FeMO2 Dive Cruise 2007
Report Day 02 -- Friday 12 October 2007 -- Probing Ula Nui with the CTD
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FeMO2 Dive Cruise 2007 |
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Sometime in the early morning we passed South Point on the island of Hawaii (the southern most point in the United States) and turned towards the northeast. Daybreak finds us approaching our Loihi station with a spectacular view of Mauna Loa, the long mountain. The haze at sea level hides the shoreline, so Mauna Loa seems to hover over the ocean with its summit in the clear as though reaching for the stars. Low along the shore, well north of the summit, is a broad plume of white steam and volcanic gasses that arise where hot lava is entering the ocean. The plume of "vog" drifts south on the trade winds. We are over a site on the lower edge of Loihi called Ula Nui.
The first task is to deploy 2 transponders that will form a navigation network for Jason. The transponders are pingers that hang from a float above the seafloor. Because the pinger must be near the bottom, and the bottom at our deepest site is nearly 5000 meters, it must be able to withstand crushing pressure. The pressure at 5000 m is 500 times atmospheric, or about 7,500 lbs per square inch! The amazing thing is that the float is made of glass. The glass sphere is about 35 cm across and 2.5 cm thick. A tough yellow plastic "hardhat" is bolted over the sphere to protect the sphere and make it easier to spot when it floats to the surface.
The transponder itself is an electronic instrument encased in a high-pressure stainless steel housing. It will respond with a ping (more of a metallic click) when "interrogated" by a ping of a specified frequency. The transponder housing is also designed with an anchor release so that it and its float may be recovered at the end of a dive series.
To rig a transponder for deployment a float has to be attached with cable above the transponder and the anchor below. The transponder is deployed with the ship moving slowly ahead. First the float is placed in the water and the cable run out to the transponder. While still moving ahead the transponder is placed. The anchor is not pushed over the side until the float and transponder are steaming astern. When the anchor weight is pushed over the stern the rig sinks anchor first followed by the transponder and finally the float.
Once the transponders are down they must be surveyed in by the Kilo Moana. The ship sails overhead "interrogating" the transponders and listening for the return ping. Multiple passes are made near each transponder and the closest approach for each pass found. The intersection of the lines for closest approach indicates the position of the transponder. Once the exact position of the transponders is known it becomes possible to navigate under water using the transponder net. Once we are finished with the transponder network we can cause the transponders to release their anchors by pinging them each with the correct release frequency. They will then rise to the surface on their glass floats. Deploying and surveying in the transponders takes all afternoon.
Night falls and Elly, Suzanna, Alexis and Rick ready the rosette sampler for the CTD cast. Brian, Alexis handle tag lines while one of the crew operates the crane and others control the winch. Elly directs the CTD over the stern of the ship in a heaving trade wind swell. The CTD cast will take nearly four hours to travel the 5000 meters to the bottom and return. The first sample bottles are triggered about 10 meters off the bottom shortly after 9pm and the rest of the bottles are triggered on the way back to the surface.
The summit of Loihi is at about 1000 meters depth and scientists are curious to see if a plume of volcanic water can be detected by the conductivity and temperature instruments of the CTD.
Following the CTD recovery, the Jason robot will be launched, but I have a shift in the Jason control van at 0400 so I climb into my bunk and try to sleep.
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