Participant Perspective - July 20, 2001

Interview with Kevin Roe
Chemical Oceanographer, NOAA/U. Washington

Jeff: What are you sampling at Axial Volcano's hydrothermal vents?

Kevin: I'm looking at the dissolved chemistry. One of the most important is sulfide. We just put a new sulfide sensor on the hot fluid sampler. It's going to be nice to have a probe down there because sulfide decays with time. There's a little lag time between when the samples come up and when I get at them so I've got to get on them right away. Other chemicals we're trying to measure decay too. Back in the lab in Seattle I do metals and other trace elements. I believe we'll get more accurate results with the probe.

Jeff: Chemically, has Axial Volcano changed since the eruption in 1998?

Kevin: Oh yeah. Silica content has decreased as well as temperature. Silica's decreasing because, as the heat decreases, so do all the chemicals in the vent fluid. I just finished with some sulfide samples and it looks like it is continuing to decrease, especially at Marker 33 on the 1998 lava flow. At other places it's still fairly high.

Jeff: Chemically, what is the difference between black smokers, white smokers and anhydrite chimneys?

Kevin: The black smokers are quite hot and have a lot of metals such as iron and zinc sulfides. That's what causes the black smoke. In the white smokers there's a lot of silica or anhydrite and they're lower in metals. The anhydrite chimney is just calcium sulfate that forms because it's less soluble in hot water than it is in cold water. Temperature really doesn't affect what type of chimney will be produced. I've seen 340 degree vents that had white chimneys. Some of the ones that are lighter in color are also lower in salinity.

Jeff: Why are there a lot more tube worm colonies around diffuse vents than black smokers?

Kevin: Probably more sulfide in the area. With the black smoker, the smoke is so buoyant that it rises 200 meters or higher above the seafloor. In a diffuse flow the sulfide stays near the bottom because the diffuse vents are cooler. Everything runs off of sulfide so the worms and other organisms are bathed in sulfide more around diffuse vents.

Jeff: Are there any venting areas that have recently been discovered.

Kevin: Yes. Along the Mid-Atlantic Ridge is a vent field called the Lost City. The vents there are over 180 ft. tall (60 meters). They were only about 70 degrees but had some pretty strange chemistry. The fluid vents at pH 11-12. It's on million year old crust in a back arc basin made of a basic rock called peridotite. Vents are usually quite acidic so this is new to me. One of the things were going to do is grind up some of the chimney pieces and find out what they are. A lot of it seems to be magnesium hydroxide and oxyhydroxide (brucite). This exciting area was discovered while doing geological mapping.