Newswise — CAMBRIDGE, Mass. — Researchers at the Massachusetts Institute of Technology and Harvard University have provided a possible explanation for how the Earth’s early atmosphere was able to support the abundance of free oxygen and iron found in today’s iron ore deposits, despite evidence suggesting that the atmosphere at that time was devoid of oxygen.
The team’s results, which are reported in the journal Nature Geoscience, suggest that the vast majority of iron in the early ocean was bound to organic molecules produced by living organisms. As these organisms died and sank to the ocean floor, the iron they had bound was locked away in oxygen-poor layers of sediment, preventing it from reacting with and removing oxygen from the atmosphere.
At higher levels of organic productivity and oxygen demand, however, the iron would have been released from the organic molecules, reacted with oxygen, and settled out of the ocean as iron ore, consistent with the deposits of iron ore that have been observed in rocks from this era.
The team was able to recreate these conditions in the lab by synthesizing organic molecules similar to those likely produced by early organisms. They then exposed the organic molecules to dissolved iron and oxygen and found that the iron was efficiently bound to the organic molecules and prevented from reacting with the oxygen.
“Our work suggests that the amount of oxygen produced by early photosynthetic organisms was sufficient to support the precipitation of iron ore deposits, even in an anoxic atmosphere,” says Dustin Trail, the Cecil and Ida Green Professor of Earth and Planetary Sciences at MIT and the paper's senior author. “This provides new evidence that the Earth’s early atmosphere may have been more oxygenated than previously thought, which has important implications for our understanding of the planet's early evolution.”
The team’s results are also significant because they suggest that the processes that led to the formation of iron ore deposits on Earth may have also occurred on other planets or moons in the universe, providing a potential new way to search for signs of life beyond Earth.
“If we find iron ore deposits on other planets, it could be an indication that there was once life there, even if the atmosphere is currently devoid of oxygen,” says Trail.
The team’s research was supported by the NASA Astrobiology Institute.
