The cornerstone of our approach to the assessment of plume 

 fluxes are the element/heat ratios determined for the highly 

 diluted proximal plume waters. To attain the desired understand- 

 ing of the linkages between venting fluids and these plumes, these 

 ratios must also be determined for the venting fluids. Because of 

 the high thermal gradients within the cm-scale vent fluid sampling 

 zone, it is difficult to ensure that collected fluids and 

 (independently) measured temperatures precisely correspond. A 

 sense of this can be seen in the Fe versus temperature data for 

 21°N, EPR (Fig. 5). The uncertainties of the element/heat ratios 

 produced by separate measurements of chemistry and temperature 

 make it impractical to use these data to track mixing processes 

 into the near- field. 



Fluid-correlated and accurate (to within 1°C) temperature data 

 is also needed for thermodynamic applications and for detecting 

 subtle temporal changes in venting behavior (Campbell et al . , 

 1988; Bowers et al., 1988). Furthermore, laboratory studies 

 (Seyfried and Janecky, 1985) and thermodynamic modeling 

 calculations (Janecky and Seyfried, 1984; Bowers et al . , 1985) 

 suggest that the compositions of some major vent fluid 

 constituents are extremely variable at temperatures near the 

 critical point for the discharging fluids. For example, the 

 experiments of Seyfried and Janecky (1985) show that a 25°C 

 increase in temperature from 400°C to 425°C at 400 bars leads to 

 approximately 4 to 10-fold increases in concentrations of Fe and 

 Mn and equally large or larger decreases for Cu, Zn, and H2S. 

 Last, the calibration of chemical geothermometers (e.g., those for 

 Na-K-Ca and Na-Li) for submarine hydrothermal systems await 

 reliable vent fluid temperature data (Von Damm, 1988). 



To address some of these sampling concerns, Submersible- 

 coupled In situ Sensing and Sampling System (SIS^) was conceived 

 by a group of vent fluid chemists (R.E. McDuff, J.E. Lupton, M. 

 Lilley of the University of Washington, and G.J. Massoth) and 

 marine engineers (H.B. Milburn and B. Walden of the Alvin 

 Group/WHOI ) and fabricated at PMEL under the auspices of the NOAA 

 VENTS Program and Office of Undersea Research. The guiding 

 concept was that fluids needed to be sampled in a way that both 

 the elemental concentration and the temperature at the precise 

 point of sampling would be known. Real-time temperature and 

 chemical data would provide valuable guidance in sampling. The 

 system would have to be effective over a wide range of 

 temperatures and flow rates. Specific improvements included: 

 1) the ability to sample and sense both high-temperature and warm 

 spring vent fluids with a single system configuration, 2) the 

 ability to insure in realtime that sample quality is maximized by 

 precise control of sampling flow rate and the monitoring of in 

 situ temperatures along the system's flow path, 3) more "pilot- 

 friendly" manipulator requirements during sampling operations, 

 4) more basket payload and space available for other operations 



46 



