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Media Relations

Scientist at Work: Enrique Merino

Earth scientists are taught to find and examine evidence of past events, and then devise the simplest explanation possible. The idea is that nature itself tends toward simplicity.

But the Earth is a dynamic planet. Sometimes its geological evolution is so complex, the results are counterintuitive.

"The supposed simplicity of nature resides rather in our mind," says Indiana University Bloomington Professor Emeritus of Geological Sciences Enrique Merino.

Throughout his career, Merino has sought evidence of complexity in geochemical phenomena recorded in rocks, and has tried to apply a dynamics-based kind of thinking to understand them. Merino, a geochemist with broad and multidisciplinary interests (in minerals and rocks and their chemical reactions with water), is retired from teaching but continues to do research.

"You cannot look at geochemical problems as simple," Merino said. "A real geochemical problem in nature most often can be understood only by combining microscopic observation, ideas of irreversible thermodynamics, and dynamic equations incorporating the feedbacks that one suspects were crucial in the workings of the problem in question. You must bring together bits and pieces from many perspectives."

Of particular interest to Merino are banding patterns, common in rocks of all kinds. This interest began in about 1980, when he started to study the formation of stylolites in the limestone rocks around Bloomington.

Stylolites are parallel, wiggly, planar seams found in many limestone formations worldwide. They form when calcium, in the form of calcite, is carried by water to new areas within the limestone. If the calcite traveled randomly, there would be no observable change in the limestone. But the calcite seems to aggregate in layers, which forms the whitened lines and seams you see. This suggests a dynamic process, in which opposing forces actually create the semblance of order.

"You can see stylolites in the limestone steps in the Monroe County courthouse," Merino said. "The rock spontaneously does two things at the same time. It dissolves in one place and another, and simultaneously precipitates in the regions in between. How and why does the dual behavior happen routinely?"

In 1984, Merino published a catalogue of several such types of dual behavior in rocks in a NATO Science Series, coining the term "geochemical self-organization." The catalogue included stylolites, agates, the repeated light and dark mineral banding of metamorphic rocks, Precambrian banded iron formations (or BIFs), and others. It became an informal plan for Merino's subsequent research.

Most recently, Merino was the coauthor of a paper in Nature Geoscience that examined the genesis of BIFs, which are as quirky and unique as they are ancient. The paper's lead investigator and author was Sandia National Laboratory scientist Yifeng Wang, who was once Merino's graduate student at IU Bloomington. University of Wisconsin-Madison geologists Huifang Xu and Hiromi Konishi are also coauthors of the article.

BIFs are found in the Precambrian cores of all continents. They are huge formations, consisting of alternating iron-rich and silicon-rich layers of sediment (magnetite and quartz). Often, the bands are very thin. BIFs are about 2 billion years old. Around 1.8 billion years ago, they stopped forming, and never formed again. And because BIFs are believed to have occurred around the same time as the appearance of the first eukaryotic (non-bacteria) life forms on Earth, some evolutionary biologists wonder whether BIFs could have influenced life's progression -- or the other way around.

"To have oscillation, you need to have some sort of feedback going on," Merino said. "In other words, the rock could not have formed close to equilibrium, chemical or physical."

In their Nature Geoscience paper, the research team proposed a novel explanation for the formation of the banding of BIFs and for their absence in the geological record and why their formation ceased 1.8 billion years ago.

Sea-water leaching of the basalts in the ancient Earth's oceanic crust would have freed silicon and iron, which could then travel into the ocean through hydrothermal systems. The scientists showed how the presence of these minerals could trigger a series of oscillatory chemical reactions (involving the scarce oxygen that photosynthetic organisms had probably already started to make) that would lead to the alternating precipitation of silicon dioxide and iron hydroxide in the water column.

For unknown reasons, oceanic crust basalts appear to have become much richer in aluminum after about 1.8 billion years ago. With more aluminum available, the basalts' hydrothermal alteration by sea water led to the formation of aluminum chlorites, which locked up the iron, preventing its leaching by hot water. Based on their intricate thermodynamic analysis, the scientists argue that the increased presence of aluminum in the crust thus ended the cycle that produced BIFs' alternating bands.

"Dual geochemical behavior, especially if expressed spatially as it is in stylolites and BIFs and other self-organized patterns, cannot result from equilibrium, because equilibrium is blind to space," Merino said. "It must result from dynamics and feedbacks. The trick is to find which ones."

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