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| chap. 10 



particularly well-marked feature in both the eastern Atlantic and the Pacific. In 

 both the Atlantic and Pacific the minimum is less marked at the equator than 

 on either side. The fact that the minimum is less pronounced at the equator 

 has been attributed to the upwelling and/or mixing that takes place across the 

 thermocline at the equator. 



The oxygen minimum zone of the eastern North Pacific includes a greater 

 body of almost oxygen-free water than any other region in the world oceans. 

 There is a layer of water whose oxygen content is less than 0.25 ml/1, in the 

 upper one thousand meters which extends out from the coast of Central 

 America some 3000 miles and at places is as much as 800 m thick (Fig. 8). 

 The layer is not destitute of life, and some animals probably live their entire 

 lives in this zone (Kanwisher and Ebeling, 1957). 



The northern boundary of this oxygen minimum is very sharp and on any 

 given section the boundary corresponds closely with the demarcation between 

 what Sverdrup has called the Pacific Equatorial water and the "transition 



Fig. 8. The thickness of the layer of water (contours are in meters) in which the dissolved 

 oxygen is less than 0.25 ml/1. The entire layer lies above 1000 m. 



region" of the California Current (Sverdrup et al., 1942. fig. 209a). There is 

 also a sharp faunal division at this line (see Johnson and Brinton. Chapter IS. 

 page 388). The southern edge of this layer is less well marked, but the thickness 

 of the layer begins to diminish at the north edge of the countercurrent. 



The steady-state oxygen distribution below the euphotic zone represents a 

 balance between oxygen consumption and the replenishment by advection and 

 diffusion (see Sverdrup et al., 1942, p. 160). The factors controlling this balance 

 need not be the same for all areas, and an inspection of the data indicates that 

 the oxygen minima cannot be simply related to a single variable. For instance, 

 the extreme minimum off the coast of Mexico does not correspond with either 

 the zone of maximum vertical stability or maximum biological productivity. 

 Both of these maxima occur somewhat to the south of the central oxygen 

 minimum. 



4. Zonal Flow — Geostrophic Currents 



The great zonal currents of the equatorial waters are in approximate geo- 

 strophic balance. Charts of dynamic topography showing the geostrophic flow 



