Tritium southern hemisphere

The convolution integral and the Exponential Piston Flow Model (EPM) were used to relate measured tracer concentrations to historical tracer input. The tritium input function is based on tritium concentrations measured monthly since the 1960s near Wellington, New Zealand. CFC and SF6 input functions are based on measured and reconstructed data from southern hemisphere sites. The EPM was applied consistently in this study because statistical justification for selection of some other response function requires a substantial record of time-series tracer data which is not yet available for the majority of NGMP sites, and for those NGMP sites with the required time-series data, the EPM and other response functions yield similar results for groundwater age. 

The contrast between these tracers is rooted in the pronounced meridional asymmetry in the tritium pattern (lower-southern-hemispheric values), compared to the more balanced CEC distribution. Another aspect is the northern midlatitude maximum in tritium compared to a poleward enhancement of CEC-11. The latter is a result of the temperature dependence of CEC solubility, but the former arises from the progressive sequestration of tritium in the subtropical gyres, a phenomenon similar to that of radiocarbon (see Broecker et al., 1985),... 

The surface waters of the ocean are the largest sink for tritium. Transfer into the surface ocean occurs by water vapor exchange, precipitation and continental run-off (e.g., Weiss et al. 1979 Weiss and Roether 1980). Most of the nuclear weapons tests were performed in the northern hemisphere, leading to a strong asymmetry in the global north-south distribution of bomb tritium in the ocean surface waters. Concentrations in the northern hemisphere are relatively high compared to those in the southern hemisphere (Weiss and Roether 1980 Fig. 1). 

Figure 1. Tritium concentration in marine precipitation at 50°S and 50°N as a function of time (after Weiss and Roether 1980). Note the different scales for the tritium concentrations in the Northern and Southern hemisphere.

Presently, maximum tritium concentrations in the surface waters of the oceans are approximately 1.5 to 2 TU in the northern hemisphere and 0.20 to 0.75 TU in the southern hemisphere. Regions that receive high concentrations of runoff such as the Arctic Ocean or locations surrounded by continents such the Mediterranean and Red seas exhibit elevated tritium concentrations. 

He/ He precision of 0.2%, the age resolution is calculated and plotted in Figure 3. For waters with tritium concentrations of about 50 TU (shallow groundwater) an age resolution of about one week can be achieved. For northern hemisphere ocean surface waters with tritium concentrations of about 2 TU the time resolution is of the order of 6 to 12 months, and for Southern Hemisphere surface waters ( 0.5 TU) the time resolution is no better than about 3 years if it is possible to separate the tritiogenic He from the total He measured in the water. The age resolution given here is that of the apparent tritium/He age and does not take into account systematic errors such as mixing. 

Tritium, however, was largely released directly to the stratosphere, where it is oxidized to tritiated water vapor. It is subseqently mixed back into the troposphere, predominantly at mid-latimdes and rapidly rained out as tritiated rainfall (e.g., see Weiss and Roether, 1980). Thus, there is a far greater asymmetry between the two hemispheres, and characteristically different deposition histories. Figure 7 shows the hrst two EOFs of the tritium in precipitation as analyzed by Doney (Doney et ai, 1992) from IAEA data. The pattern decomposes into two dominant factors a northern impulse (the largest component) and a smaller, more diffuse southern component. 

The northern component reflects the more immediate injection of bomb tritium into the northern hemispheric hydrologic system because virtually all of the major detonations occurred in the Northern Hemisphere. Prior to the bomb tests, the concentration of natural tritium in rainfall was of the order of 5-10 tritium units (1 TU = 1 tritium atom per 10 normal hydrogen atoms). During the mid-1960s, tritium concentrations of more than several thousand TU were recorded in higher latitude, mid-continental locales such as Chicago, USA or Ottawa, Canada. The southern component, on the other hand, is much weaker in amplitude and more smeared out in time... 

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