FeMO2 Dive Cruise 2007
Report Day 15 -- Thursday 25 October 2007 -- Medea and Murky Waters
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FeMO2 Dive Cruise 2007 |
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This Jason dive has become a wide-ranging tour to clarify our knowledge of Lo’ihi. I am spending a lot of time in the control van just to see the volcanic landscape. The submarine landscape looks surprisingly similar to what you can find above sea level on the big island – deep cracks run down slope from the summit. The cracks are the fissures from which molten black rock erupts red-hot. Surrounding the cracks are ripples formed while the lava was flowing from the crack. A feature not seen above water are the “pillow lavas” formed where lava squeezes through a crack as a blob that then crusts-over as a shiny black pillow. These pillows sit side-by-side one squeezing out next to another.
Another thing to watch in the control van is Medea’s monitor. Medea has a camera that looks down on Jason as Jason moves over the bottom. In Jason’s lights can be seen the rock outcrops he flies over, and his manipulator arms. Between Jason and Medea can be seen the tether that carries power to Jason and returns the data. This tether is 30 meters, only about half the length of the tether that failed 3 days ago. Because Medea is closer, Jason appears much larger in the monitors and the heave of the ship over the swell is easier to see. It is remarkable to see the surface swell because Medea’s camera is more than a kilometer below us. As the ship heaves and pitches Medea rises and falls, the slack in the tether allowing Jason to remain undisturbed to by the wind and weather at the surface.
If you have ever been swimming or diving in the coastal ocean you are aware how difficult it is to see long distances under water – things in the distance seem to get dimmer and blurrier and good visibility may be reduced to 10 meters or less. But in the deep ocean the water is so clear, it has so little suspended material in it, that Medea has no problem seeing Jason’s lights more than 30 meters (100 feet) away. Most of what makes shallow water murky are the living things – tiny phytoplankton and zooplankton – and to a lesser degree sediment stirred up by waves or blown in by wind. The seawater that most of us are probably familiar with is green, not the bright tropical blue of postcards, but phytoplankton appear green from all the little plants living in it. Tropical waters are not green because there is very little phytoplankton. The bright tropical sun allows phytoplankton to grow so fast they consume all the nutrients they need to grow. Tropical waters quickly become nutrient-poor. Like plants without fertilizer, phytoplankton in the nutrient-depleted tropics grow slowly and don’t grow as large. Consequently the total biomass of phytoplankton in tropical seas is smaller.
However, the deep ocean is not particulate-free. Most of the particulate that would cloud your vision in the deep ocean drifts down from the surface: dead or dying plankton, fecal pellets from zooplankton, and dust blown out to sea by offshore winds. As the plankton sink out of the sunlit surface water into the deep ocean they starve and die and are consumed by bacteria in the water column. Not only that, these sinkers find themselves in a much bigger room; the sunlit ocean may be only 100 meters deep, but the deep ocean is 4000 meters deep – 40 times greater – and the concentration of murk much less. If not for the darkness, visibility in the deep ocean would be at least as good as the clearest tropical seas.
Jason and Medea are recovered shortly after noon today and the Jason team begins to service their equipment and prepare for the next research cruise. The Kilo Moana begins a sonar survey to clarify the topography around Ula Nui and the Moon Mat. The region is relatively flat and has not been a focus of previous high precision surveys so the resulting maps are not very accurate. Better maps will help characterize the site and help future expeditions relocate it. There is also a discussion of the need for additional CTD casts once the survey is complete. Scientists are busy performing their last set of analyses, entering data or packing up their lab equipment. We are all looking forward to seeing friends and family in the next few days.
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