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The rates of tritium input to the world oceans
Earth and Planetary Science Letters, 49 (1980) 435-446
435
(g)Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands [21
THE RATES OF TRITIUM INPUT TO THE WORLD OCEANS WOLFGANG WEISS and WOLFGANG R O E T H E R
Institut ffir Umweltphysik, University of lleidelberg, Heidelberg (Federal Republic of Germany) Received July 5, 1979 Revised version received October 12, 1979
Mean annual rates of tritium input into the ocean averaged over 5° latitude bands are presented for the major oceans, for the period 1952 1975. The rates are obtained by converting tritium concentrations in marine precipitation into net oceanic tritium input, by means of a hydrological model. The tropospheric tritium pattern is specified on the basis of available observations, and climatological means from the literature are used for the rates of evaporation and precipitation and for the relative humidity in ship's height, that enter the model. Tritium input by water vapor exchange exceeds that by precipitation about three-fold. Tritium input by river runoff and by net tropospheric tritium outflow from the continents is also accounted for. This contribution is small except for the northern Indian Ocean and the North Atlantic. The inputs have hemispheric maxima near 50 ° latitude. The northern hemisphere inputs were strongly peaked in 1963-1964, whereas temporal changes in the southern hemisphere were much more gradual. By 1972, about 75% of the total oceanic input had been received by the northern ocean. For the Pacific, the computed total input agrees with the actual tritium inventory within the limits of uncertainty (about -+20%). The global tritium inventory is estimated at 1.9 GCi in 1972, which corresponds to an average tritium yield of 0.9 kg tritium per megaton TNT equivalent of nuclear fusion.
1. Introduction The distribution o f bomb-produced tritium in the oceans has extensively been studied during the 1 9 7 2 1974 GEOSECS field program [1]. At the present time, measured tritium concentrations are available for the Atlantic [ 2 - 4 ] and the Pacific ocean [5,6]. Whereas it is evident that much information on water transport and mixing within the ocean can be gained from the tritium data, their use so far has largely been restricted to box-model evaluations. Box models are useful as low-order approximations to the behaviour of the real ocean (e.g. [7]). A special aspect is to use such models to predict the oceanic dispersion of pollutants, e.g. of excess CO2 [8]. It is the consensus that eventually dynamical numerical-model evaluations o f the tritium data [9] will provide a means to approach the physical processes in the ocean to a much higher degree. GEOSECS Publication No. 116.
Crucial to all such evaluations is that tritium boundary conditions at the sea surface be specified. Mixed-layer tritium concentrations are available for the Pacific [10] and the North Atlantic [11]. However, a concentration boundary condition a priori largely fixes the tritium concentrations in the nearsurface ocean. For this reason, a flux boundary condition often is a preferable alternative. The purpose of the present paper therefore is to provide rates of tritium input to the oceans and their temporal and areal patterns. Results for the North Atlantic have been published previously [8]. The then used procedure was to convert tropospheric tritium concentrations into rates o f tritium input into the ocean by means o f a hydrological model. Tritium input by rainout and water vapor exchange, as well as by continental river runoff and net tropospheric tritium outflow from the continents was allowed for. Good agreement was found between the computed total tritium input and the North Atlantic tritium inventory. This agreement was taken as evidence for
436 the appropriateness o f the model. In particular, the agreement gave p r o o f to the dominant role o f water vapor exchange, as previously pointed out by Craig and Gordon [12], and repeatedly discussed in the literature [ 1 3 - 1 5 ] . In this paper the same model is used to compute tritium input rates for the remainder o f the world ocean. In order to do this, the tropospheric tritium pattern is derived first, and the hydrological tritium input model is sketched. These sections are brief, because the procedures and a justification of the model have been published previously [8]. The computed inputs are presented in tables. From this information, annual-average rates o f tritium input into the ocean can be calculated for any given latitude and year. The procedure is summarized in section 4. Finally, a comparison o f the computed total tritium input with the tritium inventory taken from oceanic observations is made for the Pacific, and various features o f the tritium input are discussed.
2. The tritium pattern in marine precipitation About 3500 monthly composite precipitation data from 49 monitoring stations on ocean weather ships (OWS), small islands and at selected coastal sites [16] (cf. Table 1) are available to document the general pattern o f tritium in marine precipitation. The fact that tritium concentrations at any two monitoring stations within each hemisphere have been varying in time in near constant proportions has led us to factorize the general hemispheric tritium pattern into temporal and areal distribution functions. The mathematical procedure is described elsewhere [ 17], and results for the northern hemisphere have previously been published [8]. The pattern for the southern hemisphere has been constructed in an identical fashion. It turns out that here the pattern is less well documented (see section 5). However, the results should still be adequate, considering that only a smaller part o f the total bomb tritium has entered the southern oceans. Results for both hemispheres are summarized in Tables 2 - 7 and in Figs. 1 and 2. The temporal patterns in 50°N and in 50°S for the period 1 9 5 2 - 1 9 7 5 are given in Fig. 1. The northern hemispheric pattern is dominated by tritium pulses in 1954, 1 9 5 8 - 1 9 5 9 and, predominantly, in 1963. The
TABLE 1 Geographical positions of tritium monitoring stations [16] used for the description of temporal and areal pattern of tritium in precipitation. The time interval during which observational data have been available for this work is also given Station
Position
Time interval
Isfjord Radio Reykjavik Groennedal Lista Adak OWSJ Milford Haven Valentia Acores OWSE Bermuda OWS V Virginia Key Cape Sable Tamiami Homestead Royal Palm Hawaii WakeIsl. Puerto Rico Johnston Isl. Guam Barbados Yap Minicoy Isl. Truck Isl. Christmas Isl. Tawara Group Canton Isl. Madang Diego Garcia Ascension Isl. FufunatiIsl. Apia, Samoa Pago Pago St. Helena Fiji Isl. Rarotonga Isla de Pascua Juan Fernandez Isl. Kaitaia, New Zealand Gough Isl. Kaitoke, New Zealand Invercagill, New Zea. Marion Isl. Stanlay Falkland Isl. Campbell Isl. Argentine Isl. Halley Bay
78°N, 14°E 64°N, 22°W 61°N, 48°W 58°N, 7°E 52°N, 177°W 52°N, 1 5 ° W 52°N, 5°W 52°N, 1 0 ° W 38°N, 2 7 ° W 35°N, 48°W 32°N, 65°W 31°N, 164°E 26°N, 80°W 25°N, 81°W 25°N, 81°W 25°N, 8 0 ° W 25°N, 80°W 20°N, 155°W 19°N, 167°E 19°N, 66°W 17°N, 170°W 14°N, 145°W 13°N, 60°W 9°N, 138°E 9°N, 73°E 8°N, 151°E 2°N, 157°W 2°N, 173°E 3°S, 172°W 5°S, 146°E 7°S, 73°E 8°S, 1 5 ° W 8°S, 181°W 14°S, 172°W 14°S, 171°W 16°S, 6°W 18°S, 179°E 21°S, 160°W 27°S, 109°W 34°S, 79°W 35°S, 173°E 40°S, 1 0 ° W 41°S, 175°E 46°S, 168°E 47°S, 38°E 52°S, 58°W 53°S, 169°E 65°S, 64°W 75°S, 27°W
1964--1969 1960 1963 1962-1965 1 9 6 1 1969 1962-1969 1964-1969 1965 1958-1975 1963 1971 1962-1969 1962-1967 1962-1969 1964-1970 1964 1969 1964-1969 1964-1965 1964 1965 1962-1969 1962 1967 1963-1969 1962 1969 1964-1971 1959-1971 1968-1969 1968 1968-1969 1962-1967 1962-1972 1962-1967 1968-1972 1964-1975 1966-1971 1966 1972 1963-1975 1970-1971 1964-1975 1967-1973 1964-1971 1965 1975 1965-1975 1963-1975 1962-1975 1963-1975 1963-1975 1962-1975 1962-1975 1964-1975 1964--1975 1966 1975
437 , ,
;rU
tO00
%cso*u;
cdso*s;
50* N~
500
,
,
,
,
o. 0
~
1955
t960
1965
0
0
50%
--~=--"--.. 19/0 1975
o
t 1952
I ,
(23
southern hemispheric pattern is considerably smoother with indications of a delayed response to the northern hemispheric peaks, and the absolute concentrations there are much smaller. This is a result of the fact that there was only minor atmospheric fusion bomb testing in the southern hemisphere and none before 1968 [18], so that most of the bomb tritium observed there is tritium originally released in the northern hemisphere. The areal pattern has been separated by using the temporal distributions of Fig. 1 for stations in the respective hemispheres. This procedure is somewhat arbitrary for the near-equatorial stations, for which the temporal pattern is intermediate to those at 50°N and 50°S. However, concentrations in this latitudinal belt are comparably low so that only minor errors are introduced. As shown previously [8], the areal variation of tritium in marine precipitation predominantly is by latitude only, and Fig. 2 gives the results for the latitudinal pattern. In view of the different te'mporal distributions in both hemispheres (Fig. 1), timeintegrated tritium concentrations over the entire nuclear period, Sp (TU • yr), were chosen as the ordinate in Fig. 2, Sp being defined by:
cp(t') exp[X(t' - t)l dr'
'
00
tO0
Fig. 1. Mean annual tritium concentrations (in TU) in marine precipitation characteristic for 50°N (solid line) and 50°S (dashed line) between 1952 and 1975 (numerical values are given in Table 2). Note the difference in ordinate for the t~vo histograms.
