Section III., 1914 [751 Trans. R.S.C. 



The Dawson Isothermal Layer of Low Temperature in the Gulf of 

 St. Lawrence. 



By H. T. Barnes, D.Sc, F.R.S. 



(Read May 27, 1914). 



It has been shown by Dr. W. Bell Dawson during his very careful 

 temperature survey of the Gulf of St. Lawrence, that there exists 

 a layer of low temperature at a depth of approximately 30 or 40 

 fathoms. This layer underspreads the whole area of the Gulf, and 

 that part of the Labrador current southeast of Newfoundland where 

 Dr. Dawson's measurements were taken. The temperature of this 

 layer remains apparently fairly constant the year round at about 

 30 °F. The interesting part of this remarkable phenomenon is that 

 at greater depths the temperature rises to 34°F approximately, which 

 is the temperature usually found in the deeper parts of the North 

 Atlantic. From 30 fathoms to the surface the temperature changes 

 with the seasons. In the summer the surface temperature rises to 

 45 or 50°F, and in the winter sinks to 28°F, the freezing point of salt 

 water. 



Up to the present no satisfactory explanation has been offered 

 to account for the phenomenon. It seems to me that it can be ex- 

 plained readily by taking account of the influence of iceberg melting. 

 Since the whole Labrador current carries large numbers of icebergs, the 

 effect of these fresh water masses must, in the course of years, be evident 

 in the distribution of temperature and density of the under water. 

 The mean depth of the North Atlantic icebergs is between 30 and 40 

 fathoms. This is shown from the positions where bergs will ground. 



Iceberg Melting. 



In the first place let me make quite clear what takes place during 

 the melting of a berg. It has been shown by Dr. Otto Pettersson that 

 when ice melts in salt water (Fig. 1) three currents are set up. A 

 horizontal current of sea water sets in towards the ice, which becoming 

 cooled falls by gravity, and constitutes the second current. The 

 third current consists of fresh melted berg water which, being lighter 

 rises and mixes with the surface layers. The result of this process 

 must be to bring about a cooling and drawing down of the surface 

 layer of the sea. Indeed Pettersson has likened an iceberg to a 



