FeMO2 Dive Cruise 2007
Report Day 06 -- Tuesday 16 October 2007 -- Pit of Death!
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FeMO2 Dive Cruise 2007 |
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Last night after a 12-hour maintenance interval Jason returns to the deep site called Ula Nui. It takes 2½ hours to get to the bottom, and another 2½ to surface, because of the distance to the bottom. At 5,000 meters, Ula Nui is about 1,000 m deeper than the average depth of the oceans. The weight of the Hawaiian volcanoes pushing down on the ocean crust causes it to bend down into the mantle, much the way piling a stack of laundry on a bedspread would push the bedspread and mattress down. The process in which the Earth's crust sinks or rises to equilibrium is called isostacy. For example the crust will sag into the mantle under the weight of rock or ice (the center of Greenland is depressed below sea level by the weight of glaciers) and will rebound when the weight is removed. The northern edges of continents have been rising (cleats for tying up Viking ships are now far above sea level) a fact that is attributed to the slow isostatic rebound of the crust following the melting of glaciers after the last ice age. The Pacific crust responding isostatically to the weigh of the Hawaiian Island chain results in a deeper than expected ocean floor around the Hawaii islands.
Even though the foot of Loihi is 4,000 meters (13,000 feet) below the summit it still shows evidence of on-going volcanic activity; the seafloor is lumpy basaltic lava that has only a trace of sediment on it and microbial mats colonize areas where iron rich water escapes from the crust. The flow of hydrothermal water at Ula Nui is diffuse so the highest temperatures recorded are only ½ degree Celsius above the ambient seawater. Temperature and electrochemistry readings are taken and the microbial mat is sampled with scoops and the slurper. Following the slurps, bacterial traps are placed in and on the mat. Exposure experiments, sometimes referred to as "charges", are placed at the mat and on a basaltic promontory. Finally, basalt samples are collected.
By early morning Jason's tasks are finished and it returns to the surface with more samples for the researchers. The Jason team begins the work to get Jason ready for an 8 pm dive into the "Pit of Death"!
Craig Moyer tells me about diving into the Pit of Death in the 3 man submersible Pisces 5. The pit is perhaps 500 meters across and its sides are very steep. The bottom of the pit is about 1,300 meters deep. Warm volcanic water flows diffusely into the bottom of the pit. As Pisces descended into the pit the scientists looked out the thick acrylic windows of Pisces to see a layer of white microbial mat floating a meter or two above the bottom of the pit. Apparently the water at the bottom of the pit was denser than seawater due to all the minerals dissolved in it. As Pisces plunged through the layer of mat they could see the bottom of the pit was littered with dead fish. The water, like all hot-spring water, contained no dissolved oxygen so fish wandering into it suffocated.
During Jason's dive there is no visible anoxic layer of water, but the in situ electrochemistry analyzer reveals high concentrations of sulfide in the sediment at the bottom of the pit. The presence of sulfide indicates there is no dissolved oxygen in the pore water in the sediments - a perfect habitat for chemosynthesizing microbes.
This is Jason dive J2-310 and unfortunately the camera for taking high-resolution still photos is not working so I will have to do some video captures. Jason takes samples in the Pit of Death and places a new site marker. Jason then rises out of the pit to search for a field of little mineral projections dubbed "finger chimneys" through which warm water is venting. The highest temperature recorded in the finger chimney field is 22 degrees Celsius, or a little more than 70 degrees Fahrenheit. The Ambient seawater is only about 38 degrees Fahrenheit.
Most of the dive work takes place after midnight and Jason returns to the surface at 8 in the morning of the 17th.
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