that we can measure via unattended in situ chemical analyzers 

 (ISCA's) the chemical variability of venting over time spans of 

 hours to months or longer. Although a deep-submersible is 

 required to deploy this instrumentation, recovery is possible from 

 a surface ship using acoustic release technology. It would be 

 difficult to obtain the temporally comparative information from 

 observations of the proximal plume over the short-term because of 

 smearing caused by local current variability (at semi-diurnal 

 frequencies and longer) and over the long-term because of the cost 

 of surface-ship time. It is this long-term monitoring, however, 

 that is especially relevant to the interpretation of distal plume 

 data. For example, along a transect normal to the Southern Juan 

 de Fuca Ridge, Massoth et al. (1985) and Massoth, Lupton et al. 

 (manuscript in preparation) observed a discontinuity in the plume 

 distributions of He, Mn, and Fe at an off-axis distance temporally 

 equivalent to at least several months transit time from the 

 nearest known source sites. It remains ambiguous whether this 

 discontinuity was due to a hiatus in vent output or was an 

 artifact of advective processes, e.g., the bisection of a gyre- 

 like plume flow-path by the sampling transect or the crossing of 

 this transect by the dispersal routes of plumes originating from 

 different sites. One way of reducing this ambiguity during future 

 off-axis plume studies is by coupling our knowledge of the 

 temporal variability of plume sources, gained by in situ 

 monitoring, with that of the three dimensional chemical structure 

 of the plume, thus allowing us to distinguish source variations 

 from advective processes. Lastly, the coupling of in situ 

 chemical monitoring instrumentation with similar geophysical 

 monitoring instrumentation may provide the data necessary to 

 better understand some of the short-term interconnections between 

 hydrothermal venting and the tectonic/volcanic processes that 

 drive it. 



Study Site 



The Axial Volcano study site, located 260 nautical miles due 

 west of Astoria, Oregon (Fig. 1) sits atop an active submarine 

 volcano which is distinguished by a relatively large rectangular- 

 shaped caldera (3 by 8 km wide, 80-100 m deep) through which the 

 central segment of the actively spreading Juan de Fuca Ridgecrest 

 is thought to pass. The geology of this site has been discussed 

 by Embley et al. (this volume). Venting at Axial Volcano occurs 

 at depths ranging between 1450 and 1600 m. This is at least one- 

 half km shallower than the venting observed at other sites along 

 the Juan de Fuca Ridge (Fig. 1; the Explorer Ridge vent fields, 

 not shown on the bathymetric profile, lie at about 1850 m) and is 

 the shallowest known high- temperature vent site along the global 

 mid-ocean ridgecrest system. 



Patchy, low-temperature venting and regional accumulations of 

 orange-yellow colored sediments demarcate the boundaries of the 

 approximately 1 km long, 200 m wide ASHES vent field. Low- 



35 



