Chemical tracers

Oxygen was seen in §3.4 to be useful as a tracer of deep water movement. Other chemicals can also be used to trace water movement, either because the compounds or isotopes concerned are inert, or because reaction, or radioactive decay, of the compound is known, at least qualitatively. These tracers have various sources: from the atmosphere, within the water column, or from the sea floor. The radioactive isotope of carbon, 14C, for instance, shows a similar pattern horizontally to oxygen. Fig. 3.20 shows the ratio of the 14C to the most common carbon isotope, 12C, at a depth of 4000 m around the world ocean. This radioactive isotope is formed in the atmosphere by the reaction of a cosmic ray from beyond the Earth with nitrogen atoms in the atmosphere. The radioactive decay of 14C through the emission of an electron (e-, or a (3 particle),

with a half life of 5700 years, means that once carbon, and hence 14C atoms,

Fig. 3.20. Distribution of 14C/C ratios at a depth of 4000 m in the main basins of the global ocean. The data is expressed as the part-per-thousand difference from the 14C/C ratio in the atmosphere prior to the onset of industrialization and normalized to a constant

60 20 20 60 IOO 140 180 140 100

Fig. 3.20. Distribution of 14C/C ratios at a depth of 4000 m in the main basins of the global ocean. The data is expressed as the part-per-thousand difference from the 14C/C ratio in the atmosphere prior to the onset of industrialization and normalized to a constant

13C/C ratio. [Fig. 5.4 of Broecker and Peng (1982). Reproduced with permission of W. S. Broecker.]

60

Longitude

60 20 20 60 IOO 140 180 140 100

13C/C ratio. [Fig. 5.4 of Broecker and Peng (1982). Reproduced with permission of W. S. Broecker.]

60 20 20 60 100 140 180 140 IOO

Longitude enter the sea there is no new source of 14C and so the proportion of the unstable isotope declines as the water mass originally in contact with the air is subducted. Fig. 3.20 therefore depicts the movement of water from the North Atlantic and Weddell Sea into the deep regions of the entire ocean. This tracer suggests that the northeast Pacific contains the oldest water and that it has taken approximately a thousand years for the water to reach this location since it was last at the surface.

Apart from oxygen and 14C there are some anthropogenic oceanic tracers which have only been added to the atmosphere, and therefore the ocean, in quantity since about 1940. These tracers can be used to track both where water is being subducted from the surface, and also where it travels once it sinks. The tracers of concern are freons, or chlorofluorocarbons, tritium and helium. The first of these are used extensively in industry as refrigerants, propellants and solvents, although their use is being phased out as part of the response to the ozone problem (see §7.2.1). Tritium, a radioactive isotope of hydrogen (3H), is a product of atmospheric testing of nuclear weapons. Its decay product, through the same process as reaction (3.14), is helium, and because of the relatively short life time of tritium (12.26 years) the combined tritium and helium concentration can be used to determine the age of the water since it was last ventilated at the surface (see §3.1 for a discussion of how tritium enters the ocean).

Use of freons as tracers is still fairly novel, and hampered by the large volumes of water required to extract experimentally detectable amounts of these gases. The principal technique for tracking water masses with these tracers stems from the different emission histories of the freon gases. The principal two that have been used industrially are Freon-11 (CFCl3) and Freon-12 (CF2Cl2). From knowing their historical atmospheric concentrations (which are mixed

Fig. 3.21. The tritium-helium age, in years, on an isopycnal that surfaces in the northern Atlantic but is below the thermocline in the sub-tropical North Atlantic. [Fig. 4a of Jenkins (1988). Reproduced with permission of The Royal Society from W. J. Jenkins, Phil Trans. Roy. Soc. London, A325,1988,43-59.]

Fig. 3.21. The tritium-helium age, in years, on an isopycnal that surfaces in the northern Atlantic but is below the thermocline in the sub-tropical North Atlantic. [Fig. 4a of Jenkins (1988). Reproduced with permission of The Royal Society from W. J. Jenkins, Phil Trans. Roy. Soc. London, A325,1988,43-59.]

throughout the troposphere globally in a few months) and examining the ratios of CFC-11/CFC-12 the age and spreading of a water mass can be determined. These gases have no natural sources to bias the observations.

