Dive and Discover : Exp. 8 : Juan de Fuca Ridge : Daily Updates

For the last eight nights, a group of graduate students from the University of Washington searching for signs of a new hydrothermal

vent field have grown increasingly excited about data collected using an instrument dangling from Atlantis.

The students look for the vents on the seafloor by “plume hunting.” Their eye in the sea is an instrument called a CTD, which is lowered and raised in the ocean to measure its electrical Conductivity, Temperature, and water Depth. The CTD detects properties in the water that signal the presence of a vent field far below. A hydrothermal plume is usually at slightly higher temperature (a few hundredths or tenths of a degree Centigrade) than the surrounding water, and is laced with very fine-grained sulfide minerals that make the plume more 'cloudy' or turbid. This signal too is picked up by a 'turbidity meter' a device that measures how 'cloudy' the water is. Chemical analyses of the water samples taken from the plume often have higher concentrations of some metals, like manganese, and gases like hydrogen and methane.

Plume hunting is the main way that oceanographers discover new hydrothermal vent fields. On the Endeavour Segment, the fields Mothra, Salty Dawg, and Sasquatch were found during night programs using a CTD, said Deb Kelley, the expedition’s chief scientist.

Around 8 p.m., the students ask the mates to position the ship about a half mile north of the Sasquatch vent field on the Endeavour Segment, an area identified in 2000 as a possible hydrothermal vent site. A crane on the side of the ship lowers the CTD into the water to begin what they call “pogo-ing.” The students send signals from their computers to the CTD, which makes measurements as it is repeatedly lowered and raised in the water, like a bouncing pogo stick.

HOW POGOING WORKS
During each cast, computers record information about temperature changes and depth. When the readings show signs of a plume—water that is warmer than usual—the students trigger one of 22 bottles on the CTD to capture a water sample. From these samples, graduate student Brooke Love analyzes methane, which is higher in hydrothermal plumes than in ordinary seawater.

Next the students plot the data collected by the CTD and from Brooke’s analyses on a bathymetric

map of the area. “By putting it all on a map, we can try to find the bulls eye of the highest temperature and highest methane, which we use to zero in on the field,” said Kris Ludwig, a doctoral student in charge of the night operations.

With just two dives left in this area of the Endeavour Segment scheduled for other research, the students won’t use Alvin to go to the seafloor to see if there is a new field. But two upcoming expeditions will allow for continued research. In June and July, researchers will return to the area with an instrument called the Autonomous Benthic Explorer—ABE for short—which is programmed to swim freely in the water to map the sea floor and collect water chemistry information that could help advance the search. With ABE, researchers may be able to show whether the pogo-ing indeed zeroed in on a new vent field.