Hot Topic Article:
Microbes at Work: A New Agent in Clay Transformations
Jay Chapman
TEM photomicrograph of Shewanella cells attached to smectite for the respiration of iron. Photo courtesy of Dr. Hailiang Dong. Click on image for larger photo. |
The smectite-to-illite (S-I) reaction is one of the most common diagenetic processes in sedimentary basins around the world. New research shows that this process may be facilitated by bacteria, bringing together the often disparate worlds of mineral transformations and microbiology.
As sediments are buried and exposed to increasing amounts of heat and pressure, smectite (a swelling clay) progressively changes to illite (a non-expanding clay). Because it occurs over the same temperature range as hydrocarbon generation, the S-I reaction is frequently used as a geothermometer, helping to reconstruct thermal and tectonic histories. The importance of the reaction extends beyond the petroleum industry, with influences on pore water pressure, fluid migration, formation of growth faults, and the overall chemical evolution of sedimentary basins.
For the last two decades, researchers have widely studied the S-I reaction and considered it to be wholly physiochemical. Jinwook Kim, of the Naval Research Laboratory at the Stennis Space Center in Mississippi, Hailiang Dong, of Miami University, Ohio, and others recently challenged that notion in the February 6th issue of the magazine Science, proving that bacteria can promote the reaction, at least in laboratory settings.
Normally, in the absence of bacteria, the S-I reaction takes place between 300 to 350 oC over a period of several months, but Kim and Dong et al. found that the metal-loving bacterium Shewanella oneidensis can transform smectite to illite at room temperature in a matter of weeks. The rate of transformation is impressive, although Dennis Eberl, a clay mineralogist with the USGS in Boulder, Colorado and co-author of the paper, maintains that, “the most interesting thing is the reaction between two realms that has never been thought to exist before.”
A secondary electron image showing mineral transformation of the oxidized, unaltered smectite (dark sheet on the right) to biogenic illite (euhedral crystals on the left). Photo courtesy of Dr. Hailiang Dong. Click on image for larger photo. |
Indeed, the discovery is indicative of the increasing overlap between the biological and geological sciences. Scientists have been researching the bacterium and the S-I reaction independently for years. Kim and Dong’s research began by looking at the role of microbes in the reduction of iron in clays for the purposes of bioremediation. During lab experiments, they “discovered a new mineral forming, a new biogenic mineral,” says Dong. After examining the unidentified clay mineral with advanced TEM and XRD techniques, the researchers determined the new mineral was illite and that the bacteria actually catalyzes the S-I reaction.
Although the experiments were performed under controlled conditions in a laboratory setting, previous studies have shown that reduced iron is common in illite and that metal-reducing bacteria can exist at depths and temperatures similar to that in diagenetic settings. The next step is determining to what extent bacteria affect the S-I reaction in nature and learn more about the conditions and locations in which these types of bacteria thrive.
The first locations to be examined will be areas with “the presence of illite… where the right [pressure and temperature] conditions are not met,” says Dong. One example of an ill-placed illite with unknown origins is glauconite (an iron-rich illite), which appears on the ocean floor.
The implications of the new discovery are widespread, but the lesson is simple: researchers must continue to examine the role of microbes and other life on geologic processes. According to Dong, “models incorporating living things are much more complex and there are things about living things that cannot be predicted, but it is for the best because taking into account the effect of microbes will improve our models and rates.”
Expect to hear more smectite-to-illite developments at the 41st Annual Meeting of The Clay Minerals Society in Richland, Washington (June 19-24). Further details may be found on our conferences page
References
1. Kim, J., Dong, H., Seabaugh, J., Newell, S., and Eberl, D., 2004, Role of Microbes in the Smectite-to-Illite Reaction: Science, v. 303, p. 830-832.
Contacts
Jay Chapman jchapman@geosociety.org
Jinwook Kim
Hailiang Dong dongh@muohio.edu
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