Keck Expedition 2004
August 23, 2004 Day 9
Cruise Update August 23, 2004 - by Chris Preston
We returned to the Middle 150°C Valley Field today with a full compliment of sampling devices in the ROV tool sled including major water samplers for collecting vent fluids, niskin bottles for bulk water sampling, push cores for recovering sediment samples down to 15cm, a biobox for bringing back rock or invertebrate samples, and temperature probes.
At Middle Valley, we visited five
sites with diffusive flows. We 
explored
the Northern part of Middle Valley including the Heineken Hollow and East
Hill. Because the diffuse flows are
sparse on a large flat field of sediment and we have only a basic map of the
seafloor, we used the sonar on the ROV to locate structures to investigate. 
In this area we passed over many inactive sites
as indicated by extinct sulfide structures, lacking diffuse flow and often
covered with dead clam shells. This
picture shows old sulfide occupied by an octopus. In contrast, when we found an active mound
it was covered with bacterial mats, live clams, and a simmering of diffusive
flow was apparent.
Then we returned south and visited several of the sites we investigated yesterday including Dead Dog Mound, Central site and Inspired Mound taking additional water samples and using the temperature probe to get an idea of the temperatures of diffuse flows. For example, at Inspired Mound we found 150°C fluids emanating from the chimney structure. Just an inch in the sediment below the chimney fluids were 98°C, and water near the structure was between 1.6 to 6°C. The temperature of the fluid drops rapidly as the cold seawater mixes with it.
Middle Valley has a very different macrofauna than the Endeavor or Mothra seqments of the Juan de Fuca Ridge. Clams dominate the diffuse flows in Middle Valley. While in Main Endeavor field and Mothra black smokers are dominated by tubeworms, limpets.
Tonight has been an early day for us. The ROV was on the deck by 5pm and we were able to begin sampling right after dinner. It’s about midnight. We finally finished processing of samples and putting together samplers for tomorrow. Tonight we steam back to the Mothra vent field and tomorrow we wake to dive along Juan de Fuca Ridge, one of our last days in the deep blue. GOOD NIGHT!