May 25, 2012
Melting Mantle Linked To Great Oxygenation Event 2.5 Billion Years Ago
Brett Smith for RedOrbit.com
Oxygen-based life evolved on Earth because of geological events that occurred over 2.5 million years ago, according to Princeton University researchers who published a report this week in the online journal Nature.
Based on geological evidence, scientists know that roughly 2.5 billion years ago, oxygen levels in the atmosphere exploded and eventually gave birth to our present atmosphere. This time period, dubbed the Great Oxygenation Event (GOE), appears to have coincided with a sharp drop in the melting of the Earth´s mantle, the Princeton research team said.
The study´s authors, C. Brenhin Keller and Blair Schoene, applied a statistical analysis of a 70,000 geological sample database and constructed a 4-billion-year geochemical evolution.
"Research in this area has been largely qualitative, but with this much data, we can pick up finer features in the geologic record, particularly a level of detail related to this sudden change 2.5 billion years ago that people had not seen with such clarity before," said Keller, the lead author and a doctoral student at the university.
Analysis of the geological data showed a dramatic change in the makeup of Earth's magma at the end of the Archean eon, which lasted from 4 to 2.5 billion years ago. Keller and Schoene focused on changes in the chemical composition of basalt, a byproduct of melting magma in the Earth's mantle, because when melting in the mantle is high, basalt contains greater concentrations of elements that are ordinarily found deep in the mantle, according to Keller. Less intense melting results in basalt with a higher content of incompatible elements that are found closer to the Earth's surface.
Based on these observations, they determined that magma was being formed at greater depths during this time and that diminished melting in the mantle decreased the depth of melting in the Earth's crust, thereby reducing the output of volcanic gases that react with and remove oxygen from the atmosphere.
"The perspective behind past efforts to connect geologic processes to the Great Oxygenation Event has been hypothetical, saying that 'If the Earth had been X, there would have been reaction Y,'" Schoene said.
"But these ideas cannot be tested experimentally because they are largely notional. In our paper, we have the evidence to say, 'The Earth was like this,' and then propose a hypothesis that can be tested by examining the same rich database of mantle and deep-crust changes we used in our work."
A decrease in the production of oxygen-reactive gases would allow for the accumulation of oxygen in the atmosphere through the photosynthetic activities of existing organisms, which were alive before the GOE. What exact mechanism drove up atmospheric oxygen levels is difficult to know – even recent fluctuations are difficult to gauge, according to climate scientists. However, the scenario painted by the Princeton study shows that a drop in volcanic activity was precursor to the GOE.
"In science, it is becoming increasingly obvious that seemingly different systems act together and the question is how," Schoene said. "Overall, this analysis strengthens emerging arguments that interaction between the solid Earth and biosphere are very intimate and important.”
"This is strong evidence of how biological and geological systems might work together, and it suggests that important planetary change is not simply the result of life dragging the rest of the planet along."