HR: 0800h
AN: V21D-0753    [Abstracts]
TI: A New Tool for Detecting Hydrothermal Plumes: an ORP Sensor for the PMEL MAPR
AU: * Walker, S L
EM: Sharon.L.Walker@noaa.gov
AF: NOAA/PMEL, 7600 Sand Point Way NE, Seattle, WA 98115, United States
AU: Baker, E T
EM: Edward.Baker@noaa.gov
AF: NOAA/PMEL, 7600 Sand Point Way NE, Seattle, WA 98115, United States
AU: Resing, J A
EM: Joseph.Resing@noaa.gov
AF: JISAO/PMEL/NOAA Univ. Washington, 7600 Sand Point Way NE, Seattle, WA 98115, United States
AU: Nakamura, K
EM: koichi.nakamura@aist.go.jp
AF: National Institute of Advanced Industrial Science and Technology, AIST Tsukuba Central 7, Tsukuba, 305-8567, Japan
AU: McLain, P D
EM: Patrick.D.Mclain@noaa.gov
AF: NOAA/PMEL, 7600 Sand Point Way NE, Seattle, WA 98115, United States
AB: Mapping hydrothermal plumes in the water column above mid-ocean ridges, submarine arc volcanoes, and back- arc spreading centers has lead to the discovery of many seafloor hydrothermal systems. Hydrothermal plumes can be identified by strong optical backscattering anomalies, which indicate particulates injected into the water above the seafloor by the vents, and by chemical anomalies coincident with the particulate signal. The chemical anomalies are often characterized by increased concentrations of dissolved reduced species such as iron, manganese, and sulfur that decrease the oxidation-reduction potential (ORP) of the plume relative to ambient seawater. Some of these reduced chemical species are short-lived relative to the particulates, so having a method for measuring the ORP along with optical backscattering has improved efforts to locate source vent fields. Greater ORP anomalies indicate "younger" or "fresher" parts of the plume, which are closer to the source. Additionally, some low temperature vent fields often have minimal particle signatures, yet their above-bottom plumes can be detected by changes in ORP. The PMEL Miniature Autonomous Plume Recorder (MAPR), an instrument with pressure, temperature and optical backscatter sensors, has been used extensively in explorations for hydrothermal systems. We have now enhanced MAPRs to include a newly-designed platinum electrode paired with a silver/silver chloride reference electrode and high impedance circuit for the measurement of ORP in seawater. Results from the laboratory and field demonstrate this new sensor system effectively detects ORP anomalies associated with hydrothermal plumes. Laboratory experiments with Fe(II) and sulfide (dissolved Na2S) show that changes in OPR are rapid when the sensors are exposed to reduced species in solution, and the magnitude of the change is proportional to the concentration of the reduced species. In the field, intercomparison of several individual OPR-capable MAPRs deployed simultaneously confirms their response is rapid when plumes are encountered and that all sensors had comparable magnitudes of response. One MAPR was recently deployed on the ABE autonomous vehicle for detailed mapping missions at Brothers Volcano, Kermadec Arc. Response of the MAPR ORP sensor mirrored that of the Japanese (AIST) ORP sensor routinely mounted on ABE. Both sensors provided high- resolution maps of near-bottom vent effluent over known vent fields on Brothers.
DE: 4832 Hydrothermal systems (0450, 1034, 3017, 3616, 8135, 8424)
DE: 4894 Instruments, sensors, and techniques
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting