Transport of carbon to the earth’s interior: researchers in Bayreuth discover highly stable carbonate structures
University of Bayreuth, Press Release No. 099/2017, 31 July 2017
How does carbon get from the surface of the earth to deep in the earth’s interior? Until now, the transport route, which represents a central stage in Earth’s carbon cycle, had puzzled scientists. Researchers at the University of Bayreuth, working together with international partners, have now succeeded in shedding light on this mystery. In the journal Nature Communications, they report on crystal structures made of iron carbonate that form at extremely high pressures and temperatures existing at around 2,000 kilometres below the earth’s surface. The iron contained in the iron carbonate oxidizes strongly in the process. The resulting structures, which are remarkably stable, enable carbon to be transported even deeper into the earth’s interior.
The crystal structures of the iron carbonate change dramatically at extremely high pressures and temperatures. On the earth’s surface, the carbon atoms (black) and oxygen atoms (red) are arranged in triangles located between octahedrons made of iron atoms. Deep in the earth’s interior, the carbon and oxygen atoms rearrange to form tetrahedrons. Illustration: Catherine McCammon.
Geoscientists believe that around 90 percent of our planet’s carbon may be found deep in the earth’s interior. From there, carbon rises up through the earth’s upper mantle, into the crust of the earth, and finally into the atmosphere; conversely, carbon travels from the atmosphere deep down into the earth’s interior. In the course of this global cycle, carbon atoms are bound up in various gasses and minerals that undergo a variety of chemical reactions and transformation processes as they travel. The processes involved over the course of their travel to the lower mantle have now been investigated by scientists at the Bavarian Research Institute of Experimental Geochemistry & Geophysics (BGI) at the University of Bayreuth using iron carbonate (FeCO3) as an example.
In a laboratory, samples of this mineral were exposed to conditions that are present around 700 kilometres below the earth’s surface and in deeper regions of the earth’s interior. Diamond anvil cells created pressures of up to 100 gigapascals, or around one million times the pressure of the earth’s atmosphere. At the same time, a laser beam heated the samples, reaching up to about 3,000 degrees Celsius. Under these conditions, the scientists exposed the samples to intensive X-ray radiation. The diffraction patterns thus obtained showed how the crystal structures of the iron carbonate had changed. “It turned out that the carbon and oxygen atoms in the lower mantle phase had adopted new crystalline structures. They rearrange to form tetrahedrons – structures familiar to us as silicon and oxygen atoms in minerals on the earth’s surface,” explained Dr. Catherine McCammon of the BGI. The experiments showed that the new structures made iron carbonate unusually stable. The carbon atoms remain embedded in these structures as the mineral sinks deeper into the lower mantle.
The scientists also reported a further discovery. The very high pressures and temperatures in the lower mantle result in a strong oxidation of the iron contained in the iron carbonate. “On the earth’s surface for instance, such oxidation processes would completely turn all the steel in an automobile into rust in a very short amount of time,” said Dr. McCammon.
The findings published in Nature Communications are the result of close international cooperation. The following partners worked together with the Bavarian Research Institute of Experimental Geochemistry & Geophysics (BGI): the European Synchrotron Radiation Facility (ESRF) in Grenoble, the University of Chicago’s Argonne National Laboratory, and the University of Milan. The BGI’s team was made up of the following members: lead author Dr. Valerio Cerantola, Dr. Elena Bykova, Dr. Maxim Bykov, Dr. Leyla Ismailova, Dr. Sylvain Petitgirard, Dr. Catherine McCammon, and Prof. Dr. Leonid Dubrovinsky, who coordinates the research project.
Valerio Cerantola et al., Stability of iron-bearing carbonates in the deep Earth’s interior,
Nature Communications (2017), DOI: 10.1038/ncomms15960.
Prof. Dr. Catherine McCammon
Bavarian Research Institute of Experimental Geochemistry & Geophysics (BGI)
University of Bayreuth
Phone: +49 (0)921 / 55-3709
University of Bayreuth
Universitätsstr. 30 / ZUV
Phone: +49 (0)921 / 55-5356