Adsorption on clay accounts for organic-rich rocks

January 24, 2002

Despite intensive research in the past several decades, the controls on the burial of organic-rich material in marine sediments are not fully understood by paleoceanographers and petroleum geologists. In the 25 January 2002 issue of Science, the University of California, Riverside's Martin Kennedy and colleagues report a new mechanism to explain organic-rich rocks common in Late Cretaceous (~60 million years ago) sediments.

The authors analyze new data from black shale deposits in South Dakota and Wyoming and show a clear relationship between the presence of organic carbon in the sediments and a particular clay mineral called smectite. The study posits that dissolved organic molecules common in seawater are drawn into the crystal lattice of smectitic clay minerals where the organic matter is protected from oxidation and metabolization.

The burial of organic-rich material plays an important role in maintaining habitable conditions in the biosphere, being ultimately responsible for the free oxygen in the atmosphere and providing a major sink for atmospheric carbon dioxide (an important greenhouse gas). The organic-rich material is also economically important as the ultimate source of oil.

"We find that smectite's abundant surface area can readily account for the large amounts of organic carbon we see in the deposits," said Kennedy. "Indeed, it looks like the adsorption onto clay mineral surfaces of molecular scale organic compounds that are abundant in sea water is really the important control." (Adsorption is the process by which gases, liquids or solutes accumulate on the surface of a solid or liquid.)

Kennedy explained that smectite crystals bear a large charged surface area that is attractive to organic molecules. As smectite sinks through sea water it adsorbs carbon-rich molecules onto its intercrystalline surfaces.

The existence of organic matter on smectite surfaces also helps understand an important process -- natural clay catalysis -- in the formation of petroleum.

"One of the big problems in catalysis is getting the reactant in intimate contact with the catalyst" coauthor David Pevear said. "Now, nature has done this for us in these deposits. With organics already on the clay surface before burial, water is not able to block access to the natural clay catalyst. The smectite-organic complex we describe may be an important player in the generation of commercial hydrocarbons."

Since clay minerals such as smectite are produced by continental weathering and are highly sensitive to local climatic conditions, the new work suggests, too, that changes in weathering could control carbon burial, thus completing the loop of a climate balancing act.

"Enhanced burial of organic carbon means consumption of carbon dioxide from the atmosphere and, ultimately, cooling of climate," said Kennedy. "This cooling could in turn change the weathering pattern and reduce the production of smectitic clay minerals capable of absorbing organic carbon.

"For years we assumed these deposits resulted from a complicated balancing act between oceanic and biological processes. It's ironic that it may turn out to be nothing more complicated than adsorption of organic carbon onto mud!"

University of California - Riverside

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