Ocean sciences Inhomogeneous distribution of tracers in the abyssal southern ocean C. T. A. CHEN and M. R. RODMAN College of Oceanography Oregon State University Corvallis, Oregon 97331 The deep and bottom waters around Antarctica have been reported to be quite homogeneous in radiocarbon con- centration and to a somewhat lesser extent in tritium content. The presence of a bottom layer with clearly marked higher tritium values than in the deep water zone [e.g., typical bottom values of more than 0.1 tritium unit (Tu) relative to deep water of about 0 Tu; 1 iu = 10 18 T/H where T/H is the isotopic hydrogen ratio 3 H/ 1 H] is well documented (Michel 1978; Weiss, Ostlund, and Craig 1979 and Jenkins et al. 1983). This situation is inter- preted to mean that bottom waters contain significant percent- ages of relatively young waters (i.e., waters of recent surface origin). Chen and Poisson (Antarctic Journal, this issue) also indicate that the antarctic bottom water (AABW) contains some excess carbon dioxide, again suggesting that a portion of AABW was recently in contact with the atmosphere. Radiocarbon content, on the other hand, has sometimes been reported to be very uniform regionally in the deeper waters of the Antarctic, below the salinity and potential temperature maxima and the oxygen minimum. Values reported are - 0.163 (Broecker 1979; Fiadeiro 1982), —0.158 (Stuiver, Quay, and Os- tlund 1983) and - 0.160 (Bolin 1983). These results may suggest that the bottom waters do not contain waters of recent surface origin. Contrary to these reports, however, our study confirms the report of Weiss et al. (1979) that radiocarbon is also non- homogeneous in deep to bottom waters, similar to the trend observed for tritium and excess carbon dioxide. The radiotracer and hydrographic data used in this study are from the antarctic GEOSECS (Geochemical Ocean Section Studies) stations in the three oceans (Ostlund, Dorsey, and Brescher 1976; Ostlund et al., 1979; Ostlund, Olson, and Brescher 1980; Stuiver and Ostlund 1980; Bainbridge 1981; Broecker, Spencer, and Craig 1982; Weiss et al., 1983). The excess carbon dioxide results are from Chen (1984). The combined antarctic GEOSECS tritium data are plotted against potential density (o) in figure 1. As mentioned above, higher tritium is found in the bottom water below cr. = 27.83. Excess carbon dioxide values for the antarctic GEOSECS stations are also plotted vs. o in figure 2. The young water signature (higher excess carbon dioxide) near the bottom is weak because of the scatter of the results. Results obtained from the 1981 Weddell Polynya Expedition (figure 3 in Chen and Poisson, Tritium (T U) Tritium (TU) 0.0 0.5 1.0 0.0 0.5 1.0 27.3 27.4 27.5 71 27.6 27.7 27.8 27.9 Figure 1. A. Composite GEOSECS potential density (if0) vs. tritium units (Tu) for Atlantic (GS76,77,78,82,89,91), Pacific (GS287), and Indian (GS430,431 ,432,433) antarctic stations. B. Curve fit by eye resulting from averaging data In figure 1-A in density bands. Points with no error bars Indicate single-datum points. Antarctic Journal, this issue), however, clearly show the increase in excess carbon dioxide in the waters below cr 8 = 27.83. A similar plot for radiocarbon (C-14) is shown in figure 3. Again, the lowest value is centered around a = 27.83, which is at or just below the maximum-temperature and maximum- salinity layers. These layers are dominated by the old circum- polar deep water low in C-14. Weiss etal. (1979) reported AC-14 values of 0.152 for the antarctic bottom water at - 0.4°C and - 0.16 for the warm deep water at 0.6°C. Figure 3 also shows an increase of 0.008 in C-14 in the bottom water relative to the minimum C-14 value in the deep water. The increase in AC-14 is in agreement with the tritium and excess carbon dioxide data, and suggests that the bottom water does contain a portion of relatively young water. 68 ANTARCTIC JOURNAL