from 'Science Daily', May 23, 2012
'The influence of the ground beneath us on the air around us could be greater than scientists had previously thought, according to new research that links the long-ago proliferation of oxygen in Earth's atmosphere to a sudden change in the inner workings of our planet.'
The latest effort highlighting geological processes to help illuminate the earth's oxygenation can be found published in the journal 'Nature'. Princeton University's research appears to be presenting the strongest data-driven correlation yet between deep Earth processes and the Great Oxygenation Event of approximately 2.5 billion years ago.
Blair Schoene and C. Brenhin Keller, Princeton, have put together a database of 70,000 geological samples (previously reported rock and trace element analyses) to construct a 4-billion-year timeline that shows how the geochemical makeup of the crust has changed through time. Rocks preserved in Earth's crust reveal a steep decline in the intensity of melting within the planet's mantle 2.5 billion years ago, which brought about ideal conditions for the GOE. This coincides with existing rock evidence of atmospheric changes related to the Event.
Keller and Schoene's database shows how the geochemical makeup of the crust has changed through time, leading to a more detailed hypotheses about how this would affect the atmosphere 2.5 billion years ago. Their findings show how volcanic gases may have suddenly been neutralized (the effect of this on atmospheric oxygen levels, unknown), suggesting that a reduction in volcanic gases brought about by a drop in mantle-melt intensity was an important precursor to oxygenation. The link shows that diminished melting in the mantle decreased the depth of melting in Earth's crust, which in turn reduced the output of reactive, iron oxide-based volcanic gases into the atmosphere. A lower concentration of these gases -- which react with and remove oxygen from the atmosphere -- allowed free oxygen molecules to proliferate.
Keller and Schoener considered the Earth's gradual cooling since its early history when focusing on changes in the chemical composition of basalt, a byproduct of melting in Earth's mantle. Around 2.5 billion years ago, the levels of compatible elements in the sampled basalt plummeted, indicating that the magnitude of melting deep in the mantle dropped off suddenly.
What does this mean?
'The researchers conclude that when melting happens at a great depth in the crust then the concentration of the iron-oxide gases in magma increases. When emitted into the air by volcanoes, these gases bond with free oxygen and essentially remove it from the air. On the other hand, when crust melting becomes shallower, as they observed, atmospheric levels of those volcanic gases drop and free oxygen molecules can flourish.'
As more data surfaces in the near future, researchers hope to eliminate room for cause-and-effect hypotheses attempting to connect geologic processes to the GOE. "Because of the complicated questions of how solid Earth changes lead to biological innovations, scientists now have to start thinking deeply and working across the boundaries of what have traditionally been pretty rigid subdisciplines in the Earth sciences."
Fischer of Caltech concludes with the following: "It's clear from research like this," he said, "that there is hay to be made by interdisciplinary efforts to connect processes and mechanisms from the solid to the fluid Earth, and to understand that interplay with an ever-evolving biology."
For the full article, please see: http://bit.ly/JVokxX