Sp(t) = f
,
¢
.-/'"
oooo° J 0 0
o.o.~
o
'
200
o.o
-~
x
s/sds0*m
(1)
•$in 0
$ in
go io ~o ,b k
io
,b. ~
ks
~o
k
8
~o 6 io'9o
Fig. 2. Latitudinal variation of the relative tritium delivery rates Sp/Sp (SO°N) in 1972 (station positions ef. Table 1). Solid line represents best fits of the data; dashed line is a linear extrapolation from the reference station in 50°S towards higher southern latitudes. Numerical values of the relative tritium delivery rates Sp in SO°N and in 50°S (*) are given in Table 2. Numerical values for the latitudinal variation of Sp, averaged over 5° latitudinal bands i, (Sp(i)/ Sp(50 ° N) 1972 are given in Tables 3- 7. where Cp tritium concentration in precipitation (TU) and X = 0.05635 yr -1 = tritium decay constant. Pre-1952 concentrations are regarded negligible [11]. The factorization assumes that in each hemisphere the latitudinal pattern has not changed in time, but only the absolute values have varied proportional to the values in Fig. 1. Apparently, concentrations were minimal between the equator and about 20°S. Straight lines have been fitted to the data points in Fig. 2, with a break at 15°S. In the northern hemisphere the fit includes all data points between the equator and 75°N. In the southern hemisphere the data between 15°S and 50°S were used. The so-found southern hemisphere line is extrapolated beyond 50°S to estimate the tritium concentrations south of 50°S. The extrapolation was chosen because the only two stations available south of 50°S (Fig. 2) report tritium concentrations which are presumably too high to represent open-ocean conditions. This may be due to the local influence of the Antarctic continent. The curve defined by these straight lines is taken as the average tritium versus latitude variation in marine precipitation. The scatter is moderate apart from the near-equatorial region;the uncertainties of the fits (1 o) amount to about -+15% north of 20°N, -+20% between 20 and 50°S, and about -+30% between 20°N and =
438 20°S. South o f 50°S the uncertainty is considerable. There are no indications o f any systematic longitudinal trends, e.g. b e t w e e n Atlantic and Pacific, within the precision o f the fits. The time-integrated tritium concentrations are therefore assumed to be constant within a latitudinal band. In 1975 the time-integrated c o n c e n t r a t i o n in 50°S, S ( 5 0 ° S ) , a m o u n t e d to about 1/7 o f the one in 50°N, S ( 5 0 ° N ) . Between 1970 and 1975 the ratio S(50°N)/S(50°S) has changed by about 3%/yr (cf. Table 2). Monitoring stations at continental sites show a similar latitudinal pattern as do the marine stations [8]. Their absolute c o n c e n t r a t i o n s are, however, higher than those over the ocean [8,17], owing to continental re-evaporation o f tritium. In the n o r t h e r n hemisphere the tritium c o n c e n t r a t i o n s o f continental
stations are typically a factor o f 4 higher than those over the ocean at the same latitude [8]. Effects o f this continental tritium excess are included in the hydrological m o d e l below. Time-integrated o p e n - o c e a n tritium concentrations ( e q u a t i o n (1), time integration up to 1972) for 5 ° latitude bands, relative to those at 50~N, are listed in Tables 3 - 7 . To obtain m e a n tritium c o n c e n t r a t i o n s for any time, t, and latitudinal bands, i, the figures for the n o r t h e r n hemisphere have to be multiplied with the m e a n annual tritium c o n c e n t r a t i o n s in 50°N listed in Table 2, c o l u m n 2. For the southern hemisphere, the conversion for the latitudinal band i is given by:
Ci(t ) = (Si/S50ON)1972
• (S50oN/S50os)1972
. CsoOs(t
) (2)
TABLE 2 Yearly averages of tritium concentrations in precipitation~ cp, characteristic for 50°N and 50 ° S, respectively, and of the tritium supply function Sp (equation (1), cf. text; 1 TU = 1018 × [T]/[H]). The tritium concentrations are based on observation from several tritium monitoring stations in -50°N [8]. Observations from several monitoring stations between 40 and 53°S have been averaged to obtain cp(50°S): 1953-1962: Kaitoke [32,33] and Campbell Island [16], after 1962: Invercargill (New Zealand), Marion Island, Stanley Falkland Islands, and Campbell Island. The values of Kaitoke have been corrected for the mean latitudinal pattern (Fig. 2). The 1952 figures are pre-nuclear concentrations [11]. Cr(50 ° N) and.Sr(50 ° N) are tritium concentrations and tritium supply function of river runoff in 50°N (model values, taken from Weiss et al. [8]) Year
cp(50 ° N) (TU)
¢p(50 ° S) (TU)
Cr(50 ° N) (TU)
Sp(50 ° N) (TU. yr)
Sp(50 ° S) ( T U ' yr)
Sr(50 ° N) (TU. yr)
1952 1953 1954 1955 1956 1957 1958 1959 1960 1961 1962 1963 1964 1965 1966 1967 1968 1969 1970 1971 1972 1973 1974 1975
2.5 6.8 90.4 13.4 43.6 30.1 145.9 136.2 44.9 43.6 274.7 1000.0 555.0 264.0 166.8 85.7 70.7 71.5 60.0 68.9 44.1 27.7 30.0 21.8
2.5 2.5 2.5 3.0 3.5 5.8 9.8 19.0 19.0 10.0 17.9 28.8 34.4 40.5 34.7 40.4 29.1 31.1 32.2 24.0 18.5 15.8 11.9 10.6
0.9 2.4 8.4 8.9 11.8 15.3 31.3 57.3 41.5 32.5 68.5 209.6 251.3 186.9 147.6 117.8 95.4 81.1 69.3 66.1 65.2 64.0 63.2 62.5
44.3 50.9 135.9 141.5 176.1 195.7 326.8 441.2 460.6 477.7 718.4 1651.1 2099.8 2241.1 2280.0 2238.0 2183.7 2133.2 2074.2 2027.2
44.3 44.3 44.3 47.2 48.0 51.0 57.7 73.0 87.4 92.4 104.7 126.9 153.4 184.3 207,9 235.8 251.1 267.5 284.1 291.8
1958.6
293.8
1877.9 1803.8 1725.9
293.0 288.4 282.9
16.0 17.5 24.7 32.0 41.7 54.3 81.7 132.9 166.0 188.5 244.7 435.0 655.4 801.1 900.6 965.6 1005.3 1028.9 1039.8 1046.9 1052.8 1057.2 1060.6 1063.1
439 TABLE3 Summary of quantities used to calculate the North Atlantic tritium delivery (from Weiss et at. [8]): E = evaporation, P = precipitation, R r = continental runoff, R V = tropospheric water vapor inflow, S p / S p ( 5 0 ° N) = tritium concentration ratio (Fig. 2), DEp = tritium deposition (eq. (3)), A = North Atlantic area of 5° latitude bands, 1 = tritium input in latitude bands. Transformation of DEp into total tritium input/EP is by multiplication with area A (103 TU - m = 3.24 ~Ci/m2);the bottom line gives timeintegrated totals of IEp, Ir, and I V for 1972 Latitude band (°N)
E (m/yr)
P (m/yr)
Rr (km3/yr)
0- 5 5-10 10-15 15-20 20-25 25-30 30-35 35-40 40-45 45-50 50-55 55-60 60-65 65-70 70-75 75-80
1.20 1.33 1.46 1.53 1.53 1.53 1.62 1.53 1.19 0.98 0.93 0.77 0.59 0.44 0.34 0.18
1.45 1.69 1.01 0.68 0.52 0.64 0.63 0.82 1.00 1.14 1.18 1.02 0.97 0.53 0.34 0.26
2345 2130 654 336 100 640 176 150 180 810 917 518 566 637 143 83
0-80
1.21 0.87 (average)
10,385
RV (km3/yr)
3300
} 3400
) 1 )
1100
Sp A Sp(50ON) (103 km 2) 0.10 0.13 0.17 0.22 0.29 0.36 0.46 0.57 0.71 0.86 1.02 1.19 1.36 1.52 1.67 1.79
653.9 1039.6 1157.2 1535.5 1925.0 2615.5 3636.8 4645.9 5049.0 5646.6 6504.8 6430.1 6163.3 4580.1 3673.0 2369.5
7800
DEp (TU. m)
IEp (MCi)
lr (MCi)
3595 2796 3857 4176 4547 4522 3629 3119 3415 2562 2303 2588 2250 2100 2116 1253
7.5 9.3 14.3 20.5 28.0 37.8 42.2 46.4 55.2 46.3 47.9 53.3 44.4 32.0 24.9 9.5
2.5 3.2 ) 1.3 j 0.9 0.4 5.9 1.0] 1.1 / 1.7 7.8 12.6 8.4 / 10.7 13.6 3.4 2.1
48,908
519.5 76.6 (total)
TABLE 4 Rates of tritium input, DEp , to the South Atlantic (explanation of the parameters cf. Table 3) Latitude band (°S)
E (m/yr)
P (m/yr)
Sp Sp(50°N)
0 5 5-10 10-15 15-20 20-25 25-30 30-35 35-40 40-45 45 50 50-55 55-60 60-65 65-70 70-75 75 - 8 0
1.30 1.48 1.65 1.62 1.48 1.40 1.29 1.12 0.89 0.70 0.57 0.44 0.29 0.17 0.12 0.07
0.72 0.36 0.25 0.26 0.39 0.51 0.65 0.84 0.95 1.00 0.92 0.82 0.58 0.45 0.42 0.35
0.0755 0.0575 0.045 0.046 0.059 0.075 0.094 0.117 0.1.42 0.169 0.200 0.230 0.265 0.295 0.320 0.390
