Gorda Ridge Cruise
August 5 - 21, 2002

August 8, 2000: Day #4

Dave Clague writes: Despite high seas and winds, we completed seven successful deployments of the rock crusher overnight and into the morning. Dive 188 began about 1 pm and dove to the "Seacliff" hydrothermal site, where we had done a short dive on August 6. We mapped the extent of the entire vent field by running transects along contours and measuring distances to a beacon we had left 2 days ago.

The field is not as large as previously thought and is elongate upslope-downslope rather than along contours, as previously mapped. The field has a central region of anhydrite chimneys venting 304 degree centigrade low-salinity fluids. The temperature probe worked perfectly, although we melted some of the plastic potting that connects the probe to the cable. We collected two gas-tight water samples easily but had difficulty triggering the Ti-major water sample bottles and the two samples collected are only partly high-temperature fluid. Surrounding the area of high-temperature vents and chimneys is an extensive region of tubeworms and other vent biota. We collected a large clump of pogonophoran tubeworms, some more of the unusual blue bacterial mats (attached to both tubeworm tubes and hydrothermal crusts), some red anemones, several types of worms, several types of gastropods, more limpets, and more pycnogonids.

At the very end of the dive we used a push-core, modified by the addition of a core-catcher, to collect some grainy hydrothermal sediment. The 23-cm core consists mostly of amorphous silica, with anhydrite near the top. We will be working on ways to make it easier to deploy the major water samplers before we arrive in Escanaba Trough, where we will be collecting additional hydrothermal vent fluids as part of Rob Zierenberg's NURP-funded program. Tomorrow's dive will be to sample and map some small cones that are the youngest expression of the President Jackson Seamount Chain.

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