We continue to track southwest. The weather continues to be moderate changing from clear to overcast but with winds between 10 and 20 kt. Seas are moderate between 5 and 10 feet. We are moving at 10 kt most of the time although we slow to as little as 5 kt when we get into sea ice.
We are seeing more patches of sea ice. Everyone is excited about seeing the ice because it means we are getting close to the Ross Sea and the start of our sampling. Some of the ice floes are several tens of feet in diameter. Most of the ice is in small clumps that are (and have been) melting. We are also seeing many more icebergs. Most are rather small, for icebergs, being 50 to a few hundred yards across. Some of the icebergs look to be 50 to 60 feet tall which means that they are 500 or more feet thick. Remember that only about 10% of floating ice is above water and 90% is below.
Our group will use the CTD (conductivity temperature depth) instrument package to make measurements of water properties. The CTD also has a sensor that measures dissolved oxygen in the water. We use dissolved oxygen to trace particular water masses. In order to get realiable measurements, the CTD has two different sensors for each property: temperature, conductivity (converts to salinity) and dissolved oxygen. Conductivity and oxygen sensors can drift in their calibration so we make measurements of salinity and oxygen from the water taken from the Niskin bottles and use these numbers to check the readings from the sensors.
The oxygen measurement involves a chemical process that traps the dissolved oxygen in the bottle in a precipipate. Once the sample warms to room temperature (after about a day), acid is added which dissolves the precipitate and colors the solution. Another chemical is then added which makes the solution clear and the amount that is added is used to calculate the dissolved oxygen in the water sample. Chemists call this process "titration". We use an automatic titrator to add this chemical. We just follow the right steps and the equipment does the sensitive work for us.
Kim, one of the marine techs, showed us how to use the oxygen titrator. The small flask sitting to the left with the yellow-colored solution in it is the oxygen sample. The sample will go from yellow to clear as the chemical is added. The gray box on the lab bench regulates the chemical addition from the bottles in the background. The laptop computer records the measurements.
(Pierre and Marco are checking the oxygen titrator.)
Measuring salinity of the water samples is a bit simpler. We use a a salinometer that takes water from a sample bottle and electronically determines the electric conductivity of the water (which is converted to salinity). Sea water is an ionic
solution which conducts electricity reasonably well (compared to fresh water). The more salt in solution, the better it conducts. The measurement is very sensitive to temperature, so the salinometer has its own insulated room on the ship which is kept at 21.0C.
Lindsey (one of the marine techs) gave us a class in how to run the salinity samples. Most of the work is done by the salinometer. Our job is to feed samples, drain and fill the measurement cell and switch the machine between "standby" and "measure". A laptop computer records the details of the measurements for later analysis. We measure the salinity of each bottle three times to make sure that we have a good measurement. Running lots of salinity samples is tedious and boring but it is a necessary job.
(Lindsay, one of the marine techs, is showing how to use the salinometer.)
(Marco (UMass-Dartmouth graduate student) is running salinity samples - the case of samples is to the right.)
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