The Techniques

Fire + Ice: Exploring for Volcanoes Beneath the Arctic
The Arctic Mid-Ocean Ridge Expedition
July 31 - October 3, 2001

Q:  How do we locate hydrothermal vents?

A:   One of the things we are looking for on the Gakkel Ridge is evidence of hydrothermal vents, the "hot springs" of the ocean floor. Hydrothermal vents on mid-ocean ridges in the Atlantic, Pacific, and Indian Oceans are host to weird and wonderful life forms. Their food chains are supported by chemical energy from the hydrothermal vent waters, in a process called chemosynthesis. In contrast, ecosystems at the Earth's surface are supported by energy from the sun, or photosynthesis. Vents are also important to understanding the chemistry of seawater, as well as the formation of ore deposits of valuable metals.

Hydrothermal plumes

People have speculated about hydrothermal vents on the Gakkel Ridge. How frequently would they occur on a ridge with such a slow spreading rate? What sorts of new species of animals might be discovered here? Yet nobody has looked for them before.

To locate hydrothermal vents, we are looking for their plumes. Think of smoke coming out of a smokestack. If you were in an airplane and looking for a train, a boat, or a factory, you might find it easiest to spot the plume of smoke and then follow it back to its source. Hydrothermal vent water is very hot (typically over 350^oC) and it is rich in many metals, such as iron. When this hot vent fluid mixes with cold seawater, the metals form particles of minerals. The mixture, with these mineral particles in it, then rises up into the oceans far above the vents, where it spreads out like the plume from a smokestack.

We can "see" the plume because the particles of minerals in it scatter more light. Our instruments can measure how a beam of light is transmitted through the water. Sometimes the hydrothermal plume has a small increase in temperature over the surrounding seawater, too. It is important to realize that the hydrothermal plume is mostly seawater, so the temperature difference is very small. The plume is about a 10,000 to 1 mixture of seawater to vent water.

Miniature autonomous plume recorders (MAPRs)

The instruments we are using to look for hydrothermal plumes are called miniature autonomous plume recorders, or MAPRs. Dr. Hedy Edmonds is putting one of these instruments on the wire above every dredge and rock corer. The MAPR records depth, temperature, and light scattering (the amount of particles in the water). Later in the lab, Dr. Edmonds downloads the data from the MAPR and plots the light scattering and temperature versus depth.

In the graphs here, you can see an example of a hydrothermal plume from dredge 21 of our cruise. The dark blue line shows the light -cattering data as the dredge was lowered to the bottom, and the light blue line shows it coming back up. The peak, between 2400 and 2700 m deep, is blown up in the middle plot. Finally, in the plot on the right, you can see the same structure in the temperature data (purple for on the way down, red for on the way up).

Notice that the peaks in the temperature profile are only about 1/100 of a degree! It is much easier to spot a plume with the light-scattering sensor. Then we know where to "zoom in" on the temperature data to look for a signal.

We saw a plume at the same depth on another dredge and a rock-core station close to dredge 21. We then lowered another instrument, called a CTD, to collect water samples from the plume. With four stations all showing evidence for this plume, we are confident that we have located a new hydrothermal vent site! Some day, we hope, we can come back to the Gakkel Ridge to study this vent site—and its chemistry and biology—in more detail.

DCP

DCP stands for direct current plasma spectrometer. This machine is used to measure the different elements in rocks. From a rock's contents, we have hints about how it formed — and where.

In order to use the DCP, we first have to dissolve the rock samples in acid. We then inject the solution of dissolved rock into a hot gas, called a plasma. The heat in the gas makes the atoms in the rock solution emit light. Each element has its own kind of atom, and each kind emits light of its own specific wavelength and energy.

A detector in the machine measures the intensity of all these different emissions. From the detector's readings, we can calculate the contents of the solution — and so of the original rock sample.

Dredge

Dredges are used to sample the ocean floor. A dredge looks like a big metal bucket. It has a bag inside made of chain mesh and an opening like a jaw. Scientists lower it into the water all the way to the ocean floor, then drag it behind the ship. The jaws scrape pieces of rock off the ocean floor, to be collected in the bucket.

For each sample, a ship drags the dredge a fair distance, from hundreds of meters to several kilometers. That means we cannot know for certain the exact location of a sample from the bucket.

Rock Core

Rock coring is another way of sampling volcanic rocks and sediments from the ocean floor. A metal cylinder filled with wax gets thrown into the water on a line. By gravity, it falls to the floor of the ocean, where it slams into the rocks, shattering them. The shattered pieces get stuck in the wax.