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2-Billion-Year-Old 'Window' to Planet's Past Found

Banded rock layers hint at past climate conditions

Oct 12, 2009 08:49 GMT  ·  By
A banded iron formation about 2.5 billion years old, near Soudan Underground Mine State Park, in Minnesota, showing alternating layers of silica-rich (red) and iron-rich (gray) minerals
   A banded iron formation about 2.5 billion years old, near Soudan Underground Mine State Park, in Minnesota, showing alternating layers of silica-rich (red) and iron-rich (gray) minerals

Banded iron formations (BIF) is the name geologists gave to strikingly banded rock formations that existed around the world and that had been formed between 3.8 and 1.7 billion years ago. Their external appearance and inner composition hint at the history of the planet's climate as it was about two billion years ago, giving experts access to a wealth of knowledge about temperature shifts that could help in predicting how the current global warming trend will go on, e! Science News reports.

The BIF are ever more important for studies when considering the fact that they formed on what billions of years ago was the bottom of the ocean. Iron-rich minerals like hematite and magnetite alternate in these layers with layers of silica-rich chert, offering a clear view into the nature of the waters, as they were when only a few basic forms of life existed on Earth.

“They are all connected. The lithosphere affects the hydrosphere, the hydrosphere affects the atmosphere, and all those eventually affect the biosphere on the early Earth,” University of Wisconsin-Madison (UWM) Geology Professor Huifang Xu says.

He reveals that how the rocks themselves formed all these years ago has been a subject of international debate ever since the deposits started being mined as a rich iron source, at the start of the industrialization age. In a new scientific study published in the October 11 issue of the respected scientific journal Nature Geoscience, Xu and his team propose a thorough explanation of how the rocks formed. His group included Sandia National Laboratories expert Yifeng Wang, Indiana University scientist Enrique Merino, and UWM postdoctoral researcher Hiromi Konishi.

The paper shows that hydrothermal fluids may have been the main elements that led to the formation of the layered deposits. The scientists argue that hot oceanic crusts from the planet's mantle interacted with seawater, to produce iron- and silica-rich minerals, in oscillating intervals. During each of these intervals, a new layer was deposited at the bottom, containing either iron or silica minerals.

“The modern-day ocean floor is basalt, common black basalt like the Hawaiian islands. But during that time, there was also a strange kind of rock called komatiites. When ocean water reacts with that kind of rock, it can produce about equal amounts of iron and silica,” Xu explains.