mixed. In siunmer, melt-water runoff tends to stratify this layer 

 and the sharp transition at 50 m disappears. 



The Pacific water is characterized by a temperatiire maximum of 

 -1.2 to -l.il-°C at 75 m. The layer extends from 50 to 200 m. This 

 water mass comes from the Pacific entering the Arctic Ocean throiigh 

 Bering Strait. Salinity increases with depth from about 50°/oo in 

 this layer. 



The Atlantic water is marked by a temperature maximum of 0.5° 

 C at 500 m and extends from 200 to 900 m in depth. This water mass 

 enters the Arctic Ocean near Spitsbergen and then spreads at depth 

 throughout the Arctic Ocean. Below the Atlantic water, Arctic deep 

 water extends to the bottom with nearly constant salinity and tem- 

 peratures of -0.3 to -0.1j-°C. 



The 12 KHz scattering layer occurring between depths of 50 and 

 200 m therefore coincides with the Pacific water mass. This is a 

 region of high stability due to a sharp increase in salinity. In 

 polar waters where temperature variations are small, density is 

 primarily controlled by salinity. 



The thin layer at 50 m recorded by the 100 KHz sounder coin- 

 cides with the top of the Pacific water where the steepest gradient 

 in water properties within the entire column is found. Detailed 

 oceanographic profiles were made on several occasions to check the 

 coincidence of this layer and the sharp change in water properties. 

 The Hansen bottles were visible on the record and their position 

 with respect to the layer could be determined exactly. The sampl- 

 ing interval between Hansen bottles was 2 m in the vicinity of the 

 thin 50 m layer. In all cases there was agreement between depth of 

 the scattering layer and the interface between the surface and 

 Pacific waters. The increase in numbers of individual scatterers 

 below this layer may be connected with the increased stability of 

 the water column. Organisms could maintain their level more easily 

 in the sharp density gradient below 50 m than in the homogeneous 

 water above 50 m. 



Origin of the Arctic Scattering Layers 



The seasonal behavior of the 12 KHz scattering layer which 

 shows considerable variation from year to year strongly suggests 

 that it is caused by animals. The marked diurnal behavior which 

 correlates with light conditions during the autvmnal equinox also 

 supports a biological cause. No known physical properties of the 

 water vary as irregularly as this scattering layer. Attempts to 

 capture the responsible animal have not been especially success- 

 f\il. The quick changes in the layer suggest a highly mobile ani- 

 mal which would be difficult to net. 



441 



