10 MARIOX EXPEDITION TO DAVIS STRAIT AND BAFFIX BAY 



The point of solidification of sea ice: i. e., the temperature at 

 which it becomes a true solid, is lioweA'er. quite different frf>m the 

 freezing point of sea water. The fact that sea ice is a conglomera- 

 tion of pure ice crystals and of particles of brine, combined in vary- 

 ing proportions, due to varying temperature and salinity, naturally 

 prevents one single definite point of solidification. Strictly speak- 

 ing, the solidification process in such ice can not be considered 

 completed until the last salt solidifies, namely CaCL, the eutectic 

 point of which is —55° C. ( — 67° F.). For general purposes of 

 calculation, therefore, the freezing point of sea ice is assumed to 

 occur at the eutectic point of its major salt XaCl (about 77 per cent 

 of the salt content of sea water), i. e., at approximately —22° C. 

 ( — 7.6° F.). The closer the solidification point of a sample of sea ice 

 lies to the freezing temperature of sea water, the more homogeneous 

 will be the composition of the ice, and the less the deposition of brine 

 and salt. The lower the temperature to which sea water is exposed, 

 the more rapidly will the net of ice crystals form. 



One of the most astonishing things about sea ice is the fact that it 

 is so fresh. Malmgren (1928, Tables 4 and 5), has made several 

 determinations of the salinity of the ice of the polar cap, showing it 

 to be subject to a relatively wide range — 0.05 0/00 to 14.59 00. A 

 salinity of 7 0/00, therefore, may be taken as the salt value of ordi- 

 nary sea ice.^ It would be interesting to learn what is the average 

 salinity of the Arctic water from which ice of 7 0/00 is formed. The 

 salinity of the surface layers of the north polar ocean, outside of the 

 continental slope, as Nansen (1928, p. 11) has pointed out. are greatly 

 diluted by the land drainage from the Eurasian side. Xansen esti- 

 mates that the inflow of fresh water from Siberian rivers alone is 

 sufficient to cover the ocean from the Asiatic coast to the pole each 

 year, with a surface film 3i4 feet thick. The distribution of salinity 

 of the water as found along the Frani's track was 28.40 0/00 to 29 0/00 

 to a depth of 30 meters, 90 miles north of the New Siberian Islands, 

 and only 31.7 0/00 to 33 0/00 at 30 meters, within 300 miles of the polo. 

 In view of these observations we may conclude that the surface hivei> 

 in the polar basin, i. e., the water subjected to freezing, vary little from 

 a mean salinity of 31 0/00." The average salinity of sea ice. ai)i)roxi 

 mately 7 0/00, shows, therefore, that only about one-fourth of the ^ 

 total quantity of salts of sea water enters its ice. Xansen. on the 

 drift of the Fimrn^ made no determinations of the salinity of sea ice. 

 but from our information on the distribution of the salinity in the 

 Arctic Ocean, the MamVs ice experiments can. without criticism, be 

 compared directly with the Frains hydrographical observations. 



The rate at which sea Avater freezes is a very important factor 

 in determining the percentage of salt imprisoned in the ice. The 

 more rapid the process, the more sudden the pure ice crystals im- 

 pregnate the water, catching wathin their meshes a large quantity 

 of brine drops. The saltiest piece of ice that Malmgren (1928, j). 



\ 



" Weyprwht (1897, p. .58) during the drift of the rajctihoff found the salinity of a 

 piece of thin iee formed rapidly under low temperature in the Arctic to he 25 000, nnil 

 this is probahly as salt as sea ice ever is. 



"No otlier ocean in the world can show sucli low salinities more than a few miles o"' 

 from land. The inflow of land water around the basin's border fails to explain a salinl'.^ 

 uniformly so low ovei- the entire expanse. Probably the absence of evaporation and tlj'' 

 I)resence of ice are the two factors chiedv respdusi'ble, combiiKHl with the seasonal oycl'' 

 of freezing and melting, repeated throughout the years since the present ca|i liecanic 

 permanent. 



