5 December, 1996

This morning I went to collect CTD data with a scientist from New Zealand, Miles LaMare. Miles, like the rest of his country mates I've met, is especially friendly and will go out of his way to be helpful. Even though we both speak English we seem to have a language barrier. I'm getting used to his accent but I still have to listen carefully to understand him.

A CTD is a device that monitors and records the pressure, temperature, and electrical conductivity of the water column as it is lowered from the surface to the bottom. When the data is recovered, the depth of the probe is calculated from the pressure on the sensor. The salinity is determined from the conductivity. Most measuring device use this technique of indirect measurement, a probe is designed which produces a voltage or current that is proportional to the quantity of interest.

A computer chip records thousands of bits of data as the CTD is lowered. When it is retrieved the data is down- loaded into a lap top computer where it can be displayed in either tabular or graphical form. The graph is useful because it aids in visualizing the structure of the water column. Miles is interested in the water temperature because of the affect it has on the fish. Small differences near the freezing point cause chemical changes in the fish's blood, which keep ice crystals from growing. An ongoing study at McMurdo is to try to investigate specific proteins that stick to ice crystals that form in the cells of fish and keep them from becoming larger. The graph shows data from a CTD cast that was taken very close to the transition between the permanent ice shelf, which is about 100 feet thick, and the 10-foot thick sea ice. It indicates that the water gets warmer below 100 feet and that the salinity increases. Warm water under cold is very unusual in the ocean except in regions where there is sea ice.

In the open oceans, the water temperature decreases with depth. The upper layers of water are heated by the sun and float on top of the colder more dense water. Around Antarctica, when the water near the surface freezes, the salinity of the remaining water increases. This warmer saline water is denser than the colder water below and sinks. The sinking water causes the bottom water to move up. The circulation brings nitrogen and phosphorous (nutrients necessary for photosynthesis) from the benthos into the photic zone. It also brings oxygen from the surface to the ocean depths. This abundance of nutrients makes primary production especially high during summer months. This bloom of phytoplankton has decreased the visibility in the water to the point where normal dive operations have been halted.

Miles lowers a CTD through a hole in the ice.

A graph of temperature vs. depth plotted from CTD data shows that the water becomes warmer with increasing depth.