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Figure 6.39 Sections of CFC-12 concentration, made along ~ 20° W, from Iceland to 20° N, (a) in 1988, from RV Oceanus, and (b) in 1998, from RV Discovery. During the intervening ten years, surface water has mixed down from the surface to varying extents, depending on the latitude. In (b), the high concentrations at 1000-2000 m depth at - 50°-55° N correspond to CFC-12 which dissolved in the surface waters of the Labrador Sea and was carried down by deep convection, before spreading across to intersect the transect along 20° W. (In both sections, the ticks along the bottom indicate the locations of the hydrographie stations.)

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1800 J 2400 f 3000 | 3600 4200

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Two other tracers that have been used extensively are ,4C (carbon-14 or radiocarbon) and 3H. tritium. Both are radioactive and are introduced into the atmosphere by nuclear explosions, whence they are carried into the ocean via precipitation and run-off. Both are also produced naturally in the atmosphere by cosmic-ray bombardment. 3H has a relatively short half-life of about 12 years; l4C. on the other hand, has a half-life of 5700 years, which makes it very suitable for studying the time-scales of oceanic mixing (though corrections have to be made to compensate for 'bomb' carbon produced in the 1950s and 1960s). Indeed, naturally occurring 14C is perhaps the tracer most used to calculate ages and mixing times in the oceans.

Chemically. ,4C behaves exactly like the dominant isotope of carbon. I2C. It therefore occurs in atmospheric C02. and is found in the oceans in dissolved and particulate form in both the hard and soft tissues of marine organisms. The highest ratio of ,4C to l2C atoms is found in the atmosphere, and in surface waters in equilibrium with the atmosphere. When the carbon is removed from the air-sea interface - either carried down in solution by sinking water or through the sedimentation of organic particles (dead plankton and other organic debris) - the ,4C : l2C ratio begins to decrease as the radiocarbon decays back to NN, Therefore, the older the seawater- i.e. the longer it has been away from the surface - the lower us IJC: '~C ratio will be. The 4C ; '-C ratio of a sample of.sea water can be measured using a mass spectrometer and the result used to calculate its age.

Carbon isotope data ha^e been used to estimate a value for the residence time for water in the deep ocean of about 1500 years. However, it is now known that carbon in particulate form sinks from surface to deep waters very much faster than was originally thought, and as a result this estimate has been considerably reused. For example, in the North Atlantic, the mean residence time of water colder than 4 'C is probably closer to 200 years. This average conceals large geographical variations: for example, the residence time for water in the European Basin Ithe north-eastern North Atlantici is about 15 years, while that for the Guinea Basin (off equatorial Africa) is more than 938 years. Calculations based on the l4C distribution in the abyssal waters of the oceans indicate that average residence (or replacement) times for water below 1500 in depth are approximately 510. 250 and 275 years in the Pacific. Indian and Atlantic Oceans, respectively. By taking into account the volumes of the w ater masses involved, these residence or replacement times may be used lo estimate the rates at which the various w ater masses form.

The implication of these finite residence times for the deep water masses is thai, ultimately, the w ater must recirculate to the surface. In summary, the deep and bottom waters spread from the polar regions along the western sides of the ocean basins (Section 6.3.2. Figures 6.20 and 6.37). They spread eastwards from these deep western boundary currents (which flow counter to the surface western boundary currents) and well up through the ocean at speeds of perhaps 3-4 x I0~ m s*1 (- 3 cm day ' i.This upwelling is a more or less uniform upw aid diffusion throughout most of the ocean, but is to some extent concentrated in the northern North Pacific. In certain well-defined regions, such as the Equatorial and Antarctic Divergences, or where there is coastal upwelling (Section 4.4). rates ot upward movement of water are enhanced, and higher in the w ater column velocities may be several orders of magnitude greater (upward velocities of 2 m day 1 have been recorded in the vicinity of the thermoclinei. On returning to the upper layers of the oceans, the water rejoins the wind-driven circulation; eventually, it returns to polar regions where it enters the cycle again f igure 6.40 shows the simplified model of the deep circulation, which was derived theoretically b\ Stommel before any deep boundary currents had actually been obser\ed. You ma\ notice that according to this model the predominant flow of deep water in the Atlantic is from the northern source regions, so that the deep western boundary current Hows Southwards in both the North and South Atlantic. As discussed in Section 4.3.2. in the 1950s and 1960s, the southward-flowing deep western boundary current in the North Atlantic was observed dtrecth using Swallow floats (Figure 4.24(b)). and indirectly by means of geostrophic calculations (Figure 4.22); it is shown schematically in Figure 6.20. (lowing south from the Labrador Sea (having originated as the overflows to the east a I Greenland). In the South Atlantic, however, the flow along the bottom is actually the northward llow uf Antarctic Bottom Water, which is concentrated on the v\csietn side of the ocean (Figure 6.251 Lind over-ridden b\ the southward-flowing western boundary current of North Atlantic Deep Water. Figure 6.37 show ed both these deep western boundarv currents most clearly (cf Question 6.12).

Figure 6.40 Stommel's simplified model of the deep circulation of the world ocean, with source regions of Deep and Bottom Water in the North Atlantic and the Weddell Sea. According to this model, in the Atlantic, flow In the deep western boundary current is southwards (apart from the flow from the Weddell Sea, which travels north and then eastward with the Antarctic Circumpolar Current); in the Indian Ocean and most of the Pacific, flow in the deep western boundary current is northwards.

Figure 6.40 Stommel's simplified model of the deep circulation of the world ocean, with source regions of Deep and Bottom Water in the North Atlantic and the Weddell Sea. According to this model, in the Atlantic, flow In the deep western boundary current is southwards (apart from the flow from the Weddell Sea, which travels north and then eastward with the Antarctic Circumpolar Current); in the Indian Ocean and most of the Pacific, flow in the deep western boundary current is northwards.

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