Although the causative organisms have not been definitely identi- 

 fied, William Hansen (personal commxonication) has suggested that the 

 Polar Cod Boreogadus saida is the most likely scatterer. This fish 

 is one of the most common nektonic species of the high Arctic seas 

 and has been caught at T-5 and at other ice stations. The fish's 

 gas -filled swim bladder woiold presijmably be the actual scattering 

 agent. Annxial migrations of these fish in large schools could ac- 

 count for the seasonal behavior of the scattering layer. The geo- 

 graphic distribution of the layer may be due to preferred migration 

 routes and feeding areas, although there is no ready explanation 

 for the particular geographic distribution observed. 



The origin of the thin scattering layer observed at 50 m depth 

 with the 100 KHz sounder is also obscure. This layer coincides so 

 precisely with a sharp change in density that a physical cause can- 

 not be ruled out immediately for this layer. It is possible that 

 sound is reflected specularly from the density interface. The Ross 

 echo soimder is now being calibrated to determine whether the layer 

 can be accounted for simply as a reflection. The other possibility 

 is that objects closely associated with this pycnocline scatter the 

 sound. Planktonic material, living or dead, might tend to accumu- 

 late at this interface. Observations in other oceans have also 

 shown scattering layers at pycnoclines (Weston, 1958). Weston 

 foimd that plankton trapped in the interfacial layer were the scat- 

 tering agent. Plankton tows at T-5 have shown a concentration of 

 the pteropod Spiratella helicina (Limacina helicina ) at a depth of 

 50 m in the Arctic Ocean and this may be the scatterer in the pres- 

 ent case (William Hansen, personal communication). The calcareous 

 shell of this planktonic mollusc would provide the contrast in 

 acotistic impedance needed to scatter sound. 



\flriatever its cause, this thin layer observed with the 100 KHz 

 instrijment is clearly a good indication of the motions of the top 

 of the Pacific water. The obsejrved period of oscillation in Figure 

 7 is about 10 min. The Vaisala or stability period was 2^ minutes 

 at 53 m, increasing to 10 minutes at 100 m. These are upper limits 

 for internal wave motions. The observed motions are thus within 

 the internal wave range and wo\Jld have a slow speed of propagation 

 at this period. 



Acknowledgements 



I am indebted to all those from the Arctic group at Lamont- 

 Doherty Geological Observatory who successfully maintained the 

 echo sounders on T-5. 



This work was supported by the Office of Naval Research and by 

 the U.S. Naval Ordnance Laboratory under contract Nonr 266(82). 



442 



