Flow Mow Electronic Cruise Report

Chemical Signals in Rising Buoyant Plumes and Tidally Oscillating Plumes at the Main Endeavour Vent Field, Juan de Fuca Ridge

Ko-ichi Nakamura, Scott Veirs, Christian P. Sarason, Russell E. McDuff, Fritz Stahr, Dana R. Yoerger and Albert M. Bradley

During the Flow Mow cruise conducted in August 2000 (Stahr et al., this meeting), we made measurements of redox potential using a sensor adjacent to the SBE CT inlets of the Autonomous Benthic Explorer (ABE) from WHOI and on the CTD rosette. The redox potential measurements were logged in the main ABE data logger simultaneously with other data sets. On the CTD package, in addition to redox potential logged through the SBE9 electronics, temperature, redox potential and voltammetric measurements were independently logged. Although the redox potential measurement on the moving vehicle (ABE and CTD) is not an equilibriated measurement, the rate of potential drops (negative values of dEh/dt, where "t" is the time) is a good indicators of plume characteristics. Steady motion of the measurement platforms, i.e., constant horizontal speed of ABE and constant vertical speed of the CTD during casts and tows, yielded smooth records of all parameters. When ABE passed through the rising buoyant plume above vents, the rate of redox potential decrease mimics the potential temperature rise even in the raw 4 Hz record, which indicates that the magnitude of dilution in the convecting plume governs the measurements. Whereas, outside of the rising plumes the correspondence of redox potential and potential temperature rises becomes poor, which indicates multiple sources of hydrothermal emission in space and time. Two stationary vertically oscillating CTD casts north and south of the Main Endeavour Vent Field recorded tidally oscillating electrochemical signals suggestive of tidally oscillating plumes. The thickness change and chemical anomaly magnitude change were well recorded both in the redox potential measurement records and in the voltammetric measurement records.

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