0-80
1.13
0.76 (average)
A (103 km 2) 3234 3032 3034 2963 3297 3585 3728 3869 3877 3605 3378 2845 2243 2000 783 276 98,013 (total)
DEp (TU. m)
IEp (MCi)
484.2 338.1 251.1 256.4 429.2 549.7 679.2 806.4 862.8 895.8 908.2 857.9 673.5 503.7 450.2 400.8
5.07 3.32 2.47 2.46 4.59 6.39 8.20 10.11 10.84 10.46 9.94 7.91 4.90 3.26 1.14 0.36 91.42 (total)
1V (MCi)
28.5
43.1
25.7
97.3
440 TABLE 5 Rates of tritium input, DEp, to the Pacific Ocean (explanation of the parameters cf. Table 3) Latitude band
E (m/yr)
P (m/yr)
Sp Sp(50°N)
65-60 ° N 60-55 55-50 50-45 45-40 40-35 35 - 3 0 30-25 25-20 20-15 15-10 10- 5 5- 0
0.24 0.34 0.48 0.67 0.93 1.13 1.34 1.51 1.62 1.60 1.46 1.30 1.20
0.61 1.15 1.41 1.46 1.34 1.17 1.01 0.82 0.83 1.13 1.75 2.57 1.81
1.36 1.19 1.02 0.86 0.71 0.57 0.46 0.36 0.29 0.22 0.17 0.13 0.10
0 5°S 5-10 10-15 15 - 2 0 20-25 25-30 30-35 35-40 40-45 45-50 50-55 55-60 60-65 65 70 70-75 75-80
1.26 1.50 1.64 1.58 1.50 1.40 1.27 1.13 0.99 0.83 0.68 0.52 0.36 0.21 0.13 0.09
1.10 1.33 1.40 1.27 1.25 1.16 1.11 1.11 1.16 1.26 1.25 1.02 0.75 0.51 0.43 0.35
0.0755 0.0575 0.045 0.046 0.059 0.075 0.094 0.117 0.142 0.169 0.200 0.230 0.265 0.295 0.320 0.390
65°N-80°S
1.20
1.29
A (103 km 2)
DEp (TU. m)
749 2528 3246 4058 4508 5419 6217 6860 7859 8667 9819 10,963 10,497
3167.1 4577.4 5089.2 5097.9 4832.3 4126.5 3521.0 2769.1 2257.7 1966.1 1569.2 1274.0 747.3
7.69 37.49 53.52 67.03 70.58 72.45 70.92 61.55 57.49 55.21 49.92 45.25 25.42
9802 8894 8376 8326 7920 7442 7119 6717 6281 5755 5083 4580 3959 3022 1755 467
528.8 451.3 351.1 341.6 533.8 645.2 755.1 873.8 988.4 1086.7 1143.2 1036.8 851.7 595.7 472.0 438.9
16.79 13.01 9.53 9.22 13.70 15.56 17.42 19.02 20.12 20.26 18.83 15.39 10.92 5.83 2.68 0.66
176,888 (total)
(average)
IEp (MCi)
883.46 (total)
TABLE 6 Rates of tritium input, DEp, continental runoff, 1r, and tropospheric vapor inflow, 1V, to the northern Indian Ocean (explanation of the parameters cf. Table 3) Latitude band (o N)
E (m/yr)
P (m/yr)
Rr (kin 3/yr)
30-25 25-20 20-15 15-10 10- 5 5- 0
1.93 1.55 1.59 1.57 1.55 1.50
0.12 0.53 0.87 1.05 1.30 1.54
20 1757 ) 501 ~ 487 117 349
0-30
1.56 1.14 (average)
3231
RV (kma/yr)
3760
3760
Sp Sp(50°N) 0.36 0.29 0.22 0.17 0.13 0.10
A (10 a km 2) 381 952 2149 2877 2855 3268 12,382
DEp IEp (TU. M) (MCi)
lr (MCi)
2885.1 2274.0 1844.9 1336.2 1069.8 792.7
0.10 6.16 ) 1.35 0.99 0.17 0.38
3.56 7.01 12.85 12.46 9.90 8.39
54.17 9.15 (total)
IV (MCi)
23.6
23.6
441 TABLE 7 Rates of tritium input, DEp, to the southern Indian Ocean (explanation of the parameters cf. Table 3) Latitude band (°S)
E (m/yr)
P (m/yr)
Sp Sp(50ON)
0- 5 5-10 10-15 15-20 20-25 25-30 30-35 35-40 40-45 45-50 50-55 55 60 60-65 65-70
1.55 1.63 1.75 1.92 1.92 1.74 1.52 1.33 1.04 0.76 0.62 0.47 0.28 0.12
1.89 1.96 1.44 0.88 0.52 0.48 0.65 0.86 1.08 1.18 1.05 0.85 0.61 0.43
0.0755 0.0575 0.045 0.046 0.059 0.075 0.094 0.117 0.142 0.169 0.200 0.230 0.265 0.295
0-70
1.24
1.03
(8i/$5oON)1972from
Tables 3 - 7 , and (SsooN/
8S0°S)1972 as well as CsoOs(t) from Table 2. This procedure defines yearly mean tritium concentrations in 5 ° latitude bands.
3. The hydrological model The hydrological model (for details see Weiss et al. [8[ and Weiss [191) converts the tritium concentrations in precipitation into absolute rates of tritium input to the ocean. Tritium input to the oceans is by rainout, water vapor exchange, and by continental tritium runoff. The net direct input by rainout and water vapor exchange, d (TU • m/yr), is calculated from equation (3):
dEp=(P + E 1.
h )
1- h
DEp (TU • m)
3749 4973 5808 4861 4238 4418 4895 5898 5675 5339 4933 4580 4104 1750
729.9 548.4 369.8 355.4 558.4 660.7 778.9 926.1 999.8 1003.8 1007.2 904.8 676.2 420.5
65,221 (total)
(average)
where
A (103 km 2)
1
' cv - E . h~ -1- ~ )
" Cs
(3)
Rainout is given by the product of the precipitation rate, P, and the tritium concentration o f marine precipitation, cv. Vapor exchange is a process which is coupled with the evaporation. The net tritium input due to water vapor exchange is given by the sec-
IEp (MCi) 8.87 8.84 6.96 5.60 7.67 9.46 12.35 17.70 18.38 17.36 16.10 13.43 8.99 2.38 154.09 (total)
ond and third term of equation (3). It is proportional to the evaporation rate E and depends on the water vapor gradient in the atmospheric boundary layer (h : water vapor content in 10 m above sea level relative to saturation, i.e. h = 1 at the air-water interface). The "ingoing" fraction (second term) is set proportional to the tritium concentration of marine precipitation, Cp. This assumption is based on observational evidence ( [ 8 ] , see also below), that the tritium concentration in marine water vapor at ship's height on the average has a tritium concentration corresponding to isotopic equilibrium with falling precipitation (isotopic fractionation factor a ~ 1.1). The third term in equation (3) is the "outgoing" fraction. It is set proportional to the tritium concentration of surface water, Cs. As Cs is in any case much smaller than co (cf. [11]), the latter term is a minor correction only. Since on the other hand o~~ 1, E ~ P, and h ~ 0.75, it follows that tritium input by water vapor exchange greatly exceeds that by rainout. For the relative humidity, we use h = 0.74, which is the mean of literature data [ 2 1 - 2 3 ] . It is to be pointed out that equation (3) basically follows the concept given previously by Craig and
442 Gordon [12], while we only neglect kinetic isotopic fractionation. What is new, however, and essential to our approach, is the somewhat unexpected finding that the marine water vapor at ship's height is in fact nearly in isotopic equilibrium with falling rain. This notion is based on tritium analyses of about one-week composites of continuously sampled marine water vapor and precipitation sampled over the same period [8]. The data may be slightly biased, because sampling periods without any precipitation are excluded from the comparison, but basically the near equilibration is an observational fact. In evaluating equation (3), we choose the tropospheric tritium concentration to be that representative for open-ocean conditions (Fig. 2). Because continental tritium concentrations are considerably larger (see section 2), air-mass exchange with continental areas leads to a net tritium inflow into marine areas from the continents. Such net tropospheric advection of tritium from continental areas is estimated from published transport rates of vapor across the continental borders [24,25] and the excess of continental over marine tritium concentrations in precipitation (i.e., threefold) [8,19]. Such tritium is supposed to be taken up by the ocean soon after the air mass in question has left the continent. The length scale of this process is estimated to be of the order of 1000 km. The additional tritium input by river runoff is calculated from published rates of river runoff [20] and the tritium concentrations in precipitation characteristic for the catchment area o f the river in question; the continental tritium concentrations are again taken to exceed the marine ones by a factor of 4. The lagging behind in time of the tritium concentrations o f the rivers relative to those in precipitation is accounted for by a model, which has been established for the Rhine river (Western Europe) [26]. Satisfactory agreement, typically within +20%, has been found between measured tritium discharges available for several rivers on the North American continent and the ones predicted by the model [19, table 9; 8, tables I and II].
