UNM Contacts: Tobias Fischer, (505) 277-0284 Steve Carr, (505) 277-1821 Scripps Contact: Mario Aguilera, (858) 534-3624

August 15, 2002 UNM AND SCRIPPS INSTITUTION OF OCEANOGRAPHY RESEARCHERS FIND CENTRAL AMERICAN VOLCANOES YIELD NEW CLUES ‘Surprise’ finding shows nitrogen is quickly recycled through subduction zones A chain of Central America’s most active volcanoes has given a team of researchers from the University of New Mexico, Scripps Institution of Oceanography, and Northern Illinois University a novel glimpse into fundamental Earth processes. Their research, published in the Aug. 16 issue of the journal Science, gives new insight into the past and possibly the future of elements cycling between Earth’s underlying mantle and surface reservoirs such as the crust, oceans and atmosphere. The collaborative study was initially conducted on Costa Rican volcanoes in January 2001 (descriptions of the expedition are available online at www.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 compared 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.” “We also show 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,” adds David Hilton, a geochemist from the Scripps Institute of Oceanography. “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 the 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 Hawai’i 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 UNM, Alison Shaw of Scripps and James Walker of Northern Illinois University. The research was supported by the National Science Foundation. # # # Photos for publication may be obtained from: http://scrippsnews.ucsd.edu/pressreleases/hilton_science_volcanoes.html

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