastened erosion was thought to stem not only from increased atmospheric oxygen levels but also from increased colonisation by new life forms.
Popular theories that the explosion of new life on the planet occurred around 545 million years ago had been fuelled by the fact that few fossils existed from earlier periods, Professor Collerson said.
"A lot of the new life around 2 billion years ago would have been soft-bodied and may simply not have been preserved in the fossil record," he said.
Their theory supports results from recent molecular studies suggesting diversification of metazoans (multi-cellular animals) also began approximately 2 billion years ago.
"Our oxygen model provides a physical reason for this diversification," Dr Kamber said.
The study also showed a progressive increase in the Nb to Thorium (Th) ratio in rocks derived from the mantle during the time period 3.9 billion years ago to the present day.
"In other words, the Earth's continents are still growing despite previous theories suggesting this process had ceased a long time ago," Professor Collerson said.
According to the researchers, the Nb to Th variation is related to the amount of continental crust present throughout Earth history.
As the Nb/Th ratio is still increasing, their data shows that more continental crust is being extracted from the mantle today than is being returned to the mantle via weathering.
Furthermore, they concluded that continental formation processes had not changed over geological time.
The study required very accurate chemical analysis of Thorium, Uranium and Niobium concentrations with many analyses carried out in the University's $7 million Radiogenic Isotope Laboratory.
This world-class analytical facility was established by Professor Collerson over the last six years using University funding, Australian Research Council (ARC) grants and mineral and petroleum exploration company support.
For more information, contact Professor Collerson (telephone 07 3365-1180 or 07-3365-8505). Photos available on the University's ftp site: http://photos.cc.uq.edu.au/ folder name: Collerson.
Element ratio changes as indicators of Earth's atmosphere evolution
The upper panel shows the evolution of the ratio between Th and U in depleted mantle through Earth history. The grey curve depicts the observed evolution. Note that between 4.5 billion years ago and 2 billion years ago, the ratio only decreased from 4.1 to 3.7. After 2 billion years ago, the ratio decreased rapidly to the present-day value of 2.5. The black curve shows a hypothetical evolution in a permanently anoxic Earth (modelled from the information in the lower panel). The curves agree well up to 2 billion years ago but then diverge strongly. This reflects recycling of U from the continents into the mantle once an oxydizing atmosphere was established.
The lower panel shows the volume of continental crust through Earth's history (reconstructed from the Nb/Th ratio of depleted mantle). Note the strong net growth from 3.5 to 2 billion years ago, followed by a markedly slower growth since 2 billion years ago. This decrease in net continental growth rate is interpreted to reflect higher erosion rates under an oxydizing atmosphere reduced growth rate because of formation of supercontinents with strongly reduced total circumference and hence reduced subduction zone length.
.jpg)
