June 19, 2009
55° 31.5449 N
When I awoke at 05:00, I found myself stretched out in a chair in the lounge with the guidebook for the Fort Schwatka National Heritage Site of Unalaska in my lap. I now know what to turn to in case of insomnia. I went next door to the galley, filled up my new Alaskan Ship Supply Company coffee mug and went down to the main lab to see what the board said. It now said 08:00 for our CTD cast.
The station we are arriving at is designated 8-UAP5. The first number is a running total of how many stops (or stations) have been made so far for sampling. Even though this will be our first sample, it is the ship's 8th overall for the TN250 cruise that started Wednesday night. The second part of the name is a series of letters that represent the rough geographic track of the sampling line the ship is taking. For example, we are on the UAP line right now, a series of sampling sites that runs from Unalaska Island to the Alaskan Peninsula. Later on in the cruise we will be running other lines (or transects) such as the CN (Cape Newenham) line and the SL (St. Lawrence Island) line. The last number is specific to the line that is being run. These stops are numbered consecutively from the shallowest point to the deepest point. Since we are heading northeast towards shallower waters, this number is decreasing. This would make the previous stop 7-UAP6 and the next stop 9-UAP4.
Things ran behind and the CTD was finally dropped over the starboard side at around 08:30. At this station the water is only 92 m deep so the CTD was stopped at 87 m to prevent any damage that might occur from striking bottom. The acronym CTD stands for conductivity, temperature, and depth and refers to the instrument package mounted in the bottom of a metal cage. Once in the water and activated, it takes various measurements such as conductivity, temperature (and if you know conductivity and temperature then you can calculate the salinity of the water), dissolved oxygen concentration, light intensity, and fluorescence (as a means of measuring the chlorophyll content). When the CTD is lowered all the way to the bottom it can provide an accurate depiction of the conditions throughout the water column.
Also housed inside the metal cage is a series of 12 30-liter capacity PVC Niskin bottles arranged in a circle. These bottles collectively make up a rosette. Each of these bottles starts the deployment with both ends open until it reaches the bottom. As the rosette is slowly raised to the surface, it makes a series of stops at predetermined depths and the bottles begin to close, one by one, triggered by the CTD operator, and trapping inside a water sample from that particular depth. Once the CTD is secure on deck, various scientists and technicians pull their water samples from the bottles. On this particular cast only 8 bottles were closed at depths of 87, 75, 50, 40, 30, 20, 10, and 1 meter. The number of bottles used and the depths they are closed at depend of a variety of things. The overall depth of the water, what the scientist ordering the cast is studying, and what volumes of water they need for their study are all taken into account when constructing what is known as a water budget. For the deeper waters later on in the cruise, two casts may be done at the same location to get a higher resolution profile of the entire water column and to make certain that there is enough water collected to satisfy everyone's needs.
In order to pull a water sample from a bottle, the release valve at the top must be loosened first so as to allow for air displacement of the water flowing from the bottom. Then a two foot long piece of tubing is fitted to the opening valve at the bottom (not necessary for all samplings) and the container in which the sample is to be collected is rinsed three times with water from the bottle. We collected one 4-liter jug from each bottle for a thorium-234 profile and the water was carried back to the main lab where the processing of the samples would start.
Into each of the 4-liter jugs a series of reagents was added. First 8 drops of concentrated ammonium hydroxide were added, followed by 25 µL of a potassium permanganate solution, and then 11 µL of a manganese chloride solution. Between each reagent, the sample jug was capped tightly and shaken to ensure full mixing. Once the chemicals had been added, the jugs were inverted and placed in a special housing awaiting filtration. There they sat for one hour to allow sufficient time for the manganese oxide to scavenge up all of the thorium present in the seawater samples.
The 4-liter samples are then filtered simultaneously for four hours (or until completion) and the manganese-thorium complex becomes trapped on the 1 micron filter. If there is any leftover sample that has yet to be pulled through the filter, the quantity of sample remaining is recorded, and it is dumped overboard. The filter containing the precipitate is then dried overnight in an oven. Once all of the water has been driven off, it is ready for ß counting after the three-day rest period.
The sample bottles, graduated cylinders, and filter housings were then rinsed with deionized water so that they will be ready for the next sampling site. We will not sample again until Sunday night, as we are moving further towards the coast and away from the group's geographic area of interest. It is now time to do some writing and then get some sleep. Tomorrow should afford me the opportunity to start making my way around to other groups on the ship. Everybody's work onboard this cruise is somehow related in the Bering Sea ecosystem. Maybe I can start to put together the puzzle.