Ocean sciencesInhomogeneous distribution of tracers

in the abyssal southern ocean

C. T. A. CHEN and M. R. RODMAN

College of OceanographyOregon State UniversityCorvallis, Oregon 97331

The deep and bottom waters around Antarctica have beenreported 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 highertritium values than in the deep water zone [e.g., typical bottomvalues of more than 0.1 tritium unit (Tu) relative to deep water ofabout 0 Tu; 1 iu = 10 18 T/H where T/H is the isotopic hydrogenratio 3H/ 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 surfaceorigin). Chen and Poisson (Antarctic Journal, this issue) alsoindicate that the antarctic bottom water (AABW) contains someexcess carbon dioxide, again suggesting that a portion of AABWwas recently in contact with the atmosphere.

Radiocarbon content, on the other hand, has sometimes beenreported to be very uniform regionally in the deeper waters ofthe Antarctic, below the salinity and potential temperaturemaxima 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 suggestthat the bottom waters do not contain waters of recent surfaceorigin. Contrary to these reports, however, our study confirmsthe report of Weiss et al. (1979) that radiocarbon is also non-homogeneous in deep to bottom waters, similar to the trendobserved for tritium and excess carbon dioxide.

The radiotracer and hydrographic data used in this study arefrom the antarctic GEOSECS (Geochemical Ocean SectionStudies) stations in the three oceans (Ostlund, Dorsey, andBrescher 1976; Ostlund et al., 1979; Ostlund, Olson, andBrescher 1980; Stuiver and Ostlund 1980; Bainbridge 1981;Broecker, Spencer, and Craig 1982; Weiss et al., 1983). Theexcess carbon dioxide results are from Chen (1984).

The combined antarctic GEOSECS tritium data are plottedagainst 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 stationsare also plotted vs. o in figure 2. The young water signature(higher excess carbon dioxide) near the bottom is weak becauseof the scatter of the results. Results obtained from the 1981Weddell Polynya Expedition (figure 3 in Chen and Poisson,

Tritium (T U) Tritium (TU)0.00.51.0 0.00.51.0

27.3

27.4

27.5 71

27.6

27.7

27.8

27.9

Figure 1. A. Composite GEOSECS potential density (if0) vs. tritiumunits (Tu) for Atlantic (GS76,77,78,82,89,91), Pacific (GS287), andIndian (GS430,431 ,432,433) antarctic stations. B. Curve fit by eyeresulting from averaging data In figure 1-A in density bands. Pointswith no error bars Indicate single-datum points.

Antarctic Journal, this issue), however, clearly show the increasein 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 isat 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-14values 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 anincrease of 0.008 in C-14 in the bottom water relative to theminimum C-14 value in the deep water. The increase in AC-14 isin agreement with the tritium and excess carbon dioxide data,and suggests that the bottom water does contain a portion ofrelatively young water.

68 ANTARCTIC JOURNAL

2

2

2

2

2'

2

2

27.5° b

27.6

27.7

08

27.8

AC-14 (l0)-150-100-50 AC-14 (I0)

-100-50

.7-.-

4.Figure 3. A. Composite GEOSECS potential density (on) vs. radiocar-bon (C-14) for Atlantic, Pacific, and Indian antarctic stations. B.Curve fit by eye resulting from averaging data in figure 3-A in densitybands. Points with no error bands indicate single-datum points.

27.9

02040 02040

excess CO2 . jjmol/kgexcess CO2 , jimol/kg

Figure 2. A. Composite potential density (os) vs. excess carbondioxide (CO 2) for Atlantic GEOSECS stations. B. Curve fit by eyeresulting from averaging data in figure 2-A in density bands. Pointswith no error bars indicate single-datum points. ("mol/Kg" denotesmicromoles per kilogram.)

This work would not be possible without the excellent GEO-SECS data. We acknowledge the financial support provided bythe Department of Energy (subcontract 19X-89608C under Mar-tin Marietta Energy Systems, Inc., contract DE-ACO5-84 or21400 with the Department of Energy) and the National ScienceFoundation (OCE 82-15053).

References

Bainbridge, A.E. 1981. GEOSECS Atlantic Expedition, Vol. 1. (Hydro-graphic data, National Science Foundation.) Washington, D.C.: U.S.Government Printing Office.

Bolin, B. 1983. Changing global biogeochemistry. In P. Brewer (Ed.),Oceanography: The present and future. New York: Springer-Verlag.

Broecker, W. S. 1979. A revised estimate for the radiocarbon age of NorthAtlantic Deep Water. Journal of Geophysical Research, 84, 3218-3226.

Broecker, W.S., D.W. Spencer, and H. Craig. 1982. GEOSECS PacificExpedition, Vol. 3. (Hydrographic data, National Science Foundation.)Washington, D.C.: U.S. Government Printing Office.

Chen, C.T. 1984. Carbonate chemistry of the Weddell Sea. (Department ofEnergy report DOE/EV/10611-4.) Washington, D.C.: U.S. Govern-ment Printing Office.

Chen, C.T., and A. Poisson. 1984. Excess carbon dioxide in the WeddellSea. Antarctic Journal of the U.S., 19(5).

Fiadeiro, M.E. 1982. Three-dimensional modeling of tracers in the deepPacific Ocean II. Radiocarbon and the circulation. Journal of MarineResearch, 40, 537-550.

Jenkins, W.J., D.E. Lott, M.W. Pratt, and R.D. Boudreau. 1983. An-thropogenic tritium in South Atlantic Bottom Water. Nature, 305,45-46.

Michel, R.L. 1978. Tritium distribution in Weddell seawater masses.Journal of Geophysical Research, 83, 6192-6198.

Ostlund, H. G., R. Brescher, R. Olson, and J.M. Ferguson. 1979. Tritiumlaboratory data report no. 8. Miami: University of Miami.

Ostlund, H.G., H.G. Dorsey, and R. Brescher. 1976. Tritium laboratorydata report no. 5. Miami: University of Miami.

Ostlund, H.G., R. Olson, and R. Brescher. 1980. Tritium laboratory datareport no. 9. Miami: University of Miami.

Rodman, M.R., and C.T. Chen. In preparation.Stuiver, M., and H.G. Ostlund. 1980. GEOSECS Atlantic radiocarbon.

Radiocarbon, 22, 1-24.Stuiver, M., P. Quay, and H.G. Ostlund. 1983. Abyssal water Carbon-14

distribution and the age of the world oceans. Science, 219, 849-851.Weiss, R. W. S. Broecker, H. Craig, D. Spencer. 1983. GEOSECS Indian

Ocean Expedition, Vol. 5. (Hydrographic data, National Science Foun-dation.) Washington, D.C.: U.S. Government Printing Office.

Weiss, RE, H.G. Ostlund, and H. Craig. 1979. Geochemical studies ofthe Weddell Sea. Deep-Sea Research, 26, 1093-1120.

1984 REVIEW 69