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Today's Weather
Lat: 35.23 N
Long: 21.47 E
Air temp: 16.9°C, 62.4°F
Bar. Pressure: 1025.61 mbar
Humidity: 61.9%
Sea surface temp: 19.7°C, 67.5°F
Winds: NNE; 8.7 knots
Visibility: unlimited

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density activity
Density Activity for grades 6-9


What are DHABs? What are DHABs?


Greek Origin
Word of the Day:


from “hypo,” meaning “under,” and “thesis,” meaning “proposition”

Twists and turns

December 1, 2011 (posted December 2, 2011)
by Cherie Winner

Yesterday the scientists and Jason team got a real-life demonstration of how the density of a liquid, like seawater, can have a big effect on objects that are in the water. (If you haven’t already checked out our Density Demo, this is the perfect time to do it!)

On its latest dive, Jason returned to the site of its first dive here at Urania Basin. It retrieved injection cores, obtained more pushcores and sediment scoops, and tried to get samples from the murky black area. As mentioned in yesterday’s dispatch, the Jason team tried to move the ROV all the way into the DHAB to gather samples.

But Jason is naturally buoyant. Unless its downward thrusters are turned on, it floats, because the blue and yellow portion is made of foam that has very low density. (This is a safety measure; if Jason loses power or its engines fail, its buoyancy will bring it to the surface where it can be recovered.) And when Jason tried to go into the high-density brine, an odd thing happened. It couldn’t make it—even with its thrusters cranked up to full power!

The bottom, mostly metal, part of the ROV got into the DHAB, but the part covered by the buoyant material could not. That meant Jason could not collect samples, because the ROV must have a stable base in order to do that work. That’s why the pilots set it down on the seafloor even though that creates a “particle cloud.” It can’t sample with its engines running full blast. So, unable to gather samples in the brine, Joan Bernhard and the rest of the sediment team decided to deploy the MC800 multicorer, a sediment-sampling device that they brought along as a backup just in case it was needed. It brings back good cores, but does not preserve them on the seafloor.

At 3:30 p.m. the sediment team gathered on the starboard deck for the launch of the multicorer, which looks like a giant metallic teepee frame. On its center column is a moveable carriage with eight core tubes mounted around it. It works by descending on a wire to the seafloor. When the legs of the teepee land, the central carriage and sample tubes keep moving downward. The carriage is weighted with slabs of lead, so if the sediments are soft, the tubes will plunge into them a foot deep or more. When the operator on the ship starts pulling the multicorer back up, caps close across the tops of the tubes to create a vacuum. When the tubes clear the surface of the seafloor, caps snap shut across the bottoms of the tubes. That keeps the sediment inside from falling out.

When the multicorer was recovered three hours later, the team had another disappointment. The tubes were empty. The instrument had reached the bottom—a few of its feet came back with gobs of dark gray gooey sediment on them. The sample tubes had dropped, but only went a couple of inches into the mud, and their top and bottom caps had not snapped into place. Joan and Ellen Roosen, the multicorer technician, didn’t know why the caps didn’t deploy. A bigger mystery was why the tubes had not penetrated any farther into the mucky seafloor.

Joan and the rest of the sediment group reluctantly turned their attention to their next sampling site, Discovery Basin, about 10 nautical miles to the northeast. But the water in Discovery is even denser than the water in Urania. If Jason couldn’t get into Urania Basin, how will it get into Discovery? And why didn’t the multicorer tubes push into the Urania sediment?

About an hour later, as Ellen was washing off the multicorer, she discovered a clue. One of the feet held several flat chunks of hard, rock-like material about the size of the little candy bars you get at Halloween. She brought them to Joan, who put them under a dissecting microscope for a quick look.

The chunks are pale grey-green in color, and they sparkle. From the side, they appear to be made of thin layers. From the top, they look like granular crystals. Joan and Dr. Konstantinos Kormas from the University of Thessaly in northern Greece hypothesized that they are calcium carbonate deposits that formed when carbon dioxide (CO2 or HCO3-) in the sediments combined with calcium ions (Ca+) in the “pore water,” which is seawater that seeps down through tiny pores in the sediments.

HCO3- + Ca+ --> CaCO3

The CO2 would have come from bacteria and archaea that oxidized methane in the sediments. Urania Basin has levels of methane millions of times higher than normal seawater (see What Are DHABs?). The same process occurs at in other areas in the oceans, where the crusty rock provides a hard surface for larvae of clams, corals, and other animals to attach to. Because DHABs are so salty and lack oxygen, there are probably no such animals attached to the rock there.

Joan, Ellen, and Kostas think this material is what stopped the multicorer tubes from going into the mud. The way the multicorer works is like those strings of Christmas lights where, if one bulb burns out, none of the bulbs work. In the multicorer all the tubes are linked together, so if one hits something hard and can’t go any further, the other tubes stop too.

In just a few hours, at 4 a.m. Friday morning, Jason heads out on its first dive to the ultra-dense Discovery Basin. Stay tuned!


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