OCEANOGRAPHIC INFORMATION FOR SUBMARINE CABLES 1075 



IV. TEMPERATURES* 



4.1 General 



The temperature of a given point on the deep sea floor is determined 

 by the system of ocean circulation. Study of deep sea circulation is still 

 in an early stage and theories which would permit prediction of changes 

 are still in a rudimentary form. In addition, observations of actual bot- 

 tom temperature are few. Thus, a study of the temperature environment 

 of submarine cables must proceed by evaluating those data that exist 

 and striving for increased understanding of the underljdng circulation 

 processes. 



The new electronic thermometer under development at the Lament 

 Geological Observator}^ determines temperature with an accuracy of 

 0.01 °C by the frequency of an oscillator employing thermistors in an 

 RC network. The oscillator is lowered on the end of a cable and its fre- 

 quency is monitored by equipment installed aboard ship. 



Bottom-temperature changes might be predicted if the rate and direc- 

 tion of circulation of the sea water could be determined. This is being 

 studied by sonar tracking of a submerged blimp designed so that it has 

 negative buoyancy at the surface but is neutrally buoyant at the level 

 where the measurement is desired. This method has been used down to 

 depths of 3,000 meters. Results have indicated much higher velocities 

 than hitherto suspected. Near the base of the continental slope off the 

 eastern United States near-bottom currents of -g- knot have recently 

 been observed by this method. 



Another method depends on the measurement of the time elapsed 

 since a given water mass was at the surface by radiocarbon dating of 

 sea-water samples. At the surface, water is in free exchange with the 

 atmosphere and acquires a radiocarbon concentration in equilibrium 

 with that of the atmosphere. As the water sinks from the surface to enter 

 the deep sea circulation system, it is cut off from the supply of fresh 

 radiocarbon, and radioactive decay reduces the content of Carbon 1-1 

 at a rate given by its half life. Thus, the measurement of the radiocarbon 

 content of a given sea water sample ideallj^ will give the time at which 

 this sample left the surface of the ocean. 



* In this section dealing with temperature, depths are given in meters rather 

 than in fathoms. Temperature has largely been of interest in physical oceanog- 

 raphy where volume is of concern. This has led to the vise of meters. Since a nauti- 

 cal mile is approximately 1,000 fathoms, the fathom has been widely used in topo- 

 graphic work. One fathom equals approximately two meters. 



