Part Three

Finding Plumes

Instruments are lowered from the ship

The principal tool that shipboard scientists use to look for plumes is a set of instruments called a CTD (Conductivity - Temperature - Depth) package. The CTD is prepared on board and towed by the ship.

The CTD package contains sensors that collect temperature and conductivity data. It may also include water-sampling bottles—the dark gray cylinders inside the metal frame in the photo—so scientists can analyze the water chemistry in a shipboard laboratory. The plume is both warmer and chemically different from the surrounding seawater, and the CTD package allows scientists to look for both chemical and thermal signatures.

When all of the equipment is checked and in place, the package is slowly lowered into the ocean. This requires a lot of teamwork. The captain and crew navigate; the bosun and marine technicians supervise the launch, the operation of the instruments, and the recovery of the package; and researchers and students monitor the data feed from the underwater instruments.

"Needle in a haystack"

As the scalding hot fluid comes out of a vent, it mixes with the cold seawater, cools significantly, and is moved upward and spread out by ocean currents. This mixed water ends up being only a fraction of a degree warmer than the surrounding seawater. The key to locating this plume is to measure the ocean temperature continuously, in the hopes of finding some temperatures (called anomalies) that are slightly higher than normal. Imagine pouring a cup of boiling water into a swimming pool. The pool water will become a tiny bit warmer in that spot, and this can only be detected by using a really good thermometer in just the right place.

On board, computers display continuous temperature readings in the form of graphs, which scientists study for temperature anomalies. Even very small differences are significant. There might be only one part of hot hydrothermal fluid (at 350°C) diluted in 300,000 parts of seawater (2°C), so a difference as small as .001°C is enough to warrant a closer look.

The "tow-yo" pattern intersects with plumes

Because the shape of a plume is irregular, it's easy to miss an anomaly completely. To increase the odds of encountering a plume, crew members drag the CTD in a vertical sawtooth pattern, up and down through the deep waters above the ridge. The pattern is called a "tow-yo," and can be several kilometers long. Several 'clouds' of heat may be detected during the course of a tow. They may represent plumes from several different vents.

Part Four

Finding Vents

A vertical drop gives scientists a closer look

Once a significant anomaly has been found, the next job is to find the vent itself. Scientists might select one point inside the 'cloud' and lower the CTD directly down to the seafloor, recording temperature measurements along the way. Repeating this yields a vertical cross section of the plume, which might be 200 meters thick and several kilometers long. These temperature-anomaly profiles help pinpoint the vent more precisely. A temperature anomaly of just .4°C might indicate a position directly over a smoking vent!

Once the ship locates a vent, scientists send down a submersible or ROV (Remotely Operated Vehicle) to explore the environment and collect samples, specimens, images, and more data. It's a long and detailed process—and an exciting one.