4. The rates of tritium input
tion by precipitation and vapor exchange (equation (3)), net tritium inflow from continental areas, and river runoff. The latter two modes are of minor importance, except for the North Atlantic and the northern Indian Ocean. Annual-average tritium input rates for any year, n, and any 5 ° latitude belt, i, are obtained as follows: (a) Open-ocean input is defined by equation (3), which for c~ = 1.12 and h = 0.74 converts to:
dEe(n, i) : [P(i) + 2.541E(01 • cp(n, i)
3.434Cs(n, 0
(4) P and E for any ocean are listed as 5 ° latitudes averages in Tables 3 7, and are assumed to be constant in time. ce(n, i) for the northern hemisphere is:
cp(n, i) = Sq)]S5oo N .Cp, 50°N(n)
(5)
where Sq)/SsooN is listed in Tables 3 - 7 and Cp,5OON(n) in Table 2 ; for the southern hemisphere the extended equation (2) has to be used. Cs is often negligible; for the Pacific and North Atlantic data are available [8,10,11], data for other areas must be guessed. An example is dEp (1964, 3 5 - 4 0 ° N ) = (0.82 + 2.541 • 1.53) • 0.57 - 555 3.434 • 17 = 1431 TU • m/yr for the Atlantic. (b) Tritium outflow from the continents. Only areas of major steady air flows are explicitly considered. Vapor fluxes, Rv(i) are given in Table 3 (North Atlantic) and 6 (Indian Ocean), in km3/yr. The corresponding inputs are:
dv(n, i) = Rv(i)/A(i). 3cv(n, i)
(6)
where A(i) is the area of the respective latitude belt and ocean, and the factor 3 accounts for the larger continental tritium concentration. This procedure distributes the tritium excess observed in continental air over the entire latitude belt. Alternatively, a more localized input could be considered, using prevailing winds and the presumed characteristics length scale for air-ocean transfer (see section 3). (c) River runoff. As before, Tables 3 and 6 list river runoff, Rr. The inputs are:
dr(n, i) = Rr(i)[A(i) . 4or(n, i)
(7)
Cr(n, i) is obtained from equation (5) if Cp,s0°N(n) is As outlined in the previous section, we consider three modes of tritium input, i.e., open-ocean addi-
replaced by or,soON(n) (Table 2). For ocean basins where the latter two contribu-
443 tions are minor, they are taken into account by increasing dEp by an appropriate factor (equation (4)). This means that the tritium from the two sources is being added uniformly over the entire ocean basin area. In these cases, a more detailed account clearly would fall well within the margins of uncertainty of our input assessment. The present procedure gives discrete input data, dtot(n, i), in the form o f products of data vectors, e.g. P(i) and c(n). We regard this as more flexible than a procedure presenting the input in the form of an analytical function in space and time (as e.g. in Fine and Ostlund [10]).
5. Check of the input by comparison to oceanic inventories
TABLE 8 Total tritium input to the world ocean up to the end of 1972. The totals of the direct input, IEp, have been taken from the Tables 3-7; the data for the North Atlantic are taken from Weiss et al. [8]. The calculation of the continental tritium input, IRV, is described in the text. The ratios of the total input,/tot = IEp + IRV, to the open-ocean input, IEp, are 1.34 (North Atlantic), 1.12 (South Atlantic), 1.09 (North Pacific), 1.04 (South Pacific), 1.61 (northern Indian Ocean), and 1.03 (southern Indian Ocean) Northern hemisphere
Southern hemisphere
IEp (MCi)
IRV (MCi)
IEp (MCi)
IRV (MCi)
173.9 60 32.8
91.4 208.9 154.1
11 7 4
Atlantic 519.5 Pacific 674.5 Indian Ocean 54.2
Total (MCi)
795.8 950.4 245.1 1991.3
Tritium observations for the Atlantic and the Pacific Oceans made during the GEOSECS field program allow to estimate the oceanic tritium inventories of these oceans for the time 1 9 7 1 - 1 9 7 3 . These estimates can be used as a constraint to the tritium input that we have derived above, insofar as the integrated model input has to agree with the oceanic inventories. Time-integrated inputs are obtained by replacing in equations (4) to (7) the tritium concentrations by the corresponding time-integrated concentrations, S, according to equation (1). Numerical values for S in 50°N and 50°S, respectively, are given in Table 2. Total time4ntegrated inputs (integration up to 1972) are presented in Tables 3 - 7 . The hemispheric totals of the open-ocean tritium input, DEp, into the individual ocean basins are summarized in Table 8, together with the amounts of the tritium added from the continents. For the North Atlantic, where apparently the continent-derived tritium amounts to about 25% of the total input, the situation has been discussed elsewhere [8]. The relatively high fraction of continental tritium is a result o f the small ocean surface area of the North Atlantic as compared to the size of, and the length o f the border lines to, the adjacent continents. The North Atlantic in this respect certainly is an extreme case compared to most parts of the world oceans. The other special case is the northern Indian Ocean where the continental tritium input is estimated to be comparable to that o f the direct input. This results from high river and vapor runoff in this
area and the small surface area of the northern Indian Ocean. The estimate of the continental tritium for the northern Indian Ocean is believed to be the most uncertain value in Table 8. This is because the hydrological regime of rivers like the Ganges is quite different to that of rivers in Europe or North America, so that the applicability of the river runoff delay model remains questionable. We believe that the uncertainty of the ocean basin input totals does not exceed -+20% in the northern hemisphere and -+30% in the southern hemisphere [ 19]. The largest source of uncertainty is the humidity, h (equation (3)). The uncertainty of the continental contributions may well exceed +30%. Table 9 compares the input totals for the North Atlantic and the Pacific with the corresponding oceanic tritium inventories. The Atlantic inventory has been reported previously [8]. Local Pacific inventories were obtained by vertical integration of available tritium versus depth profiles [5,14]. Multiplying local-inventory averages in 10 ° latitude bands by the band surface areas, and summing up latitudinally, gave the total tritium inventories in Table 9. The quoted uncertainty of +-6% includes the variability of the local inventories within the 10 ° latitude bands and the blank uncertainty of the tritium measurements [8]. For both oceans, the total inputs and the inventories agree within the limits o f uncertainty. The uncertainties of the inventory are distinctly
444
TABLE 9 Comparison of the total tritium input and the tritium inventories of the North Atlantic and the Pacific Ocean. North Atlantic data are taken from Weiss et al. [8] ; procedure to obtain the inventories see Weiss et al. [8]. Of the Pacific input 734.5 MCi reside in the North Pacific
5000 Q
~ o O ]
TU.m
~000
O D
North Atlantic (0-80 ° N) Pacific Ocean
Total tritium input * (MCi)
Tritium inventory (MCi)
693.4
665 -+ 8%
950.4
741 -+ 6%
~000
O
O
"[--112o
°<
2000
1000
* Estimated uncertainty -+20%. I
60
smaller than those o f the inputs; one might therefore tentatively correct the input rates down accordingly by a common factor. Such adjustment, however, presumably is within the uncertainty o f the input rates as we have defined them. In summary, the comparison o f the ocean basin time-integrated tritium inputs and oceanic tritium inventories, strongly supports our assessment of the tritium input. The comparison naturally can only be made on the totals. However, a comparison for the Pacific by latitude is presented in the next section, together with a general discussion.
6. Discussion As an illustration, Fig. 3 shows the comparison of the time-integrated input and the observed, local inventories for the Pacific by latitude. In general the correspondance is gratifying, although it can neither prove nor disprove the latitude dependence o f our tritium input. In detail, the local inventories appear to be highest in 2 0 - 4 0 ° N , whereas the input maximum is found at higher latitudes. Because the bulk of the tritium has had about 10 years travel time in the ocean between input and observation, a latitudinal displacement between the input and the inventories is to be expected. In fact, a general net southward transport o f tritium for the northern North Pacific has been postulated previously [27]. South of about 40°S the model input seems to be systematically
i
t
,~0
i
L
20*N
i
i
0
i
I
20*'3
I
I
gO
I
i
6O
i
I
8O
Fig. 3. Local tritium inventories, in TU . m (103 TU - m = 3.24 ~Ci/m2), for the Pacific Ocean, obtained by vertical integration of tritium profiles reported in Ostlund et al. [5] (circles) and in Michel [14] (squares). The data have been time-corrected to represent the conditions in 1972. The histogram gives the input of tritium per 5 ° latitudinal band (numerical values cf. Table 3). A corresponding comparison for the North Atlantic is given in Weiss et al. [8].
higher than the local inventories. Part of the latter discrepancy may be caused by the uncertainty o f the input in these areas. It is to be noted that no latitudinal shift between input and inventories is apparent for the North Atlantic, which agreement, however, is regarded as fortuitous [8, fig. 1 ]. For the Pacific tritium totals, the contribution o f the higher latitudes is smaller than may appear at first glance from Fig. 3, because the areas involved are relatively small. The latitudinal variation of the tritium input (Fig. 3 and [8, fig. 1]) has its northern hemispheric maxima polewards of those of the 9°Sr deposition [28]. This is believed to be caused by the net deposition rates in the case o f tritium being very different for land and sea areas, contrary to the situation for 9°Sr: The principal marine deposition mechanism for tritium is vapor exchange (driven by evaporation, see equation (3)), whereas on the continent tritium is reevaporated from the ground. This results in the marine net deposition rates being roughly one order of magnitude larger than continental ones. For 9°Sr
445 such differences are minor, because the dominant deposition mechanism is rainout. Therefore tropospheric tritium appreciably accumulates over continental areas. Because the atmospheric circulation is primarily zonal, relatively more tritium than 9°Sr is available in the troposphere for oceanic deposition, in latitudes of a larger land to sea surface area ratio * The observed shift results because the land/sea area ratio considerably increases with latitude north of ~40°N. We can, of course, not exclude further causes such as differences in stratosphere-troposphere transfer. If our proposed mechanism is effective, it is further to be expected that the latitudinal shift in the tropospheric concentrations should still be larger, because the evaporation drops off faster with latitude than does precipitation. The predicted values of total tritium input are 734 MCi for the North Pacific and 693 MCi for the North Atlantic. Considering that the surface area of the North Pacific exceeds that of the North Atlantic by almost a factor of two, the results of comparable totals of tritium input surprises. This discrepancy is mainly a result of differences in evaporation and precipitation in both oceans, north of about 30°N, and of the limited northward extension of the North Pacific. A third, though minor, aspect is the difference in the continental contributions, which are almost a factor 3 higher in the North Atlantic. From the data given in Table 8 the global oceanic tritium inventory can be estimated: our input total at the end of 1972 gives 2.0 GCi of tritium to be found in the oceans; 75% of this amount has been added to the northern, 25% to the southern ocean. As Table 9 indicates the input calculations to be on the high side, we correct the 2.0 GCi tentatively down by 10%, In other words, we recalibrate our input totals by comparison to the North Atlantic and Pacific inventories. Furthermore, the tritium remaining in the continental hydrosphere can be estimated, by using the abovementioned river-runoff delay model, to be about 5%. Neglecting any atmospheric contributions, a global tritium inventory of 1.9 GCi (for end-1972) results. The uncertainty of this global inventory is believed to
* Apart from the tritium excess considered in our calculations, the continent-derived tritium accumulation presumably also penetrates into higher tropospheric layers, which have much longer transfer times into the ocean.
be no larger than +_20%. Similar inventory figures have been published previously [6,14,30]. Our estimate corresponds to an average tritium yield in nuclear fusion of 0.9 kg/Mt fusion (see Miskel [31 ]iAcknowledgements We are grateful to H. G6te 0stlund, Miami, for letting us use his GEOSECS Pacific tritium results. This work was supported by the Deutsche Forschungsgemeinschaft and in part also by the Heidelberger Akademie der Wissenschaften.
