Figure 6.37 Sections of (a) dissolved oxygen * 000

concentration (ml H) and (b) silica concentration (limol H) between South America and the Mid-Atlantic Ridge. The sections were made along 30° S (approximately) by the research vessels Melville and Atlantis in November 1976 and May 1959 respectively.

distance (km)

QUESTION 6.12 Given that North Atlantic Deep Water is characterized by low silica concentrations and Antarctic Bottom Water by high silica concentrations, can you distinguish these two water masses in Figure 6.37(b)? What striking aspect of their flow patterns may he inferred from Figure 6.37?

Silica concentration is particularly useful for studying the mixing of North Atlantic Deep Water with Antarctic waters. This is because the silica concentration of North Atlantic Deep Water is very uniform, in contrast to its temperature-salinity characteristics which, because it is the sum of various contributions, are fairly broad.

Dissolved oxygen and silica are of limited value as tracers, because without further information about their production and/or consumption in the water column we cannot use them to get an indication of when the water mass concerned was formed at the surface - that is, of its 'age'. However, during the latter part of the twentieth century, oceanographers were supplied with a number of useful tracers for which they know the exact time of entry (or at least the earliest possible time).

One group of substances that began to enter the oceans relatively recently is the chlorofluorocarbons (CFCs), which can be used in refrigeration systems and as propellants in aerosols. Chlorofluorocarbons, sometimes referred to as 'Freons' (their US trade name), are particularly useful to the oceanographer because they are relatively easy and cheap to measure at sea, in relatively small amounts of seawater. Some tracers - radioactive 39Ar and 85Kr, for example - may only be accurately measured by sampling hundreds of litres of water.

There is no natural source of chlorofluorocarbons. They were first manufactured in the 1930s, and up until the 1990s their concentrations in the atmosphere were increasing almost exponentially. Different chlorofluorocarbons have increased in concentration at different rates: for example, the atmospheric ratio of CC13F (known as CFC-11) to CC12F2 (CFC-12) rose from near-zero in the mid-1940s to between 0.5 and 0.6 in'the 1970s (Figure 6.38(a) and (b)). The CFC-11 : CFC-12 ratio of water now at depth in the ocean is a measure of the relative concentrations of the two CFCs in the atmosphere when the water was last at the surface (after allowance is made for their different solubilities). It is thus possible to 'age' the water; that is. to work out when it was last in contact with the atmosphere.

The CFC-11 : CFC-12 ratio stopped increasing in about 1977, but significant production of a third CFC - CFC-113 - started in 1975 (Figure 6.38(a) and (b)). Since then, its atmospheric concentration has increased so rapidly that it can be used to pinpoint the actual year that a water mass was last at the surface (providing it was post-1975). CFC-113 has proved particularly useful for the study of water masses that sink and spread very rapidly - North Atlantic Deep Water, for example.

The concentrations of chlorofluorocarbons may also be used simply as a 'fingerprint'. Figure 6.38(c), plotted from data collected in the late 1980s, shows concentration profiles for CFC-11 and CFC-12 (and, for comparison, dissolved oxygen) for the South Georgia Basin in the South Atlantic. The increased concentrations below 4000 m show the presence of Antarctic Bottom Water which had only recently sunk down away from the atmosphere. By tracking these elevated CFC (and oxygen) concentrations, it is possible to observe the northward spread of Antarctic Bottom Water. (The inert chemical sulphur hexafluoride (SF6) is often used similarly in short-term experiments involving the tracking of bodies of water; in this case, the marker chemical is added directly to the water to be tracked.)

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