Not All Subduction Zones Are Equal In Carbon Dioxide Generation

"Models have a tendency to treat all subduction zones the same," says Dr. Derrill Kerrick, professor of geosciences at Penn State. "The question is, how efficient is the return of carbon dioxide to the atmosphere at various subduction zones?"

A subduction zone occurs when two of the Earth's tectonic plates meet and one moves beneath the other. As the bottom plate continues, it moves downward and eventually, pressure and heat cause the rocks to release volatiles, which permeate into the overlying mantle, cause melting and produce arc volcanoes. Carbon dioxide and water vapor also escape with the magma. Most of the time, most of the subducted rock continues down and joins the Earth's mantle.

"The amount of carbon dioxide emitted from arc volcanism appears to be less than that subducted, which implies that a significant amount of carbon dioxide either is released before reaching the depth at which arc magmas are generated or is subducted to deeper depths," the researchers report today (May 17) in the journal, Nature.

Until recently, determining carbon dioxide release for specific arc volcanoes and specific rock sources was difficult if not impossible, so only generalized models were used. Now, using a computer program created by Dr. Jamie A. D. Connolly, Swiss Federal Institute of Technology and a Penn State alumnus, researchers can model the way specific mixtures of rocks behave in a subduction zone and determine where carbon dioxide and water are released.

"The location of volatile release depends on the temperature regime of the subduction zone and the type of rocks being subducted," says Kerrick. "This study looks only at marine sediments, but previous work looked at serpentinites and future work will discuss metabasalts, the three types of carbon containing rocks that are involved with subduction."

The computer program allows the researchers to model real rock compositions. For this study, they used three real formations, a clay carbonate or marl from the Antilles and siliceous limestone from the Marianas and from Vanuatu, all marine sediment-derived formations. The fourth "rock" used was a global average of marine sediments.

Marine sediments are important in carbon dioxide studies, because the sediments consist of the calcium carbonate shell bodies of marine organisms. These organisms remove carbon dioxide from the ocean near the surface and sequester it on the ocean bottom where it cannot easily return to the atmosphere. Only when the sedimentary rocks formed by the marine organisms are heated in the subduction of a tectonic plate can the carbon dioxide escape.

However, the researchers found that most of the carbon dioxide in marine sediments does not escape back to the atmosphere during subduction. At the highest temperatures in the subduction zone range, the clay-rich marls completely lose their carbon dioxide and water before they reach the depths at which arc magma forms. In the middle range, the percentage of water and carbon dioxide released is less. At the lowest temperatures, the marls do not devolatilize at all and the carbon dioxide and water continue down to the mantle. Siliceous limestones devolatilize negligibly throughout the range of temperatures in the subduction zone.

"Most of the initial carbon dioxide and water in subducted marine sediments will not be released beneath volcanic arcs," say Kerrick and Connolly.

Volatiles retained within marine sediments may explain the apparent discrepancy between subducted and volcanic volatile emissions and represent a mechanism for return of carbon to the Earth's mantle, according to the researchers. Carbon that reaches the mantle may eventually return to the surface in mantle lava plumes that flow from the mantle up to volcanos not near subduction zones.

The National Science Foundation's MARGINS program supported this work.