References 1 H. Craig, Thc GEOSECSprogram: 1970--71, Earth Planet. Sci. Lett. 16 (1972) 47-49. 2 H.G. 0stlund, H.G. Dorsey and R, Brescher, GEOSECS Atlantic radiocarbon and tritium results (Miami), Rosenstiel Sch. Mar. Atmos. Sci., Univ. Miami, Data Rep. No. 5 (1976). 3 H.G. Ostlund, H.G. Dorsey and C.G. Rooth, GEOSECS North Atlantic radiocarbon and tritium results, Earth Planet. Sci. Lett. 23 (1974) 69. 4 W. Roether and W. Weiss, A transatlantic tritium section near 40°N, 1971, "Meteor" Forsch.-Ergeb. Reihe A: 20 (1978) 101-108. 5 H.G, 0stlund, R. Brescher, R. Oleson and M.J. Ferguson, GEOSECS Pacific radiocarbon and tritium results (Miami), Rosenstiel Sch. Mar. Atmos. Sci., Univ. Miami, Tritium Lab, Data Rep. No. 8 (1979). 6 H.G. 0stlund and R.A. Fine, Oceanic distribution and transport of tritium, in: The Behaviour of Tritium in the Environment (International Atomic Energy Agency, Vienna, 1979) 303-314. 7 W.H. Peterson and C.G. Rooth, Formation and exchange of deep water in the Greenland and Norwegian Seas, Deep Sea Res. 23 (1976) 273. 8 W. Weiss, W. Roether and E. Dreisigacker, Tritium in the North Atlantic Ocean: inventory, input and transfer into the deep water, in: The Behaviour of Tritium in the Environment (International Atomic Energy Agency, Vienna, 1979) 315-336. 9 J.L. Sarmiento, A 3-D model of bomb tritium and carbon-14 in the North Atlantic, Abstract, Fall Meeting, American Geophysical Union, San Francisco, Calif., Trans. Am. Geophys. Union 59 (1978) 1101. 10 R.A. Fine and H.G. (gstlund, Source function for tritium transport models in the Pacific, Geophys. Res. Lett. 4 (1977) 461. 11 E. Dreisigacker and W. Roether, Tritium and Sr-90 in North Atlantic Surface Water, Earth Planet. Sci. Lett. 38 (1978) 301. 12 H. Graig and L.I. Gordon, Deuterium and oxygen-18
446
13
14 15 16
17
18
19
20 21
22
variations in the ocean and the marine atmosphere, in: Stable Isotopes in Oceanographic Studies and Paleotemperatures, Spoleto 1965, E. Tongiorgi, ed. (Consiglio Nazionale delle Richerche, Laboratorio di Geologia Nucleaxe, Pisa, 1965) 19-130. E. Eriksson, An account of the major pulses of tritium and their effects in the atmosphere, Tellus 17 (1965) t18. R.L. Michel, Tritium inventories of the world oceans and their implications, Nature 263 (1976) 103. H.J. Simpson, Tritium in Crater Lake, Oregon, J. Geophys. Res. 75 (1970) 27. International Atomic Energy Agency, Tech. Rep. Ser. Nos. 96,117,129, 147, 165,192 (IAEA, Vienna, 1969, 1970, 1971, 1973, 1975, 1979). W. Weiss, J. Bullacher and W. Roether, Evidence of pulsed discharges of tritium from nuclear energy installations in central European precipitation, in: The Behaviour of Tritium in the Environment (Int_ernational Atomic Energy Agency, Vienna, 1979) 17-30. C.B. Taylor, Influence of 1968 French thermonuclear tests on tritium fallout in the southern hemisphere, Earth Planet. Sci. Lett. 10 (1971) 196 -198. W. Weiss, Die Tritiumbilanz des Nordatlantik, Doctoral Dissertation, University of Heidelberg, Heidelberg (1975) unpublished. A. Baumgartner and E. Reichel, Die Weltwasserbilanz (Oldenbourg Verlag, Mtinchen, 1975) 179 pp. H.L. Crutcher and J.M. Meserve, U.S. Naval Weather Service Command: Selected level heights, temperatures and dew points for the northern hemisphere, NAVAIR 50-K52 (1970). World Meteorological Organization, Climatological Normals (Clino) for Climat and Climat Ship Stations for the period 1931-1960, WMO/OMM-No. 117, TP.52 (1962).
23 R.E. Newell, K.W. Kidson, D.G. Vincent and G.J. Boer, The General Circulation of the Tropical Atmosphere and Interactions with Extratropical Latitudes, Vol. 1 (MIT press, Cambridge, Mass., 1972). 24 J.P. Peixoto and A.R. Crisi, Hemispheric humidity conditions during ICY, Planet. Circ. Proj. Sci. Rep. No. 6 (1965) 166. 25 E.M. Rasmussen, A study of eastern North America using atmospheric vapor flux data, Mon. Weather Rev. 95 (1971) 403. 26 W. Weiss and W. Roether, Der Tritiumabfluss des Rheins 1961-1973, Dtsch. Gewfisserkd. Mitt. 19, Heft 1 (1975) 1-5. 27 R. Michel and H. Suess, Bomb tritium in the Pacific Ocean, J. Geophys. Res. 80 (1975) 4139. 28 A.B. Joseph, P.F. Gustafson, I.R. Russell, E.A. Schwert, H.A. Volchok and A. Tamp/in, Sources of radioactivity and their characteristics, in: Radioactivity in the Marine Environment (U.S. Government Printing Office, Washington, D.C., 1971) 6. 29 S.L. Kupferman, H.D. Livingston and V.T. Bowen, A mass balance for 137Cs and 9°St in the North Atlantic Ocean, J.Mar. Res. 37 (1979) 157. 30 K.J.P. Schr6der and W. Roether, The releases of Kr-85 and H-3 to the environment and H-3 to Kr-85 ratios as source indicators, in: Isotope Ratios as Pollutant Source and Behaviour Indicators (IAEA, Vienna, 1975) 231. 31 J.A. Miskel, Production of tritium by nuclear weapons, in : Tritium, A.A. Moghissi and M.W. Carter, eds. (Messenger Graphics, Phoenix, Ariz., 1973) 79. 32 G.B. Allison and M.W. Huges, The history of tritium fallout in southern Australia as inferred from rainfall and wine samples, Earth Planet. Sci. Lett. 36 (1977) 334. 33 G.B. Taylor, Tritium in southern hemisphere precipitation 1953-1964, Tellus 18 (1966) 105.
435
(g)Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands [21
THE RATES OF TRITIUM INPUT TO THE WORLD OCEANS WOLFGANG WEISS and WOLFGANG R O E T H E R
Institut ffir Umweltphysik, University of lleidelberg, Heidelberg (Federal Republic of Germany) Received July 5, 1979 Revised version received October 12, 1979
Mean annual rates of tritium input into the ocean averaged over 5° latitude bands are presented for the major oceans, for the period 1952 1975. The rates are obtained by converting tritium concentrations in marine precipitation into net oceanic tritium input, by means of a hydrological model. The tropospheric tritium pattern is specified on the basis of available observations, and climatological means from the literature are used for the rates of evaporation and precipitation and for the relative humidity in ship's height, that enter the model. Tritium input by water vapor exchange exceeds that by precipitation about three-fold. Tritium input by river runoff and by net tropospheric tritium outflow from the continents is also accounted for. This contribution is small except for the northern Indian Ocean and the North Atlantic. The inputs have hemispheric maxima near 50 ° latitude. The northern hemisphere inputs were strongly peaked in 1963-1964, whereas temporal changes in the southern hemisphere were much more gradual. By 1972, about 75% of the total oceanic input had been received by the northern ocean. For the Pacific, the computed total input agrees with the actual tritium inventory within the limits of uncertainty (about -+20%). The global tritium inventory is estimated at 1.9 GCi in 1972, which corresponds to an average tritium yield of 0.9 kg tritium per megaton TNT equivalent of nuclear fusion.
1. Introduction The distribution o f bomb-produced tritium in the oceans has extensively been studied during the 1 9 7 2 1974 GEOSECS field program [1]. At the present time, measured tritium concentrations are available for the Atlantic [ 2 - 4 ] and the Pacific ocean [5,6]. Whereas it is evident that much information on water transport and mixing within the ocean can be gained from the tritium data, their use so far has largely been restricted to box-model evaluations. Box models are useful as low-order approximations to the behaviour of the real ocean (e.g. [7]). A special aspect is to use such models to predict the oceanic dispersion of pollutants, e.g. of excess CO2 [8]. It is the consensus that eventually dynamical numerical-model evaluations o f the tritium data [9] will provide a means to approach the physical processes in the ocean to a much higher degree. GEOSECS Publication No. 116.
