U.S. Dept. of Commerce / NOAA / OAR / PMEL / Publications
Both megaplume discoveries have come during extensive hydrographic and chemical surveys designed to map the distribution of hydrothermal emissions over and around the crest of the southern JDFR. Deep-two data were collected by a Sea-Bird conductivity-temperature-depth sensor (CTD) and a Sea Tech 0.25-m path length transmissometer continuously cycled through the bottom 400–1000 m of the water column while towed from a surface ship. Effective precision of the measurements was ±0.001°C for temperature, ±0.002‰ for salinity, and ±0.001 m−1 for light attenuation. Hydrothermal plumes were identified in real time by the presence of light attenuation and temperature anomalies. The plume temperature anomaly (ΔT) is the deviation along the potential temperature (θ ) axis from the normally linear relationship between potential density (σθ) and θ [Lupton et al., 1985]. The temperature anomaly thus calculated is relative to ambient water of the same potential density as the neutrally buoyant plume.
Primary navigation was by Loran C; position relative to the axial bathymetry was determined by Sea Beam and by acoustic navigation where transponders were within range. The time, duration, and distance of each tow is given in Table 1.
|Time, UT||Date||Time, UT||Date||km|
|S2||0730||Aug. 20, 1986||1048||Aug. 21, 1986||121|
|S3||1745||Aug. 21, 1986||0440||Aug. 22, 1986||21|
|S23||0740||Aug. 23, 1986||1500||Aug. 23, 1986||16|
|T5||1230||Sept. 12, 1987||1803||Sept. 13, 1987||79|
|T9||0630||Sept. 19, 1987||1540||Sept. 19, 1987||24|
|T10||1805||Sept. 19, 1987||0110||Sept. 20, 1987||14|
|T11||0303||Sept. 20, 1987||0850||Sept. 20, 1987||16|
|T14||0220||Sept. 23, 1987||1507||Sept. 23, 1987||35|
Water samples were collected during some of the tows by rosette-mounted 30-L Niskin bottles internally coated with Teflon and with silicon tubing springs. The suspended particles were isolated by N2 pressure filtration through 0.4-μm pore size Nuclepore polycarbonate filters. Elemental particulate chemistry was determined by X ray primary and secondary emission spectrometry using a Kevex Super 8000-770 X ray spectrometer with a Rh source and a Ge secondary target [Feely et al., 1987]. Individual grain identification and sizing were performed on an International Scientific Instruments DS130S scanning electron microscope coupled to the Kevex spectrometer. Dissolved Mn in acidified samples was determined onshore by flameless atomic absorption spectrometry after Klinkhammer . Nutrient samples were collected in 250-mL plastic amber bottles and analyzed spectrophotometrically at sea using a Technicon Autoanalyzer. Helium samples were collected and stored in copper tubes after cold-weld sealing [Young and Lupton, 1983] and measured by mass spectrometry at the University of California, Santa Barbara.
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