Monterey Bay Aquarium Research Institute

West Coast Expedition
July 20 - August 30, 2002
West Coast of North America
Logbook

August 21, 2002: Day #33


Figure 1 is a closeup of the biological communities that inhabit the chimney walls within the hydrothermal fields. Note the chemosynthetic tubeworms and the snails.

Debra Stakes writes: Today’s dive number 468 was planned to place a seismometer borehole east of the Endeavour Main Field and then to go collect fluids and equipment within the Hydrothermal Field. Tne Endeavour Main Field was the first hydrothermal site discovered on the Endeavour Segment of the Juan de Fuca Ridge. It has been studied since the 1980’s because of the robust high temperature smokers and their dense populations of chemosynthetic communities (Fig. 1). The Main Field is composed of a number of sulfide edifices, which are composite structures with multiple vents that tower 10’s of meters above the seafloor. The structures each have whimsical names (Dante, Dudley, Puffer) and each has a unique chemistry that had not changed in a decade—until a magmatic-tectonic event changed all the hydrothermal sites in 1999. The active vents are all located between the main fault that delimits the western wall of the axial valley and the central fissure system. We hoped to place a seismometer on the central fissure/fault to monitor the smallest changes in the underlying hydrothermal network of fractures.

Our previous experience had served us well. We avoided all active fault scarps and talus ramps. Instead we chose a gently sloping lava field that had a few pillows with dimensions adequate to place the coring sled (Fig. 2). These fractures crosscut a thick massive lava unit that fills in the inner valley floor (Fig. 3). After several hours parked in front of the pillow we produced an excellent borehole station for our short period seismometers (Fig. 4).


Figure 2. shows the approach to the coring site south of the "Dante" sulfide edifice. This coring site is just west of the central fault that appears to play a major role in the location of the sulfide structures.

With the borehole finally completed, we headed to the sulfide structures to collect fluids and instruments left two years ago. The markers left to guide us were obscured by the dense biological and mineralogical precipitates that we scraped off on the front of the toolsled Fig. 5). This rapid encrustation is one of the foremost problems for all instrumentation placed anywhere near the vent. This is an especially frustrating technical problem for any sensors that we would like to deploy for an extended time (months to years). Temperature measurement packages left here for only two years were almost impossible to remove from the chimney material that surrounded them (Fig. 6).



Figure 3 is the massive flow that is the observed basement rock for most of the Main Field structures. Such a flow morphology implies rapid fissure eruptions.

Figure 4 shows the borehole in the pillow basalt after several hours of coring.


Figure 5. One of the pitfalls of leaving anything in the vent field is the rapid growth of biological and mineralogical crusts on almost every surface. We had to scrape these markers to confirm our precise location in the vent field.

Figure 6. Hobos are autonomous temperature recorders with Ti-probes that can withstand the high temperatures and corrosive chemistry of the hydrothermal fluids. The packages can be left out for several years, but by then the problem becomes how to extricate them from the constantly growing chimney wall.

 

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