Thursday, December 3, 2009

WHY HUGE BANDS OF IRON FORMED BILLIONS OF YEARS AGO ON EARTH’S SURFACE



No one knows why massive formations of banded iron — some ultimately hundreds of kilometers long, like a sleeping giant’s suspenders — mysteriously began precipitating on Earth’s surface about 3.5 billion years ago. Or why, almost 2 billion years later, the precipitation ceased.

Because these deposits carry information about early Earth’s surface conditions and climate changes, as well as provide much of modern industry’s iron resources, interested researchers have cast a wide net in trying to explain why and how these bands formed. But attempts to explain their existence based on seasonal variations, surface temperature changes and episodic seawater mixing all have foundered on assumptions requiring the unexplained oscillations of external forces.

None of these theories could satisfactorily explain all the observations made by geologists, particularly the existence of alternating structural bands of silica-rich layers with iron-rich layers in these deposits.

A new approach proposed in an October issue of Nature Geoscience by Sandia National Laboratories principal investigator Yifeng Wang and colleagues elsewhere may have the answer.

A key component of the process, the researchers found in computer simulations, may have been the absence of aluminum in early oceanic rocks. That absence chemically favored the formation of banded iron formations. The continual enrichment of oceanic crust by aluminum as Earth evolved ultimately ended the era of iron band formation.

A complete thermodynamic explanation by the research team suggests that iron- and silicon-rich fluids were generated by hydrothermal action on the seafloor. The team’s calculations show that the formation of bands was generated by internal interactions of the chemical system, rather than from external forcing by unexplained changes such as ocean surface temperature variations.

“This concept of the self-organizational origin of banded iron formations is very important,” said Wang. “It allows us to explain a lot of things about them, like their occurrence and band thickness.”

Wang’s Ph.D. advisor, Enrique Merino at Indiana University, may have been the first to consider banded iron formations as formed through self-organization, Wang said: “We started to work on the issue about 15 years ago.” But difficulties in pinning down an actual mechanism persisted.

“Last year, Huifang Xu [at the University of Wisconsin at Madison] and I happened to talk about his work on astrobiology and then we talked about banded iron formations,” said Yifeng. “After that, I got interested again in the topic. Luckily, I came across a very recent publication on silicic acid interactions with metals. With these new data, I did thermodynamic calculations. I looked at the results and talked to both Huifang and Enrique. The whole banded-iron-formation puzzle started to fit together nicely.”

Merino and Xu coauthored the paper with Wang, along with Hironomi Konishi, also at the University of Wisconsin at Madison.

“Our work has two interesting implications,” said Wang. “The Earth’s surface can be divided into four interrelated parts: atmosphere, hydrosphere, biosphere and lithosphere. Our work shows that the lithosphere, that is, the solid rock part, plays a very important role in regulating the surface evolution of the Earth. This may have an implication on the studies of other planets such as Mars. Our work also shows that to understand such evolution requires a careful consideration of nonlinear interactions among different components in the system. Such consideration is important for prediction of modern climatic cycles.”

“After all,” he said, “Earth’s system is inherently complex and the involved processes couple with each other in nonlinear fashion.”

(Photo: Randy Montoya)

Sandia National Laboratories

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