12 



MAEIOX EXPEDITION TO DAVIS STRAIT AND BAFFIN BAY 



liquid undergoes dilution continually as a result of mixing with the 

 surrounding waters; or (c) that freezing is retarded to a very slow 

 rate. 



One of the most interesting features connected with the physical 

 properties of sea ice is the seasonal change in salinity which develops 

 with age. Young ice; that is, the variety which has formed during 

 the autumn and winter, is observed to decrease slightly in salinity 

 at all depths shortly after freezing; it then continues nearly constant 

 in this i-espect throughout the winter, with a second and greater 

 loss on the arrival of the succeeding summer. When air tempera- 

 tures over northern seas rise above —4° or —5° C. (24.8° or 23° F.) 

 the minute brine particles and salt crystals that have lodged through- 

 out the interstices of the pure ice crystals begin to melt away from 

 the more solid structure. Since the heat which comes from tlu^ 

 sun is too feeble to melt the ice crystals in the permanent polar 

 cap, at least to any great exent, the brine drains oif and down leav- 



Z50 



J 2 3 4 



8%, 



The Change in the salinity of Sea Ice With Age 



FiGUSE 2. — Young ice formed in October in the north polar 

 basin has a thiclvness of about 45 centimeters and contains 

 approximately eight parts per thousand of " salt." The 

 same ice by" the following summer has increased about 

 three times in thickness, but its salinity has decreased 

 about four times the original proportions. ^Figure from 

 Malmgren, 1928.) 



ing the tops of the floes of a composition closely approaching that 

 of fresh ice. The structure at the same time gradually becomes 

 granular and after a few years it is difficult to distinguish an old 

 floe from glacier ice. The penetration of solar warmth into the 

 ice in the Arctic regions by the end of sunmier, is sufficient to reduce 

 the salinity 2 or 3 parts per thousand to depths of 3 to 5 feet. The 

 pressure ridges .so prevalent in the regions of paleocrystic ice are 

 found to be more completely washed free of salts than any other 

 northern sea ice. The brine "that drains out of the uppermost layers 

 is prevented from settling directly downward by the bottom layers 

 which, constantly immersed in cold water, remain so solid that the 

 only drainage is through the many narrow-cut channels that furrow 

 the floes. If a series of careful measurements of the thickness of 

 the ice cover be made during the early part of summer when melting 

 begins, they will show at first a gradual decrease, then a brief thick- 

 ening as the water thawing from the tops of the floes and from the 



