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Partly cloudy
Lat: 35.23 N
Long: 21.47 E
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Humidity: 68%
Sea surface temp: 19.7°C
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Learning as they go

November 29, 2011 (posted November 30, 2011)
by Cherie Winner

The current version of the remotely operated vehicle (ROV) Jason and the 10-person team that operates it have made 608 dives to the deep sea since 2002. But every new site presents new challenges, especially when it’s in a habitat that has not been explored before. Fortunately, the Jason team is great at learning from their first visit to a site and adapting fast so the scientists can get the samples and data they need.

Jason started its first dive of this cruise on Saturday night and returned 17 hours later with its basket (cargo area) loaded with sediment samples from the Urania Basin and the area around it. It also left some experiments in place at the seafloor, which it retrieved on its dive today.

Yesterday Joan Bernhard, the WHOI geobiologist who is supervising the sediment sampling on the cruise, talked about what worked well on the first dive and where she and the Jason team will try to make improvements during today’s dive.

One problem that is just part of visiting the seafloor is that whenever Jason touches down on the bottom, it stirs up a big cloud of sediment that blocks the view. “It’s like Pigpen in Charlie Brown,” said Joan. Then the scientists have to wait for the sediment to settle before they can get to work, which is hard to do when you’re eager to see what’s down there.

Everyone was happy to see three separates zones: a black and murky one that appeared to be the Deep Hypersaline Anoxic Basin (DHAB), its bathtub ring, which was very pale, and normal seafloor, which was reddish-beige.

However, Jason’s lights didn’t extend as far as Joan wanted to see. The sampling equipment on Jason’s basket is tall, so the area that can be viewed with Jason’s cameras is farther out from the front of Jason than it is on most dives. To improve the view on the rest of the dives on this cruise, the Jason team added two powerful lights to the “eyebrow” bar on the front of Jason. They also added a downward-facing video camera so the Jason team and scientists on the ship can see what’s directly beneath the ROV, in addition to what’s in front of it.

The team also bolted on a CTD, a device for measuring conductivity, temperature, and depth. The main reason for adding the CTD was to get the conductivity measurement, which tells them the concentration of salt in the water. (The more salt there is, the more efficiently the water will conduct electricity.) The CTD is on a “swing arm” that swings out and forward until it is near the front of the basket. Since Jason tilts slightly downward, the pilot can poke the CTD into the halocline layer of water and measure how saline it is. Earlier today, in Jason’s second dive, the CTD confirmed that what looks like a DHAB really is a DHAB.

Today the team even tried to move Jason’s main body into the brine, despite the danger of corrosion—but Jason is so buoyant and the water was so dense that Jason could not penetrate it! [See Density Demo] Now Joan thinks the black area is not the DHAB itself, but the halocline on top of the DHAB. It’s not clear what the pale area between the normal seafloor and the halocline is.

A problem that will probably not be fixed on this cruise is caused by the DHAB itself. Joan had planned to spend several hours at the start of the dive running Jason above the DHAB to map it in detail with the onboard RESON multibeam sonar. But the Jason team didn’t know if its sonar would be able to map the seafloor through the DHAB brine, because salt changes how fast sound waves travel. (In fact, sonar can be used to measure the salinity, or salt concentration, in different parts of the ocean.)

As it turned out, at the settings it was used, the RESON couldn’t “see” through the brine. With such a short research cruise, the team doesn’t have time to experiment with different settings that might solve the problem. So the researchers must wait for a future cruise to learn exactly what the depth and contours of the DHAB are, and whether there are interesting features on the seafloor inside the basin.

The best result from Jason’s first DHAB dive was all three kinds of sediment samplers Jason took to the bottom worked well. We’ll talk about those, and show Jason in action with them, in tomorrow’s dispatch.

During its first dive of the cruise, the remotely operated vehicle Jason moved along and just above the border between the brownish normal seafloor (on the left) and murky black water that appeared to be the Deep Hypersaline Anoxic Basin (on the right). The white strip between them appeared to be where the halocline, or transition zone between normal seawater and hypersaline water, meets the seafloor. During Wednesday’s dive, the scientists found that the dark water is probably the halocline. They don’t know yet what the white strip is. The structure at the left edge of the frame is one of Jason’s manipulator arms. The items at the bottom of the frame are sediment samplers.

Read the new interview

Hugh Popenoe

Hugh Popenoe: Engineer Hugh Popenoe works on three major projects at WHOI: the remotely operated vehicle Jason, the human occupied vehicle Alvin, and the Martha's Vineyard Coastal Observatory. Last year he saw a Deep Hypersaline Anoxic Basin with his own eyes, from the portholes of Alvin during a dive in the Gulf of Mexico. Read the interview »

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