The use of tritium/helium is somewhat similar to freons. It has been used as a chemical tracer since the 1970s. The atmospheric concentration, which has a geographical bias due to the location of hydrogen bomb tests and the limited time over which they occurred (late 1950s-1960s), are known. Therefore the ratio of tritium to the total tritium and helium level can be used to find the age, and spreading, of water in the intermediate and deep water. This tracer is complicated by the existence of helium in both the atmosphere and ocean naturally. The atmospheric concentration of helium is extremely low, as can be inferred from its absence from Table 1.1, but it will contribute a weak background signal. Also, helium is released from areas of geothermal activity under water, particularly along oceanic ridges. This adds to the background, and will be particularly handicapping for study of very deep water movement. Therefore the helium measured in a tracer sample has to be corrected for these background levels. Fig. 3.21 shows the tritium-helium age for an isopycnal surface in the main thermocline of the North Atlantic Ocean. This shows that water requires about ten years to move half way around the sub-tropical

Another, non-anthropogenic, tracer which has recently been used to provide information about oceanic flow patterns is the ratio of the rare isotope of oxygen, 18O, to the common isotope, 16O, in ordinary sea water. 18O is not radioactive, but as we will see in §6.1.1, there is a temperature-dependence of the 18O:16O ratio in sea water ultimately related to preferential evaporation of 16O at the expense of 18O. The 18O abundance in sub-surface sea water can therefore be related to its surface origin. Fig. 3.22 shows a plot of S18O versus salinity (see (6.1) for the definition of S18O) for global ocean waters. Several patterns are discernible in this plot which illustrate the conservative nature of this tracer. Much of the ocean's water, particularly in the Atlantic, lies along the extra-tropical mixing line. This shows mixing between waters of polar origin (Antarctic Intermediate Water and, ultimately, run-off from polar regions with a 518O of around -21%c)

gyre.

Fig. 3.22. Scatterplot of S18O versus salinity for global ocean waters with a salinity >32 psu and a S18O > -4%o [Fig. 3a of Bigg and Rohling (2000). Reproduced with permission of the American Geophysical Union.]

Fig. 3.22. Scatterplot of S18O versus salinity for global ocean waters with a salinity >32 psu and a S18O > -4%o [Fig. 3a of Bigg and Rohling (2000). Reproduced with permission of the American Geophysical Union.]

32.5 33.0 33.5 34.0 34.5 35.0 35.5 36.0 36.5 37.0 37.5 38.0 38.5 39.0 39.5 40

Salinity

32.5 33.0 33.5 34.0 34.5 35.0 35.5 36.0 36.5 37.0 37.5 38.0 38.5 39.0 39.5 40

Salinity and the evaporative waters of the sub-tropical gyres, where the surface water's 518O has been enriched. In highly evaporative confined sub-tropical seas further enrichment in S18O occurs, and a Mediterranean mixing line arises with the ocean sub-tropical water. In the North Pacific a further type of mixing occurs in subducting waters between the main mixing line and relatively fresh, but subtropical, waters in the Kuroshio Current. There is also a separate, steeper, mixing line between Antarctic Bottom Water and the main mixing line, starting near a salinity of 35 and a S18O of 0%c, but the scatter of points makes this hard to see in Fig. 3.22. As equatorial waters have rainfall from locally derived evaporation they show a mixing line with practically zero gradient, along S18O = 0%c. These mixing lines should not be taken to infer that surface water is determined purely by local evaporation-precipitation balances. Several studies have shown the importance, and often dominance, of advection in determining local near-surface S18O.

This tracer has been used to suggest that Antarctic Bottom Water formed in the Weddell Sea may have two sources with similar temperature and salinity: one close to the shelf-ice edge, where melting of very 18O-depleted glacial ice has lowered the local sea water's 18O:16O ratio, and another further off-shore at the sea-ice edge, where more characteristic oceanic 18O:16O ratios for these latitudes are found. It has also helped to show how river water enters the ocean, particularly in the Arctic, and to show that significant sea-ice melt contributes to the northwest Atlantic through advection and melting in the East Greenland Current.

Further reading

A complete reference list is available at the end of the book but the following is a selection of the best books or articles to follow up particular topics within this chapter. Full details of each reference are to be found in the Bibliography.

Broecker and Peng (1982): An invaluable guide to ocean chemistry. Well written with a very comprehensive list of pre-1982 references. Also a very good guide to the chemical tracers section.

Ludlam (1980): A comprehensive book on cloud processes.

Mcllveen (1992): A very readable section on cloud microphysics.

Open University Oceanography Series (1989-2001): The Sea Water and Ocean Chemistry volumes of this series give an excellent introduction to the properties of sea water, general ocean chemistry, and the links between chemistry and ocean sediments.

Pinet (1992): A general oceanography text with good introductory material on oceanic chemistry.

Rogers and Yau (1991): A good text for the cloud microphysicist.

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