12 



MARIOK EXrEDITION TO DAVIS STRAIT AND BAFFIN BAY 



liquid undergoes dilution continually as a result of mixing with the f 

 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 durin-T 

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

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

 in this respect throughout the winter, with a second and greater 

 loss on the arrival of the succeeding summer. When air tem])era- 

 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 throush- 

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

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

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

 cap, at least to any great exent. the brine tirains off and down leav- 



Oxn 



Z50 



J X 3 4 



> 6 7 8%« 



The Change in the Salinity of Sea ice With age 



FiGUKE 2. — Young ice formed in October iu tlie north polar 

 basin has a thicl?ness 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 tlie same time gradually becomes 

 granular and after a few years it is diiHcult to di.stinguish an old 

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

 ice in the Arctic regions by the end of summer, is suHicient to reduce 

 the salinity 2 or 3 parts i)er 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 layer'^ i 

 is prevented from settling directly downward by the bottom layers^ 

 which, constantly immer.sed in cold water, remain so solid that the, 

 only drainage is tlii-ough tlie many narrow-cut i-hannels that furrow j 

 the floes. If a series of careful measureiin'iits of the thickiu'ss t)i 

 the ice cover be made during the early part of summer when nu'ltiiiil. 

 begins, they wall show at flrst a gradual dei-rease. then a brief thick- 1 

 oning as the water thawing from the tops of the floes and from the f 



