The Sea-water and its Physical and Chemical Properties 67 



the atmosphere and produced by photosynthesis. Usually this addition is not exceeded 

 by removal of the respiration of the organisms present and by the oxidation of dead 

 material. Super-saturation by oxygen is thus quite possible and is occasionally found. 

 The surface layer is generally, however, the layer which is nearest to equilibrium with 

 the air. In the deeper layers of the ocean, below the assimilation layer, the oxygen is 

 provided almost exclusively by transport of the water from the surface by vertical 

 and horizontal movements. On the trajectories which the water particles perform 

 there is a continuous progressive consumption of oxygen so that the oxygen supply 

 in deeper layers depends either on the distance covered since the water mass left the 

 surface or on the speed with which it moved. A stationary state is only possible when 

 the supply of oxygen by renewal of the water mass and the oxygen consumption are in 

 equilibrium. Estimation of the oxygen distribution in the deeper layers of the ocean, 

 especially of the vertical and horizontal differences in saturation, until very recently 

 gave only the "age" of the water mass, i.e. the time since it left the surface layers. 

 After that some clarification had been obtained of conditions for similar processes in 

 lakes, the chemical-biological processes of oxygen depletion in the sea were further 

 elucidated by Seiwell (1937, 1938), Sverdrup (1938) and Wattenberg (1938). The 

 last one has discussed in detail the relevant chemical-biological factors in the ""Meteor'"' 

 Report and has pointed out its great importance for a proper understanding of the 

 distribution of oxygen in the ocean. 



This distribution within the ocean shows that the explanation given can account 

 qualitatively for the oxygen producing and consuming factors mentioned above. The 

 maximum oxygen content is always found in the surface layers; in this skin layer mix- 

 ing by the wind and the waves and the turbulence due to ocean currents gives a more 

 or less even distribution that normally differs little from equilibrium with the atmos- 

 phere. The lower limit of this oxygen-rich layer, which coincides with the assimilation 

 layer, follows essentially the thermocline in the general oceanic structure. At this 

 transition layer, when it is strongly developed as is always the case in lower latitudes, 

 the oxygen content falls to a minimum. According to the geographical position of the 

 part of the ocean and the range of the annual convection at that point the depth of 

 this minimum varies between 100 and 1500 m. This oxygen-poor intermediate layer 

 is the most prominent feature of the oxygen distribution of the ocean in middle and low 

 latitudes. Below this minimum layer there is always oxygen-rich water with up to 

 70-90% saturation. As is explained later, this oxygen content of the deep-sea circula- 

 tion of the oceans originates from the major convection areas of the subpolar and polar 

 regions of the ocean where the water masses in the surface layers can sink to great 

 depths, and from there also fill the depths at middle and lower latitudes. In spite of 

 the long path travelled by these water masses there is little depletion because of the 

 low temperature and the small amount of organic material present, and the oxygen 

 content shows only a slight decrease. Figure 37 shows as an example the vertical distri- 

 bution of oxygen at about 10° S. in the South Atlantic; the vertical variation of density 

 is also shown and the density transition can be clearly seen. The right-hand side of the 

 figure shows the vertical changes in percentage oxygen saturation and in density a, at 

 a station in the North Atlantic near Greenland in the area where, according to a view 

 expressed by Nansen (1912), the North Atlantic deep water is formed and sinks 

 during the late autumn and winter. The almost constant value of the density down to 



