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Lost City Expedition: Mapping

Water Column

Discovering New Hydrothermal Fields-A CTD Package

Two of our goals on this cruise will be to characterize hydrothermal plumes formed from active chimneys at the Lost City Field and to explore for new fields. Because the hydrothermal fluids are warmer, less dense, and more buoyant than the surrounding cold ocean water, plumes rise in the water column above the fields similar to those emitted from smoke stacks or cooling towers on a cold winter day. Because of their anomalous chemistries, temperatures, and (sometimes) particulate content, the hydrothermal plumes serve as valuable guides to discovering new vent fields. During this expedition we will prospect for these plumes using an instrument package called a CTD (short for conductivity-temperature-and depth; some of the parameters sensors on this instrument measure).

The instrument we will use is a Sea-Bird CTD system that is mounted on a tall, round metal frame, which hosts 24, 10-liter bottles for sampling fluids in real time. The instrument will also host oxygen, methane, and eH sensors, and a transmissometer that measures the amount of particulates in the water. This instrument is connected to the ship by a conducting wire, which allows an electronic signal to be sent down and up the wire. When the instrument is in the water, data is continuously transmitted to the ship, which allows us to measure the chemical, thermal, and particulate character of the water that the instrument is in or passing through.

Each bottle mounted on the frame has two spring-loaded end caps that are fixed in the open position when the instrument package goes into the water—i.e. the bottles are a hollow tube with both ends open to the ocean. When we see an interesting anomaly in the water column we can electronically snap the end caps closed, trapping fluids immediately. In this way we can bring the instrument back onboard and retrieve the sampled hydrothermal plume water, which is later analyzed for its chemistry and also for its microbial content.

What Does the Plume Look Like at Lost City?

During this cruise we will use the CTD in two different ways. The first way, is by lowering the CTD into the water such that it goes directly beneath the side of the ship in a vertical path to just a few meters above the seafloor. During this process we keep the ship in a stationary position. Linkages between onboard computers, thrusters, and GPS satellites are so good and “smart”, that commonly when we do these casts the entire piloting of the ship is done under computer control. We use these “vertical water casts” to do pinpoint analyses of the water column. This will be a critical measurement when we first get to Lost City because it will provide us with a sound-velocity profile of the water column, which is important in acquiring good navigation. In addition, it will also give us a very good image of what the plumes looks like above Lost City—something we know very little about.

How Will We Find New Fields?

The mid-ocean ridge spreading network spans a distance of ~60,000 km. To find new sites of hydrothermal vents can sometimes be like looking for a needle in a haystack. To efficiently look for new fields we use first make educated guesses on where we might expect to find them based on the geology of the area. Usually, in a new area this means that we spend a lot of time looking at bathymetry maps, which may provide hints to where the fields are.

We then lay out a track-line, or survey line for the ship and instrument to follow. Because we have a lot of ground to cover in an exploration mode, it is not efficient to do vertical water casts. Instead we use the CTD in a “tow-yo” mode of operation. During this process we lower the CTD into the water and then start driving the ship very slowly (<1 knot) along the survey line. The CTD is first lowered close to the seafloor, and then the wire is pulled in to raise the instrument 200-500 m above the seafloor. As the ship continues on its survey path this process is repeated over and over again—the CTD follows a zig-zag or saw-toothed pattern through the water column. In this manner, we can map out significant areas of the water column to search for new sites of venting. The “hunts” will take place during night operations.