West Coast Expedition
July 20 - August 30, 2002
West Coast of North America
August 20, 2002: Day #32
Debra Stakes writes: Our dive plan for today was ambitious, requiring us to carry down the milk crate full of samplers and tools attached to the siderail of to the drillsled. We planned to core both sulfide and basalt and brought down plugs for both boreholes. We also wanted to collect sediment in a push core and glass in two wax cores. There were two fluid samplers and the temperature gauge. All of this had to be carried in the red plastic crate, which was also the only place to stow rock grab samples we happened to pick up.
The Mothra vent field is the southernmost hydrothermal site on the Endeavour Segment of the Juan de Fuca Ridge. In 1998, the top ~two meters of chimney tops were recovered as part of an expedition jointly sponsored by the American Museum of Natural History, and the University of Washington. The successful recovery of pristine, 4000 lb pieces of chimney is allowing detailed examination of these sulfide-microbial habitats, which support rich and diverse microbial communities that thrive within the warm, nutrient rich walls of the sulfides. Novel organisms and microbes that grow at temperatures greater than 90°C have been cultured from within the interiors of all four of the structures recovered.
After a bit of searching we located the vent field and a quick survey showed that several of the structures had changed since a magmatic-tectonic event in 1999. Even the flat top of the chimney Roane, left after the Edifice Rex expedition, sported a new top hat of tube worms (Figure 1).
One of the myriad goals was to core a large diameter borehole in an active black smoker, which was successfully completed on the structure called "Giraffe" (Fig. 2). The sulfide wall proved to be much easier to core into, with the effort completed in less than one hour! Much like tapping a maple tree for syrup, the new hole leaked high temperature hydrothermal fluids. The sulfide borehole was plugged with a hollow insert that we planned to recover in a couple of days (Fig. 3). We were pleased that the borehole in this actively-growing sulfide edifice stayed open long enough for us to put in the plug!
After creating two boreholes and collecting hydrothermal fluids from a recently developed black smoker, and a smoker previously sampled in 1998, Tiburon set off across the floor of the axial valley to find another site for a seismometer. We were hoping to place this corehole into fresh basalt with easy access for the ROV, but dive time was becoming a premium. As we hurried along looking for a flat spot, spectacular exposures of different lava generations in large fissures on the valley floor were imaged (Fig. 4). They were so beautiful that I was compelled to stop and grab a couple of samples and into the crate they went. As the clock ticked onward we realized that we had abandoned our acoustic marker back near the sulfide. We would need this to mark any borehole site and so we turned around and flew back. By this time the only prospect for a corehole was to drill into a piece of inactive sulfide that looked much like an old growth log - the corehole was cut in an hour and marked with the beacon.
As we pulled away to mark the borehole we were horrified to discover that our milkccrate full of samplers and samples was no longer connected to the ROV! It was lying on its side in the dust at the base of the borehole (Fig 5). What to do? We discussed the possibility of sending down an elevator to lift the crate back to the ship. However, this would take time, first to put together the elevator and then wait for it to transit from the ship to the seafloor and back again. We decided to heave the crate onto the drillsled and hold it in place with the Schilling arm. This worked fine and everything returned safely (Fig. 6) The dive ended a bit earlier but we did not lose any equipment, so we were satisfied.