48 



Atomic Radiation and Oceanography and Fisheries 



to a depth of 200 meters is equivalent to about 

 200 years. Gololobov (1949) has computed 

 the mean age of the deep water on the basis of 

 the annual contribution of phosphorus in the 

 river inflow and the quantity accumulated in 

 the depths. This computation indicates an 

 accumulation time of 5600 years. 



The Arctic Basin receives its major inflow 

 north of Scotland and a much smaller inflow 

 through the Bering Strait. Additional sources 

 are from the river runoff and excess of precipi- 

 tation over evaporation. The outflow is pri- 

 marily through the Denmark Strait (Sverdrup 

 et al., 1942, p. 655). These flows would pro- 

 vide a volume equal to that of the Arctic Ocean 

 in about 160 years. The Arctic is also stratified 

 because of the addition of fresh water from 

 rivers and melting ice, and it is not known how 

 isolated some of the waters in the deeper basins 

 may be. However, recent analyses have shown 

 that the deeper water in the Arctic Ocean is 

 far from anaerobic, so that it seems unlikely 

 that this water can be considered as isolated 

 from the circulation. 



The Mediterranean is a basin in which 

 evaporation exceeds precipitation and runoff^. 

 Through the Strait of Gibralter there is an 

 inflow of oceanic surface water and a sub- 

 surface outflow of high salinity Mediterranean 

 water. The exchange is sufficiently rapid to 

 replace the entire Mediterranean in about 75 

 years (Sverdrup et al., 1942, p. 647). The 

 Mediterranean is divided into eastern and west- 

 ern basins by a 500-meter sill between Sicily 

 and Tunisia, and it is not know to what extent 

 the deep waters of these basins are involved 

 in the over-all exchange. 



Deep circulation 



Most of our present knowledge of the inter- 

 mediate and deep circulation (see Sverdrup 

 et al., 1942, ch. 15) has been obtained in- 

 directly from the observed distribution of prop- 

 erties. The general uniformity of temperature 

 and dissolved substances in deep water suggests 

 that deep currents are very slow, perhaps at 

 most a few centimeters per second. But deep 

 currents cannot be computed by the geostrophic 

 method because only relative velocities can be 

 thus obtained. Furthermore, small errors in 

 the measurement of salinity or temperature 

 produce uncertainties in velocity of the same 

 magnitude as the currents being computed. The 



direction of movement in the deep and bottom 

 water has been deduced from the observed 

 distribution of properties such as salinity and 

 potential temperature, but little can be learned 

 about current speeds from such observations. 



Existing direct measurements of subsurface 

 currents have been summarized by Bowden 

 (1954). Such measurements have been made 

 since the time of the CHALLENGER Expedi- 

 tion (1873-76), but because of practical diffi- 

 culties (such as the problem in the open sea 

 of referring observations to a fixed frame of 

 reference) they have taught us little about the 

 deep oceanic circulation. The few successful 

 measurements at depths greater than 1000 me- 

 ters reported by Bowden showed mean speeds 

 ranging from "negligible" to about 13 cm/sec. 

 At nearly all stations and depths at which 

 current measurements have been made, semi- 

 diurnal tidal currents of the order of 10 cm/sec. 

 have been recorded. 



Recently measurements of subsurface currents 

 have been made in the North Atlantic by track- 

 ing for three days a neutral-buoyant float sta- 

 bilized at a given depth (Swallow, 1955 and 

 unpublished). These measurements show small 

 resultant speeds (1.7 to 9.1 cm/sec or 0.8 to 

 4.2 miles/day at depths from 600 to 1900 

 meters), tidal components of about 10 cm/sec, 

 and in two successive three-day measurements 

 at 1900 meters, a change in direction of 124°. 

 Thus it seems likely that motion below the 

 pycnocline is characterized by more variation, 

 periodic or otherwise, than previously supposed 

 and indeed that the mean drift may represent 

 only a small part of the total motion. 



Little is known about the nature and extent 

 of lateral and vertical mixing in the deep sea. 

 It is generally believed, however, that flow and 

 mixing take place along surfaces of constant 

 potential density (isentropic surfaces) and that 

 below the upper layer vertical mixing is very 

 slow except near coastlines and areas where 

 upwelling may occur (Montgomery, 1938). An 

 observation supporting this belief was reported 

 by Revelle, et al. (1955). Introduction of 

 mixed fission products below the pycnocline led 

 to the formation of a lamina of high radio- 

 activity about one meter thick and 100 or more 

 square kilometers in area. The radioactive water 

 apparently spread out along an isentropic sur- 

 face and resisted destruction by vertical mixing 

 for at least three days. 



