Banded rocks may reveal Earth's environment 2 billion years ago

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Washington, October 12 (ANI): Scientists have determined that the strikingly banded rocks scattered across the upper Midwest and elsewhere throughout the world can offer clues to the environment of the early Earth more than 2 billion years ago.

Called banded iron formations or BIFs, these ancient rocks formed between 3.8 and 1.7 billion years ago at what was then the bottom of the ocean.

The stripes represent alternating layers of silica-rich chert and iron-rich minerals like hematite and magnetite.

First mined as a major iron source for modern industrialization, BIFs are also a rich source of information about the geochemical conditions that existed on Earth when the rocks were made.

However, interpreting their clues requires understanding how the bands formed, a topic that has been controversial for decades, according to Huifang Xu, a geology professor at the University of Wisconsin-Madison.

Previous hypotheses about band formation involved seasonal fluctuations, temperature shifts, or periodic blooms of microorganisms, all of which left many open questions about how BIFs dominated the global marine landscape for two billion years and why they abruptly disappeared 1.7 billion years ago.

With Yifeng Wang of Sandia National Laboratories, Enrique Merino of Indiana University and UW-Madison postdoc Hiromi Konishi, Xu developed a BIF formation model that offers a more complete picture of the environment at the time, including interactions between rocks, water, and air.

"They are all connected," Xu explained. "The lithosphere affects the hydrosphere, the hydrosphere affects the atmosphere, and all those eventually affect the biosphere on the early Earth," he added.

Their model shows how BIFs could have formed when hydrothermal fluids, from interactions between seawater and hot oceanic crust from deep in the Earth's mantle, mixed with surface seawater.

This mixing triggered the oscillating production of iron- and silica-rich minerals, which were deposited in layers on the seafloor.

The researchers used a series of thermodynamic calculations to determine that the source material for BIFs must have come from oceanic rocks with a very low aluminum content, unlike modern oceanic basalts that contain high levels of aluminum.

BIFs dominated the global oceans 3.8 to 1.7 billion years ago, a time period known to geologists as the Archaean-Early Proterozoic, then abruptly disappeared from the geologic record.

Their absence in more recent rocks indicates that the geochemical conditions changed around 1.7 billion years ago, according to Xu.

This change likely had wide-ranging effects on the physical and biological composition of the Earth. (ANI)

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