7.5 years was required for it to be transported from its source region. The age resolution for the 3He/85Kr ratio is 1± to ±4 years.
In the Southern Hemisphere, tritium/3He dating is severely limited, due to the low tritium concentration in surface waters and a relatively high mantle-3He component. In addition, the CFC-11/CFC-12 ratio is fairly insensitive for the time after about 1975. This situation limits the palette of available tracer ratios for dating of young waters in the Southern Hemisphere. However, recent measurements of tritium and CFC-11 in the Weddell Sea indicate that the CFC-11/tritium ratio might be a useful parameter for tracer-ratio dating in this region (Schlosser et al., 1991b). The ratio must be corrected for radioactive decay of tritium. The decay-corrected CFC-11/tritium ratio increases as a function of time (Figure 14) at a rate of roughly 10 percent per year, yielding a time resolution of about ± 1 to ± 2 years.
Since concentrations of transient tracers are continually increasing in the ocean, a time series of a single tracer usually does not reveal ocean variability directly. To determine whether there is variability, a model must be applied to the data. Depending on the hydrographic situation, the models can range from simple box models to complex general circulation models. If the model parameterizes water-mass formation (including exchange with the atmo-
sphere), circulation, and mixing in a reasonable way and can fit the time series with a set of constant (with respect to time) parameters, then little or no variability is suggested. If one or more of the parameters used in the model (for example, exchange rates between the individual water masses) must be changed as a function of time to fit the data, then there is variability. In special cases qualitative information can be derived from a single tracer time series. For example, if the concentration of a tracer with increasing surface concentrations increases in the deep water for a certain period of time and then suddenly stays at a constant level, there is a strong indication that the deep-water formation rate has changed.
Time series of tracer/tracer ratios might be used in special cases to determine whether there is variability without a model. If there is no variability in formation, circulation, or mixing (mixing has to be weak), water-mass ages estimated from the ratios should not change with time. Changes in the age would indicate variability in the formation rate of a specific water mass, its exchange rate with other water masses, or its transfer time from the formation region to the location of observation.
Straightforward application of tracer-ratio dating might work best on isopycnals in the thermocline (Thiele and Sarmiento, 1990) or in advectively dominated boundary currents (Smethie, 1993; see below).
CFC-11 and CFC-12 observations in the deep core of the DWBC in the Atlantic between 32° and 44°N were taken in 1983, 1986, and 1990 (Figure 15). Four sections across the DWBC were obtained in 1983, three sections in 1986, and five sections in 1990, with some overlap between the three surveys. In all of these sections there were two high-CFC-concentration cores adjacent to the western boundary: an upper core with a potential temperature of about 4.5°C at about 850 m depth, and a deep core with a potential temperature of about 2°C at about 3500 m depth (Figure 16). The upper core appears to be formed by wintertime convection in the southern Labrador Sea region (Pickart, 1992a), but is too warm to be classical Labrador Sea Water (Pickart, 1992a; Fine and Molinari, 1988). The lower core consists of the waters that flow over the Denmark Strait sill and the Scotland-Iceland Ridge (Smethie, 1993). The source waters for both cores interact extensively with the atmosphere and thus become tagged with high levels of CFCs. The high CFC concentrations within the cores indicate that the water in these cores formed more recently than adjacent deep water. This is also apparent in the CFC-11/CFC-12 ratios, which are higher in the cores than in the adjacent water (Figure 16).