The collaborative study was initially conducted on Costa Rican volcanoes in January 2001 (descriptions of the expedition are available online at http://scripps.ucsd.edu/volcano). A second expedition concentrated on Guatemalan volcanoes in May 2001.
These volcanoes and others throughout Central America are created by subduction, a process by which the offshore Cocos plate slips beneath the Caribbean plate and is driven down into Earth's mantle. The researchers sampled gases, or volatiles, discharged from the volcanoes, and analyzed the nitrogen and helium compositions to trace their sources.
The heart of the study concerned comparing gases from the two volcanic regions. Through samples of sediment taken offshore, the researchers knew the materials being driven into the subduction zone were virtually the same between the two areas. The volatiles that came out, specifically nitrogen gas, were "a big surprise," however.
In the Guatemalan volcano system, nitrogen was driven down deep into the subduction zone before rising back to Earth's surface. Costa Rica was a different story. Here the scientists found no evidence that nitrogen was cycling from the subducting plate. In this respect, the nitrogen characteristics were consistent with other lines of geochemical evidence showing that the top section of the sedimentary veneer on the Cocos plate had been removed by the subduction process (for example, by plating to the overriding Caribbean plate) before it reached the zone of arc magma generation.
"The research is significant because, first of all, not much is known about nitrogen coming out of volcanoes," said Tobias Fischer, a volcanologist at UNM's Department of Earth and Planetary Sciences specializing in volcanic gas chemistry. "People thought it was just air mixed in with volcanic gases. We clearly show that in the Guatemalan subduction zone up to 95 percent of the nitrogen in volcanic gases comes from subducted marine sediments. The situation is different in Costa Rica, where the nitrogen is predominantly of mantle origin," said Fischer.
"We also show," adds David Hilton, a geochemist from Scripps, "that the sedimentary nitrogen is recycled back to the atmosphere through arc volcanism in Central America and does not get subducted into the deeper mantle. We have clearly identified the transport medium for nitrogen and show that it comes straight back out to the Earth's surface through active volcanoes."
The study, one of the first clear descriptions of how and where nitrogen travels during subduction, gives researchers a new glimpse into the long-term evolution of Earth's mantle.
"This is the kind of basic research that is important to investigate the nitrogen cycle in the earth and the evolution of nitrogen in the mantle and in the atmosphere," said Fischer. "We hope that our work will contribute to the general understanding of the earth's global geochemical cycles that have been operating for millions of years and continue to influence the composition of the atmosphere."
"Volcanoes are basically transfer conduits between the earth's surface and the underlying mantle," explains Hilton. "Consequently, they are very important to geochemists who want to understand the evolution of the earth. They're unique in allowing access to a remote region of the earth."
The researchers now plan to address nitrogen and helium relationships in other subduction zones mainly found bordering the Pacific Ocean (the so-called "Ring of Fire") as well as at mid-ocean ridges and oceanic hotspots such as Hawaii in an effort to elicit additional clues about how gases move and cycle between earth's mantle and atmosphere.
In addition to Fischer and Hilton, coauthors of the study include Mindy Zimmer and Zachary Sharp of the University of New Mexico, Alison Shaw of Scripps, and James Walker of Northern Illinois University.
The research was supported by the National Science Foundation.
Scripps Institution of Oceanography at the University of California, San Diego, is one of the oldest, largest, and most important centers for global science research and graduate training in the world. The scientific scope of the institution has grown since its founding in 1903. A century of Scripps science has had an invaluable impact on oceanography, on understanding of the earth, and on society. More than 300 research programs are under way today in a wide range of scientific areas. Scripps operates one of the largest U.S. academic fleets with four oceanographic research ships and one research platform for worldwide exploration. Now plunging boldly into the 21st century, Scripps will celebrate its centennial in 2003.
Additional Contact Information:
University of New Mexico Contact: Steve Carr, 505-277-1821
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