Chapter 4 



Transport and Dispersal 



49 



Age of intermediate and deep waters 



It is generally accepted that intermediate and 

 deep waters in most parts of the oceans acquired 

 their characteristics while at or near the surface. 

 Thus the low temperature and relatively high 

 oxygen content of deep water can only be ex- 

 plained by assuming an exchange between deep 

 and surface waters. The problem of the dis- 

 posal of radioactive wastes in the deep sea has 

 stimulated the oceanographer's natural curiosity 

 as to the rate of this exchange. 



The North Atlantic receives surface waters 

 from the South Atlantic and loses deep water 

 to the South Atlantic. Assuming a surface flow 

 from the South to the North Atlantic of 6 

 million cubic meters per second (Sverdrup 

 et al., 1942, p. 685), and considering only 

 the upper kilometer of the North Atlantic to 

 be affected, the mean replacement time is about 

 140 years. The gyral in the North Atlantic, 

 which includes the Gulf Stream, carries about 

 ten times the volume of water exchanged be- 

 tween the South and North Atlantic, so that 

 the mean circulation time is only about one- 

 tenth the replacement time. 



This surface exchange between the North and 

 South Atlantic is balanced by a deep current 

 from North to South. The mean displacement 

 time for the deep water of the North Atlantic 

 (2000-4000 meters) is calculated as about 250 

 years. This time is in reasonable agreement 

 with more recent estimates of the age of the 

 deep water discussed below. 



Between these surface and deep layers are 

 the intermediate waters which appear to circu- 

 late even more rapidly. Deacon (1933) calcu- 

 lated rapid rates of northward flow of the 

 Antarctic intermediate water in the South At- 

 lantic, based upon alternate maxima and minima 

 in the concentrations of oxygen in the oxygen 

 minimum layer. These were interpreted as rep- 

 resenting annual cycles when the waters were 

 formed at the surface. He estimated a transit 

 time of about 4^ years between the Antarctic 

 convergence and the equator. Seiwell (1934) 

 has similarly computed rapid flows and a mean 

 transport time of 7-8 years for the drift of the 

 oxygen minimum layer of the North Atlantic 

 Ocean. Deacon's and Seiwell's interpretations 

 have been questioned (see Riley, 1951, p. 77) 

 on various grounds. However, their rates of 

 flow agree with direct current measurements 



at comparable depths (see earlier) which also 

 indicate rapid rates of circulation. 



The deep outflow from the Mediterranean 

 sinks from sill depth to 1000-1500 meters in 

 the North Atlantic Ocean. This water, although 

 much diluted by Atlantic water, is characterized 

 by relatively high salinity and temperature, and 

 spreads out in a sheet which may be identified 

 in most of the temperate North Atlantic, and 

 some spreads into the South Atlantic. It can be 

 readily identified near Bermuda, 2500 miles 

 from its source. Iselin (1936) computed that 

 sufficient excess salt would be produced by the 

 Mediterranean outflow to produce the observed 

 anomaly in 12-15 years. He pointed out that 

 the actual replacement would be more rapid 

 because he neglected admixture of Atlantic 

 water in the immediate vicinity of the Straits 

 of Gibraltar. Defant (1955) has evaluated the 

 mixing processes involved in dissipating the 

 Mediterranean water within the Atlantic Ocean, 

 and has concluded that the total accumulation 

 in the Atlantic Ocean represents the contribu- 

 tion resulting from six years of flow through 

 the Straits of Gibraltar. The rapid dissipation 

 of this large water mass at mid depths suggests 

 a more rapid circulation than had been gen- 

 erally accepted for intermediate waters. 



During recent years other lines of investiga- 

 tion have led to the belief that the overturn of 

 water in the ocean basin takes place in less 

 than a thousand years and probably in 200 

 years or less. Evidence supporting this belief 

 follows. (Carbon-l4 and carbon dioxide ex- 

 change estimations are discussed in greater 

 detail by Craig elsewhere in this report.) 



1. Heat fiow measurements: Measurements re- 

 ported by Revelle and Maxwell (1952) have 

 shown a heat flow through the floor of the 

 Pacific Ocean of 1.2x10"'' calories per square 

 centimeter per second, or 38 calories per square 

 centimeter per year. If not dissipated by circu- 

 lation and mixing, this heat flow would lead to 

 warming of the deep and bottom water during 

 its passage from the Antarctic to the equator. 

 From considerations of meridional circulation, 

 observed temperature gradients and mixing in 

 the deep sea, Revelle and Maxwell estimate 

 that the deep water is replenished in less than 

 1000 years. 



2. Secular change of oxygen: Worthington 

 (1954) has shown that the North Atlantic 

 Deep Water has suffered a loss of dissolved 