Crucial to all such evaluations is that tritium boundary conditions at the sea surface be specified. Mixed-layer tritium concentrations are available for the Pacific [10] and the North Atlantic [11]. However, a concentration boundary condition a priori largely fixes the tritium concentrations in the nearsurface ocean. For this reason, a flux boundary condition often is a preferable alternative. The purpose of the present paper therefore is to provide rates of tritium input to the oceans and their temporal and areal patterns. Results for the North Atlantic have been published previously [8]. The then used procedure was to convert tropospheric tritium concentrations into rates o f tritium input into the ocean by means o f a hydrological model. Tritium input by rainout and water vapor exchange, as well as by continental river runoff and net tropospheric tritium outflow from the continents was allowed for. Good agreement was found between the computed total tritium input and the North Atlantic tritium inventory. This agreement was taken as evidence for
436 the appropriateness o f the model. In particular, the agreement gave p r o o f to the dominant role o f water vapor exchange, as previously pointed out by Craig and Gordon [12], and repeatedly discussed in the literature [ 1 3 - 1 5 ] . In this paper the same model is used to compute tritium input rates for the remainder o f the world ocean. In order to do this, the tropospheric tritium pattern is derived first, and the hydrological tritium input model is sketched. These sections are brief, because the procedures and a justification of the model have been published previously [8]. The computed inputs are presented in tables. From this information, annual-average rates o f tritium input into the ocean can be calculated for any given latitude and year. The procedure is summarized in section 4. Finally, a comparison o f the computed total tritium input with the tritium inventory taken from oceanic observations is made for the Pacific, and various features o f the tritium input are discussed.
2. The tritium pattern in marine precipitation About 3500 monthly composite precipitation data from 49 monitoring stations on ocean weather ships (OWS), small islands and at selected coastal sites [16] (cf. Table 1) are available to document the general pattern o f tritium in marine precipitation. The fact that tritium concentrations at any two monitoring stations within each hemisphere have been varying in time in near constant proportions has led us to factorize the general hemispheric tritium pattern into temporal and areal distribution functions. The mathematical procedure is described elsewhere [ 17], and results for the northern hemisphere have previously been published [8]. The pattern for the southern hemisphere has been constructed in an identical fashion. It turns out that here the pattern is less well documented (see section 5). However, the results should still be adequate, considering that only a smaller part o f the total bomb tritium has entered the southern oceans. Results for both hemispheres are summarized in Tables 2 - 7 and in Figs. 1 and 2. The temporal patterns in 50°N and in 50°S for the period 1 9 5 2 - 1 9 7 5 are given in Fig. 1. The northern hemispheric pattern is dominated by tritium pulses in 1954, 1 9 5 8 - 1 9 5 9 and, predominantly, in 1963. The
TABLE 1 Geographical positions of tritium monitoring stations [16] used for the description of temporal and areal pattern of tritium in precipitation. The time interval during which observational data have been available for this work is also given Station
Position
Time interval
Isfjord Radio Reykjavik Groennedal Lista Adak OWSJ Milford Haven Valentia Acores OWSE Bermuda OWS V Virginia Key Cape Sable Tamiami Homestead Royal Palm Hawaii WakeIsl. Puerto Rico Johnston Isl. Guam Barbados Yap Minicoy Isl. Truck Isl. Christmas Isl. Tawara Group Canton Isl. Madang Diego Garcia Ascension Isl. FufunatiIsl. Apia, Samoa Pago Pago St. Helena Fiji Isl. Rarotonga Isla de Pascua Juan Fernandez Isl. Kaitaia, New Zealand Gough Isl. Kaitoke, New Zealand Invercagill, New Zea. Marion Isl. Stanlay Falkland Isl. Campbell Isl. Argentine Isl. Halley Bay
78°N, 14°E 64°N, 22°W 61°N, 48°W 58°N, 7°E 52°N, 177°W 52°N, 1 5 ° W 52°N, 5°W 52°N, 1 0 ° W 38°N, 2 7 ° W 35°N, 48°W 32°N, 65°W 31°N, 164°E 26°N, 80°W 25°N, 81°W 25°N, 81°W 25°N, 8 0 ° W 25°N, 80°W 20°N, 155°W 19°N, 167°E 19°N, 66°W 17°N, 170°W 14°N, 145°W 13°N, 60°W 9°N, 138°E 9°N, 73°E 8°N, 151°E 2°N, 157°W 2°N, 173°E 3°S, 172°W 5°S, 146°E 7°S, 73°E 8°S, 1 5 ° W 8°S, 181°W 14°S, 172°W 14°S, 171°W 16°S, 6°W 18°S, 179°E 21°S, 160°W 27°S, 109°W 34°S, 79°W 35°S, 173°E 40°S, 1 0 ° W 41°S, 175°E 46°S, 168°E 47°S, 38°E 52°S, 58°W 53°S, 169°E 65°S, 64°W 75°S, 27°W
1964--1969 1960 1963 1962-1965 1 9 6 1 1969 1962-1969 1964-1969 1965 1958-1975 1963 1971 1962-1969 1962-1967 1962-1969 1964-1970 1964 1969 1964-1969 1964-1965 1964 1965 1962-1969 1962 1967 1963-1969 1962 1969 1964-1971 1959-1971 1968-1969 1968 1968-1969 1962-1967 1962-1972 1962-1967 1968-1972 1964-1975 1966-1971 1966 1972 1963-1975 1970-1971 1964-1975 1967-1973 1964-1971 1965 1975 1965-1975 1963-1975 1962-1975 1963-1975 1963-1975 1962-1975 1962-1975 1964-1975 1964--1975 1966 1975
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southern hemispheric pattern is considerably smoother with indications of a delayed response to the northern hemispheric peaks, and the absolute concentrations there are much smaller. This is a result of the fact that there was only minor atmospheric fusion bomb testing in the southern hemisphere and none before 1968 [18], so that most of the bomb tritium observed there is tritium originally released in the northern hemisphere. The areal pattern has been separated by using the temporal distributions of Fig. 1 for stations in the respective hemispheres. This procedure is somewhat arbitrary for the near-equatorial stations, for which the temporal pattern is intermediate to those at 50°N and 50°S. However, concentrations in this latitudinal belt are comparably low so that only minor errors are introduced. As shown previously [8], the areal variation of tritium in marine precipitation predominantly is by latitude only, and Fig. 2 gives the results for the latitudinal pattern. In view of the different te'mporal distributions in both hemispheres (Fig. 1), timeintegrated tritium concentrations over the entire nuclear period, Sp (TU • yr), were chosen as the ordinate in Fig. 2, Sp being defined by:
cp(t') exp[X(t' - t)l dr'
'
00
tO0
Fig. 1. Mean annual tritium concentrations (in TU) in marine precipitation characteristic for 50°N (solid line) and 50°S (dashed line) between 1952 and 1975 (numerical values are given in Table 2). Note the difference in ordinate for the t~vo histograms.
