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Exciting New Discoveries in Submarine Hydrothermal Systems, Commonwealth of the Northern Mariana Islands
An interdisciplinary team of scientists explored submarine volcanoes of the Mariana Arc during a recent National Oceanic and Atmospheric Administration (NOAA) cruise to the U.S. Commonwealth of the Northern Mariana Islands (CNMI). Called "Submarine Ring of Fire, 2004," the cruise produced several fundamental and exciting new discoveries about hydrothermal systems in active volcanic arcs. The most spectacular discoveries—such as the first observation of a deep-sea volcanic eruption—have been reported in numerous newspaper articles and radio and TV programs. The cruise took place in March and April 2004 aboard the research vessel Thomas G. Thompson. It was the second NOAA cruise to the CNMI; the first, conducted in 2003, completed extensive swath mapping of the arc and discovered 10 new submarine hydrothermal systems. During this year's follow-up cruise, 34 scientists from the United States, Canada, Japan, and New Zealand, representing 13 institutions, worked in four research groups and one operations group:
Scientist Jim Hein of the U.S. Geological Survey (USGS)'s Western Coastal and Marine Geology Team in Menlo Park, CA, was part of the geology and mineralization group, along with Cornel de Ronde (Institute of Geological & Nuclear Sciences, New Zealand), Bob Stern (University of Texas at Dallas), and Bob's graduate student Neil Basu. The main cruise operations included dives by the Canadian ROV ROPOS (Remotely Operated Vehicle for Ocean Science), conductivity-temperature-depth (CTD) measurements throughout the water column, and plankton tows. The CTD instrument also had sensors for measuring oxidation-reduction potential (Eh), particle size and volume, and amounts of dissolved oxygen (O2), hydrogen (H2), and methane (CH4), as well as equipment for collecting water samples for laboratory chemical analyses.
In general, each site required at least two ROV dives, one to collect biologic and geologic samples and map geologic features and biologic habitats, and another to collect vent fluids and gases. Fourteen ROV dives were completed on seven volcanic edifices. One or two dives were canceled near the end of the cruise because of the need to get to port in Japan before the super typhoon Sudal passed between us and our destination. This frontier exploration program was designed to increase our understanding of hydrothermal systems in volcanic arcs, which are poorly known in comparison with hydrothermal systems at spreading centers. Hydrothermal systems in volcanic arcs, unlike those at spreading centers, occur at a wide range of water depths and in varying rock compositions, with magmatic gases being likely common constituents of the hydrothermal fluids. Shallow-water vents are important because they can discharge hydrothermal products directly into the shallow ocean, including the photic zone, where biologic productivity is high, and the shallow vent sites may host unique and interesting biologic communities. The volcanoes chosen for study were identified by examining CTD data collected during the 2003 cruise. For the 2004 cruise, more detailed CTD operations were undertaken at each volcanic edifice to pinpoint the areas of most intense hydrothermal activity on the basis of various diagnostic plume tracers, such as particulate matter and methane content. The first ROV dive on each volcano searched for local sites of venting. This technique was remarkably successful in finding hydrothermal fields at each volcano. Some of the major shipboard results and observations include
At NW Rota-1 volcano, we discovered a 20-m-deep pit crater (dubbed "Brimstone Pit") at about 550-m water depth that displayed thick, billowing yellow plumes of molten sulfur and volcanic-rock fragments as much as several centimeters in size. This hydrothermal-magmatic event plastered the ROV with molten sulfur that had to be scraped off the vehicle after it was back on deck. This discovery clearly demonstrates that magmatic degassing dominates the hydrothermal plume and contributes to the resulting mineral deposits. The contribution of magmatic degassing to hydrothermal systems has long been an area of controversy and intense discussion. Because sulfur is a relatively minor volatile component of arc magmas in comparison with water and CO2, arc hydrothermal systems emitting abundant sulfur must also be supplying large fluxes of water and CO2 to the oceans and atmosphere. The CO2 magmatic contribution was directly observed at the NW Eifuku volcano vent site (named "Champagne vent") at 1,600-m water depth, much deeper than the Brimstone Pit at NW Rota-1 volcano. At the NW Eifuku vent site, the higher pressures and lower temperatures stabilize CO2 as a liquid, and the liquid CO2 can be seen exiting the sea floor as large bubbles that look very different from gas bubbles (think lava lamp). Those liquid CO2 bubbles were collected in a plastic core liner, and changes in their characteristics were observed as the ROV ascended and the bubbles were subjected to decreasing pressure and increasing temperature. The dominant hydrothermal mineral deposited at both NW Rota-1 and NW Eifuku is elemental sulfur, including spherules in volcaniclastic sediment, massive deposits, fracture fill, chimneys as much as 1 m tall, cement in breccia, and flat slabs forming at the sea floor in areas of diffuse venting.
All the hydrothermal systems observed in the Mariana arc are associated with either volcano summits or resurgent domes inside a volcanic caldera. The main characteristics of the hydrothermal systems along the Mariana arc are shallow-water, highly permeable rocks, gas-rich discharge, and mineralization and fluid/gas composition controlled by the boiling point of the hydrothermal fluids (phase separation), which, in turn, depends on water depth (pressure). These characteristics result in the subsurface deposition of sulfides, except in the more mature hydrothermal systems, such as at East Diamante volcano.
To learn more about the cruise, visit the Submarine Ring of Fire 2004 Web site. Hundreds of samples and gigabytes of data will keep Ring of Fire scientists busy for many years, unraveling the mysteries of the workings of volcanic arcs. The shipboard results are spectacular, and those that will follow from laboratory analyses are bound to be even more remarkable. Science Team: Ed Baker, Neil Basu, Sheryl Bolton, Dave Butterfield, Bill Chadwick, Cornel de Ronde, John Dower, Bob Embley (Chief Scientist), Leigh Evans, Jim Hein, Kim Juniper, Ben Larson, Geoff Lebon, John Lupton, Susan Merle, Anna Metaxas, Shunsuke Miyabe, Craig Moyer, Ko-ichi Nakamura, Shannon Ristau, Kevin Roe, Jonathan Rose, Bob Stern, Verena Tunnicliffe. For scientists' backgrounds and affiliations, visit Submarine Ring of Fire 2004 Explorers Web page.
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in this issue:
Sea-Otter Numbers at Record High
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U. S. Department of the Interior | U.S. Geological Survey
