limit, while in other cases a definite ice concentration may 

 have defined it. Ice plots made on successive days by the 

 USS BOARFISH in the region around 72°N and l68°W would 

 indicate that the ice limits were not fixed and that the ice was 

 moving in a southeasterly direction. 



From these ice observations, it can be stated that during 

 the period 1 to 6 August 1947 the southern limits of the ice 

 lay near 72°N latitude in the range included between longi- 

 tudes l6l°W and 169°W. The extreme southern limits of the 

 drift ice at that time were found at 71°50'N latitude, as re- 

 ported by the USS BOARFISH. The southern limits of ice 

 found in this particular year were much farther north than 

 those given in the H. O. Ice Atlas. *" 



TRANSPARENCY MEASUREMENTS 



Transparency measurements of the surface layer were 

 obtained at each hydrographic station in the Bering and Chukchi 

 Seas by a visual method. A standard Secchi disc (a white 

 disc 30 centimeters in diameter) was lowered from the main 

 deck, 25 feet above sea level, and a measurement was taken 

 of the depth at which the disc disappeared from sight. It was 

 lowered further and then raised so that a second measure- 

 ment could be taken of the depth at which the disc reappeared. 



Since the readings were taken well above sea surface, the 

 measured depths are shallower than those taken from a ship 

 with a low freeboard. The depths, furthermore, are some- 

 what less in rough sea than in smooth sea, and less during 

 dim light than during bright light. These factors are believed 

 to be secondary, however, and amount to corrections not ex- 

 ceeding 20 to 25 per cent. 



The measurements shown in figure 54 show that an opaque 

 (low-transparency) region was observed to extend through 

 Norton Sound, to the eastern edge of Bering Strait. This 

 opaque region may be the effect of sediment-laden water 

 supplied by the Yukon and other Alaskan rivers. Another 

 low-transparency region occurs just north of Bering Strait 

 and is probably caused by sediment-laden water from the 

 north Siberian shelf. 



There are two main regions of relatively high trans- 

 parency: one in the northcentral portion of the Bering Sea, 

 the other in the Chukchi Sea, north of latitude 69°N. The 

 boundary between the opaque and the highly transparent water 

 in Bering Strait is remarkably sharp and coincides with the 

 region of maximum horizontal temperature gradient. 



