FeMO2 Dive Cruise 2007
Report Day 13 -- Tuesday 23 October 2007 -- Pohaku's Missing Marker
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FeMO2 Dive Cruise 2007 |
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When I begin my watch in the control van at 4 am, Jason is moving back and forth along a wall near Pohaku looking for a marker that was put down years ago. Markers occasionally disappear, either because they are buried by rock falls or their anchor lines wear through or perhaps they are carried away by currents. While searching Jason flies over a spectacular terrain of deep rifts and cliffs much like what you see walking around the volcanoes of Mauna Loa and Kilauea. Pohaku’s marker is not found and the scientists in the van decide to sample a hot vent at the base of another cliff. Yellow-orange mat cover the rocks near the vent and fills cracks between surrounding rocks. The water measures 22 degrees Celsius. Pohaku becomes a new study site to look at the variability of microbes across distances. This site is off the summit plateau and down the south rift. It is the only hot vent we’ve visited this year that is not on the summit plateau. This is contrary to Ula Nui’s vent water temperature which is only slightly above ambient seawater temperature. Garcia et al.’s paper points to indications that Lo’ihi’s volcanism is moving south so perhaps Pohaku will get hotter.
Jason enters Pele’s Pit through the “spillway” on the south side of the crater and locates the elevator. The Jason pilot exchanges samples collected at Naha and Pohaku for instruments and samplers needed in the pit. Two “osmo samplers” are grabbed. Greg Horn has spent the last couple days building and preparing the osmo samplers for this dive. The osmo samplers are a way to record changes in vent water chemistry for an entire year. They consist of a salt chamber attached to a long thin tube. The tube is about 100 meters long with a hole about 1 mm in diameter running through it. The salt chamber and tubing need to be filled with water prior to the dive so that they are not crushed by the pressure. The outer end of the tubing will be placed in the opening of a hydrothermal vent. Once in place, osmotic action slowly draws vent water toward the salt through the 100 meters of fine tubing. It works as a super slow pump; vent water will slowly migrate down the tube toward the salt. A year later, when the osmo sampler is recovered, the tubing can be cut into sections to analyze the chemical changes in the vent water that occurred during the year. Water at the outer end of the tubing will carry the most recent vent water, and the inner end of the tubing will carry chemicals erupted from the vent a year ago.
A full suite of samples are collected at marker 39, the cliff face venting sit at the eastern edge of Pele’s Pit. The site is incredibly difficult to work for the Jason pilots. The scientists want their samples from vents near the top of a cliff so traps and samplers and rocks occasionally slide from the site and tumble down the cliff. Jason then has to dive to the bottom of the cliff and retrieve the equipment and attempt again to place it on the cliff. Given Lo’ihi’s reputation for earthquake swarms and rockfalls this looks to me like a precarious site.
One surprising thing in the images from this dive is the distinct ripple marks in the black sand at the bottom of the pit. Ripples in sand are usually seen in rivers with strong current and below waves breaking on beaches. If you dive into the waves and swim to out to deeper water you find that the ripples quickly disappear as the wave energy diminishes. The deep ocean is known for its slow steady circulation far removed from the driving forces of wind and tide – I can’t imagine that currents this strong exist at the bottom of a pit 1300 meters below sea level. Hubert Staudigal, a geologist with experience diving in manned submersibles at Lo’ihi and other seamounts assures me that currents in the crater are strong enough to form ripples. He tells me that the sub pilots frequently complain of cross currents. The cause seems to be tidal. As the moon orbits the Earth causing the tidal bulge in the oceans, water within the ocean also rises and falls. Tidal forcing can cause waves inside the water column, internal waves, 300 meters high because the density difference between deep water at 1200 meters and deep water at 1500 meters is so slight. In other words, 1500 meter water could be pulled to 1200 meters to spill over the rim of the crater. The slightly denser and colder water spilling into the crater is probably responsible for the ripples.
Dive 315 ended at 8 pm when Jason was brought back aboard, but the elevator with all the samples still needed to be recovered. It was dark and the wind and swell were rising. The elevator sits very low in the water but has a strobe light on top for spotting at night. The Kilo Moana also has a very cool feature to make recoveries easier, the bridge controls can be moved to a station at the stern beside the A-frame. Standing next to the A-frame the person controlling the ship has an excellent view of the action, but it is strange to stand at the very back of the ship and drive it into the waves which is what was required this night.
It is a crazy recovery. Captain Meyer stood at the stern controls driving the ship backwards into the night swell. Because the wind has come the elevator and ship were moving 12-15 feet relative to each other. The task is made more difficult because the work deck is so high above the water. Elly, the deck tech, tries to catch a loop on the top of the elevator with a hook and line on a 10 foot pole. The elevator is plunging up and down with waves breaking over it, and the stern of the ship is plunging up and down enough that it is easy to lose your balance. Trying to catch the loop on the elevator with a hook on a pole is like trying to hit moving target with a pole-vaulter’s pole. Elly misses several times, which really wears you out, then hooks it. But the hook is caught under the lifting loop and in trying to get it to pull straight up the hook comes off. The captain steers the ship near the elevator again and it drifts between the hulls disappearing under the after deck. Captain Meyer repeats the approach and Elly hooks it again. It's still plunging up and down every couple seconds and the ship's still moving up and down and side to side. The winch operator lifts the elevator part way out of the waves and Elly and Brian begin poking at the elevator with their long poles to hook tag lines to it. The tag lines are attached fairly quickly and they finally get it aboard. The scientists swarm over the elevator’s cargo to get their samples. Given the power of the waves that have been washing over the elevator it seems incredible that any samples remain, but there they are, strapped securely in the sample boxes.
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