NO. 9 MARINE INVERTEBRATES, ALASKA — MacGINITIE II 



years there, and some at Point Barrow may be more than four years 

 old. There is good evidence for considering the Arctic pack as a great 

 moving field, the direction of which is from northeast Siberia to the 

 east of Greenland. 



One factor that has not been given the attention it deserves is the 

 rate of melting. The writer believes that if it were not for the piling 

 up of the ice during storms and its subsequent incorporation in the 

 Arctic pack there would be little ice in the Arctic Ocean by Septem- 

 ber of each year. Probably nowhere in the Arctic does the water 

 freeze to a thickness of more than 6 feet (certainly not more than 7 

 feet) in any one winter. The winter of 1949-50 at Point Barrow was 

 very cold. The average daily temperature for the month of February 

 was the coldest on record, —23.8° F., yet the ocean ice alongshore 

 was only 68 inches thick. The thousands of lakes and ponds freeze 

 to a depth of about 7 feet and this ice melts entirely in summer. In 

 cruising among the floebergs (pi. 6, fig. 2) one often sees floes that are 

 only I or 2 feet thick. These are from leads that froze over without 

 subsequent piling up of the ice, and such floes entirely disappear by 

 September. However, there is no way of knowing how thick they 

 were originally. 



Ice can form deeper in regions where fresh water runs under the 

 ocean ice — for example, in and near Elson Lagoon it may be as much 

 as a foot thicker than nearby shore ice. Since fresh water floats on 

 salt water, when it is carried out under the ice sheet the lack of wind 

 disturbance allows it to spread out under the ocean ice for a con- 

 siderable distance. However, fresh water freezes at a higher tempera- 

 ture than ocean water and, since the fresh water is between the cold 

 ice above and the below-zero water beneath, it does not travel more 

 than a few miles alongshore before freezing. Hence some of the 

 alongshore ice near Elson Lagoon thickens more rapidly than that 

 offshore. 



A discussion of how the ice forms offshore, beginning in October 

 and continuing until the ice goes out the following July, may be of 

 interest. To begin with, ocean ice is different from fresh-water ice. 

 A slush forms on the surface of the ocean alongshore and gradually 

 creeps oceanward. When this slush becomes about 4 inches thick it 

 begins to solidify on top and a great sheet of ice is formed for perhaps 

 one-fourth to one-half mile seaward. Some time later an onshore wind, 

 with or without swells in the ocean water, breaks up this sheet, and 

 chunks of ice, large and small, are slid over the shoreward ice until 

 they are piled up into a ridge. (In 1949 the first ice alongshore 

 broke up and was carried in to the beach itself, forming a ridge 