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8
~o 6 io'9o
Fig. 2. Latitudinal variation of the relative tritium delivery rates Sp/Sp (SO°N) in 1972 (station positions ef. Table 1). Solid line represents best fits of the data; dashed line is a linear extrapolation from the reference station in 50°S towards higher southern latitudes. Numerical values of the relative tritium delivery rates Sp in SO°N and in 50°S (*) are given in Table 2. Numerical values for the latitudinal variation of Sp, averaged over 5° latitudinal bands i, (Sp(i)/ Sp(50 ° N) 1972 are given in Tables 3- 7. where Cp tritium concentration in precipitation (TU) and X = 0.05635 yr -1 = tritium decay constant. Pre-1952 concentrations are regarded negligible [11]. The factorization assumes that in each hemisphere the latitudinal pattern has not changed in time, but only the absolute values have varied proportional to the values in Fig. 1. Apparently, concentrations were minimal between the equator and about 20°S. Straight lines have been fitted to the data points in Fig. 2, with a break at 15°S. In the northern hemisphere the fit includes all data points between the equator and 75°N. In the southern hemisphere the data between 15°S and 50°S were used. The so-found southern hemisphere line is extrapolated beyond 50°S to estimate the tritium concentrations south of 50°S. The extrapolation was chosen because the only two stations available south of 50°S (Fig. 2) report tritium concentrations which are presumably too high to represent open-ocean conditions. This may be due to the local influence of the Antarctic continent. The curve defined by these straight lines is taken as the average tritium versus latitude variation in marine precipitation. The scatter is moderate apart from the near-equatorial region;the uncertainties of the fits (1 o) amount to about -+15% north of 20°N, -+20% between 20 and 50°S, and about -+30% between 20°N and =
438 20°S. South o f 50°S the uncertainty is considerable. There are no indications o f any systematic longitudinal trends, e.g. b e t w e e n Atlantic and Pacific, within the precision o f the fits. The time-integrated tritium concentrations are therefore assumed to be constant within a latitudinal band. In 1975 the time-integrated c o n c e n t r a t i o n in 50°S, S ( 5 0 ° S ) , a m o u n t e d to about 1/7 o f the one in 50°N, S ( 5 0 ° N ) . Between 1970 and 1975 the ratio S(50°N)/S(50°S) has changed by about 3%/yr (cf. Table 2). Monitoring stations at continental sites show a similar latitudinal pattern as do the marine stations [8]. Their absolute c o n c e n t r a t i o n s are, however, higher than those over the ocean [8,17], owing to continental re-evaporation o f tritium. In the n o r t h e r n hemisphere the tritium c o n c e n t r a t i o n s o f continental
stations are typically a factor o f 4 higher than those over the ocean at the same latitude [8]. Effects o f this continental tritium excess are included in the hydrological m o d e l below. Time-integrated o p e n - o c e a n tritium concentrations ( e q u a t i o n (1), time integration up to 1972) for 5 ° latitude bands, relative to those at 50~N, are listed in Tables 3 - 7 . To obtain m e a n tritium c o n c e n t r a t i o n s for any time, t, and latitudinal bands, i, the figures for the n o r t h e r n hemisphere have to be multiplied with the m e a n annual tritium c o n c e n t r a t i o n s in 50°N listed in Table 2, c o l u m n 2. For the southern hemisphere, the conversion for the latitudinal band i is given by:
Ci(t ) = (Si/S50ON)1972
• (S50oN/S50os)1972
. CsoOs(t
) (2)
TABLE 2 Yearly averages of tritium concentrations in precipitation~ cp, characteristic for 50°N and 50 ° S, respectively, and of the tritium supply function Sp (equation (1), cf. text; 1 TU = 1018 × [T]/[H]). The tritium concentrations are based on observation from several tritium monitoring stations in -50°N [8]. Observations from several monitoring stations between 40 and 53°S have been averaged to obtain cp(50°S): 1953-1962: Kaitoke [32,33] and Campbell Island [16], after 1962: Invercargill (New Zealand), Marion Island, Stanley Falkland Islands, and Campbell Island. The values of Kaitoke have been corrected for the mean latitudinal pattern (Fig. 2). The 1952 figures are pre-nuclear concentrations [11]. Cr(50 ° N) and.Sr(50 ° N) are tritium concentrations and tritium supply function of river runoff in 50°N (model values, taken from Weiss et al. [8]) Year
cp(50 ° N) (TU)
¢p(50 ° S) (TU)
Cr(50 ° N) (TU)
Sp(50 ° N) (TU. yr)
Sp(50 ° S) ( T U ' yr)
Sr(50 ° N) (TU. yr)
1952 1953 1954 1955 1956 1957 1958 1959 1960 1961 1962 1963 1964 1965 1966 1967 1968 1969 1970 1971 1972 1973 1974 1975
2.5 6.8 90.4 13.4 43.6 30.1 145.9 136.2 44.9 43.6 274.7 1000.0 555.0 264.0 166.8 85.7 70.7 71.5 60.0 68.9 44.1 27.7 30.0 21.8
2.5 2.5 2.5 3.0 3.5 5.8 9.8 19.0 19.0 10.0 17.9 28.8 34.4 40.5 34.7 40.4 29.1 31.1 32.2 24.0 18.5 15.8 11.9 10.6
0.9 2.4 8.4 8.9 11.8 15.3 31.3 57.3 41.5 32.5 68.5 209.6 251.3 186.9 147.6 117.8 95.4 81.1 69.3 66.1 65.2 64.0 63.2 62.5
44.3 50.9 135.9 141.5 176.1 195.7 326.8 441.2 460.6 477.7 718.4 1651.1 2099.8 2241.1 2280.0 2238.0 2183.7 2133.2 2074.2 2027.2
44.3 44.3 44.3 47.2 48.0 51.0 57.7 73.0 87.4 92.4 104.7 126.9 153.4 184.3 207,9 235.8 251.1 267.5 284.1 291.8
1958.6
293.8
1877.9 1803.8 1725.9
293.0 288.4 282.9
16.0 17.5 24.7 32.0 41.7 54.3 81.7 132.9 166.0 188.5 244.7 435.0 655.4 801.1 900.6 965.6 1005.3 1028.9 1039.8 1046.9 1052.8 1057.2 1060.6 1063.1
439 TABLE3 Summary of quantities used to calculate the North Atlantic tritium delivery (from Weiss et at. [8]): E = evaporation, P = precipitation, R r = continental runoff, R V = tropospheric water vapor inflow, S p / S p ( 5 0 ° N) = tritium concentration ratio (Fig. 2), DEp = tritium deposition (eq. (3)), A = North Atlantic area of 5° latitude bands, 1 = tritium input in latitude bands. Transformation of DEp into total tritium input/EP is by multiplication with area A (103 TU - m = 3.24 ~Ci/m2);the bottom line gives timeintegrated totals of IEp, Ir, and I V for 1972 Latitude band (°N)
E (m/yr)
P (m/yr)
Rr (km3/yr)
0- 5 5-10 10-15 15-20 20-25 25-30 30-35 35-40 40-45 45-50 50-55 55-60 60-65 65-70 70-75 75-80
1.20 1.33 1.46 1.53 1.53 1.53 1.62 1.53 1.19 0.98 0.93 0.77 0.59 0.44 0.34 0.18
1.45 1.69 1.01 0.68 0.52 0.64 0.63 0.82 1.00 1.14 1.18 1.02 0.97 0.53 0.34 0.26
2345 2130 654 336 100 640 176 150 180 810 917 518 566 637 143 83
0-80
1.21 0.87 (average)
10,385
RV (km3/yr)
3300
} 3400
) 1 )
1100
Sp A Sp(50ON) (103 km 2) 0.10 0.13 0.17 0.22 0.29 0.36 0.46 0.57 0.71 0.86 1.02 1.19 1.36 1.52 1.67 1.79
653.9 1039.6 1157.2 1535.5 1925.0 2615.5 3636.8 4645.9 5049.0 5646.6 6504.8 6430.1 6163.3 4580.1 3673.0 2369.5
7800
DEp (TU. m)
IEp (MCi)
lr (MCi)
3595 2796 3857 4176 4547 4522 3629 3119 3415 2562 2303 2588 2250 2100 2116 1253
7.5 9.3 14.3 20.5 28.0 37.8 42.2 46.4 55.2 46.3 47.9 53.3 44.4 32.0 24.9 9.5
2.5 3.2 ) 1.3 j 0.9 0.4 5.9 1.0] 1.1 / 1.7 7.8 12.6 8.4 / 10.7 13.6 3.4 2.1
48,908
519.5 76.6 (total)
TABLE 4 Rates of tritium input, DEp , to the South Atlantic (explanation of the parameters cf. Table 3) Latitude band (°S)
E (m/yr)
P (m/yr)
Sp Sp(50°N)
0 5 5-10 10-15 15-20 20-25 25-30 30-35 35-40 40-45 45 50 50-55 55-60 60-65 65-70 70-75 75 - 8 0
1.30 1.48 1.65 1.62 1.48 1.40 1.29 1.12 0.89 0.70 0.57 0.44 0.29 0.17 0.12 0.07
0.72 0.36 0.25 0.26 0.39 0.51 0.65 0.84 0.95 1.00 0.92 0.82 0.58 0.45 0.42 0.35
0.0755 0.0575 0.045 0.046 0.059 0.075 0.094 0.117 0.1.42 0.169 0.200 0.230 0.265 0.295 0.320 0.390
0-80
1.13
0.76 (average)
A (103 km 2) 3234 3032 3034 2963 3297 3585 3728 3869 3877 3605 3378 2845 2243 2000 783 276 98,013 (total)
DEp (TU. m)
IEp (MCi)
484.2 338.1 251.1 256.4 429.2 549.7 679.2 806.4 862.8 895.8 908.2 857.9 673.5 503.7 450.2 400.8
5.07 3.32 2.47 2.46 4.59 6.39 8.20 10.11 10.84 10.46 9.94 7.91 4.90 3.26 1.14 0.36 91.42 (total)
1V (MCi)
28.5
43.1
25.7
97.3
440 TABLE 5 Rates of tritium input, DEp, to the Pacific Ocean (explanation of the parameters cf. Table 3) Latitude band
E (m/yr)
P (m/yr)
Sp Sp(50°N)
65-60 ° N 60-55 55-50 50-45 45-40 40-35 35 - 3 0 30-25 25-20 20-15 15-10 10- 5 5- 0
0.24 0.34 0.48 0.67 0.93 1.13 1.34 1.51 1.62 1.60 1.46 1.30 1.20
0.61 1.15 1.41 1.46 1.34 1.17 1.01 0.82 0.83 1.13 1.75 2.57 1.81
1.36 1.19 1.02 0.86 0.71 0.57 0.46 0.36 0.29 0.22 0.17 0.13 0.10
0 5°S 5-10 10-15 15 - 2 0 20-25 25-30 30-35 35-40 40-45 45-50 50-55 55-60 60-65 65 70 70-75 75-80
1.26 1.50 1.64 1.58 1.50 1.40 1.27 1.13 0.99 0.83 0.68 0.52 0.36 0.21 0.13 0.09
1.10 1.33 1.40 1.27 1.25 1.16 1.11 1.11 1.16 1.26 1.25 1.02 0.75 0.51 0.43 0.35
0.0755 0.0575 0.045 0.046 0.059 0.075 0.094 0.117 0.142 0.169 0.200 0.230 0.265 0.295 0.320 0.390
65°N-80°S
1.20
1.29
A (103 km 2)
DEp (TU. m)
749 2528 3246 4058 4508 5419 6217 6860 7859 8667 9819 10,963 10,497
3167.1 4577.4 5089.2 5097.9 4832.3 4126.5 3521.0 2769.1 2257.7 1966.1 1569.2 1274.0 747.3
7.69 37.49 53.52 67.03 70.58 72.45 70.92 61.55 57.49 55.21 49.92 45.25 25.42
9802 8894 8376 8326 7920 7442 7119 6717 6281 5755 5083 4580 3959 3022 1755 467
528.8 451.3 351.1 341.6 533.8 645.2 755.1 873.8 988.4 1086.7 1143.2 1036.8 851.7 595.7 472.0 438.9
16.79 13.01 9.53 9.22 13.70 15.56 17.42 19.02 20.12 20.26 18.83 15.39 10.92 5.83 2.68 0.66
176,888 (total)
(average)
IEp (MCi)
883.46 (total)
TABLE 6 Rates of tritium input, DEp, continental runoff, 1r, and tropospheric vapor inflow, 1V, to the northern Indian Ocean (explanation of the parameters cf. Table 3) Latitude band (o N)
E (m/yr)
P (m/yr)
Rr (kin 3/yr)
30-25 25-20 20-15 15-10 10- 5 5- 0
1.93 1.55 1.59 1.57 1.55 1.50
0.12 0.53 0.87 1.05 1.30 1.54
20 1757 ) 501 ~ 487 117 349
0-30
1.56 1.14 (average)
3231
RV (kma/yr)
3760
3760
Sp Sp(50°N) 0.36 0.29 0.22 0.17 0.13 0.10
A (10 a km 2) 381 952 2149 2877 2855 3268 12,382
DEp IEp (TU. M) (MCi)
lr (MCi)
2885.1 2274.0 1844.9 1336.2 1069.8 792.7
0.10 6.16 ) 1.35 0.99 0.17 0.38
3.56 7.01 12.85 12.46 9.90 8.39
54.17 9.15 (total)
IV (MCi)
23.6
23.6
441 TABLE 7 Rates of tritium input, DEp, to the southern Indian Ocean (explanation of the parameters cf. Table 3) Latitude band (°S)
E (m/yr)
P (m/yr)
Sp Sp(50ON)
0- 5 5-10 10-15 15-20 20-25 25-30 30-35 35-40 40-45 45-50 50-55 55 60 60-65 65-70
1.55 1.63 1.75 1.92 1.92 1.74 1.52 1.33 1.04 0.76 0.62 0.47 0.28 0.12
1.89 1.96 1.44 0.88 0.52 0.48 0.65 0.86 1.08 1.18 1.05 0.85 0.61 0.43
0.0755 0.0575 0.045 0.046 0.059 0.075 0.094 0.117 0.142 0.169 0.200 0.230 0.265 0.295
0-70
1.24
1.03
(8i/$5oON)1972from
Tables 3 - 7 , and (SsooN/
8S0°S)1972 as well as CsoOs(t) from Table 2. This procedure defines yearly mean tritium concentrations in 5 ° latitude bands.
3. The hydrological model The hydrological model (for details see Weiss et al. [8[ and Weiss [191) converts the tritium concentrations in precipitation into absolute rates of tritium input to the ocean. Tritium input to the oceans is by rainout, water vapor exchange, and by continental tritium runoff. The net direct input by rainout and water vapor exchange, d (TU • m/yr), is calculated from equation (3):
dEp=(P + E 1.
h )
1- h
DEp (TU • m)
3749 4973 5808 4861 4238 4418 4895 5898 5675 5339 4933 4580 4104 1750
729.9 548.4 369.8 355.4 558.4 660.7 778.9 926.1 999.8 1003.8 1007.2 904.8 676.2 420.5
65,221 (total)
(average)
where
A (103 km 2)
1
' cv - E . h~ -1- ~ )
" Cs
(3)
Rainout is given by the product of the precipitation rate, P, and the tritium concentration o f marine precipitation, cv. Vapor exchange is a process which is coupled with the evaporation. The net tritium input due to water vapor exchange is given by the sec-
IEp (MCi) 8.87 8.84 6.96 5.60 7.67 9.46 12.35 17.70 18.38 17.36 16.10 13.43 8.99 2.38 154.09 (total)
ond and third term of equation (3). It is proportional to the evaporation rate E and depends on the water vapor gradient in the atmospheric boundary layer (h : water vapor content in 10 m above sea level relative to saturation, i.e. h = 1 at the air-water interface). The "ingoing" fraction (second term) is set proportional to the tritium concentration of marine precipitation, Cp. This assumption is based on observational evidence ( [ 8 ] , see also below), that the tritium concentration in marine water vapor at ship's height on the average has a tritium concentration corresponding to isotopic equilibrium with falling precipitation (isotopic fractionation factor a ~ 1.1). The third term in equation (3) is the "outgoing" fraction. It is set proportional to the tritium concentration of surface water, Cs. As Cs is in any case much smaller than co (cf. [11]), the latter term is a minor correction only. Since on the other hand o~~ 1, E ~ P, and h ~ 0.75, it follows that tritium input by water vapor exchange greatly exceeds that by rainout. For the relative humidity, we use h = 0.74, which is the mean of literature data [ 2 1 - 2 3 ] . It is to be pointed out that equation (3) basically follows the concept given previously by Craig and
442 Gordon [12], while we only neglect kinetic isotopic fractionation. What is new, however, and essential to our approach, is the somewhat unexpected finding that the marine water vapor at ship's height is in fact nearly in isotopic equilibrium with falling rain. This notion is based on tritium analyses of about one-week composites of continuously sampled marine water vapor and precipitation sampled over the same period [8]. The data may be slightly biased, because sampling periods without any precipitation are excluded from the comparison, but basically the near equilibration is an observational fact. In evaluating equation (3), we choose the tropospheric tritium concentration to be that representative for open-ocean conditions (Fig. 2). Because continental tritium concentrations are considerably larger (see section 2), air-mass exchange with continental areas leads to a net tritium inflow into marine areas from the continents. Such net tropospheric advection of tritium from continental areas is estimated from published transport rates of vapor across the continental borders [24,25] and the excess of continental over marine tritium concentrations in precipitation (i.e., threefold) [8,19]. Such tritium is supposed to be taken up by the ocean soon after the air mass in question has left the continent. The length scale of this process is estimated to be of the order of 1000 km. The additional tritium input by river runoff is calculated from published rates of river runoff [20] and the tritium concentrations in precipitation characteristic for the catchment area o f the river in question; the continental tritium concentrations are again taken to exceed the marine ones by a factor of 4. The lagging behind in time of the tritium concentrations o f the rivers relative to those in precipitation is accounted for by a model, which has been established for the Rhine river (Western Europe) [26]. Satisfactory agreement, typically within +20%, has been found between measured tritium discharges available for several rivers on the North American continent and the ones predicted by the model [19, table 9; 8, tables I and II].
4. The rates of tritium input
tion by precipitation and vapor exchange (equation (3)), net tritium inflow from continental areas, and river runoff. The latter two modes are of minor importance, except for the North Atlantic and the northern Indian Ocean. Annual-average tritium input rates for any year, n, and any 5 ° latitude belt, i, are obtained as follows: (a) Open-ocean input is defined by equation (3), which for c~ = 1.12 and h = 0.74 converts to:
dEe(n, i) : [P(i) + 2.541E(01 • cp(n, i)
3.434Cs(n, 0
(4) P and E for any ocean are listed as 5 ° latitudes averages in Tables 3 7, and are assumed to be constant in time. ce(n, i) for the northern hemisphere is:
cp(n, i) = Sq)]S5oo N .Cp, 50°N(n)
(5)
where Sq)/SsooN is listed in Tables 3 - 7 and Cp,5OON(n) in Table 2 ; for the southern hemisphere the extended equation (2) has to be used. Cs is often negligible; for the Pacific and North Atlantic data are available [8,10,11], data for other areas must be guessed. An example is dEp (1964, 3 5 - 4 0 ° N ) = (0.82 + 2.541 • 1.53) • 0.57 - 555 3.434 • 17 = 1431 TU • m/yr for the Atlantic. (b) Tritium outflow from the continents. Only areas of major steady air flows are explicitly considered. Vapor fluxes, Rv(i) are given in Table 3 (North Atlantic) and 6 (Indian Ocean), in km3/yr. The corresponding inputs are:
dv(n, i) = Rv(i)/A(i). 3cv(n, i)
(6)
where A(i) is the area of the respective latitude belt and ocean, and the factor 3 accounts for the larger continental tritium concentration. This procedure distributes the tritium excess observed in continental air over the entire latitude belt. Alternatively, a more localized input could be considered, using prevailing winds and the presumed characteristics length scale for air-ocean transfer (see section 3). (c) River runoff. As before, Tables 3 and 6 list river runoff, Rr. The inputs are:
dr(n, i) = Rr(i)[A(i) . 4or(n, i)
(7)
Cr(n, i) is obtained from equation (5) if Cp,s0°N(n) is As outlined in the previous section, we consider three modes of tritium input, i.e., open-ocean addi-
replaced by or,soON(n) (Table 2). For ocean basins where the latter two contribu-
443 tions are minor, they are taken into account by increasing dEp by an appropriate factor (equation (4)). This means that the tritium from the two sources is being added uniformly over the entire ocean basin area. In these cases, a more detailed account clearly would fall well within the margins of uncertainty of our input assessment. The present procedure gives discrete input data, dtot(n, i), in the form o f products of data vectors, e.g. P(i) and c(n). We regard this as more flexible than a procedure presenting the input in the form of an analytical function in space and time (as e.g. in Fine and Ostlund [10]).
5. Check of the input by comparison to oceanic inventories
TABLE 8 Total tritium input to the world ocean up to the end of 1972. The totals of the direct input, IEp, have been taken from the Tables 3-7; the data for the North Atlantic are taken from Weiss et al. [8]. The calculation of the continental tritium input, IRV, is described in the text. The ratios of the total input,/tot = IEp + IRV, to the open-ocean input, IEp, are 1.34 (North Atlantic), 1.12 (South Atlantic), 1.09 (North Pacific), 1.04 (South Pacific), 1.61 (northern Indian Ocean), and 1.03 (southern Indian Ocean) Northern hemisphere
Southern hemisphere
IEp (MCi)
IRV (MCi)
IEp (MCi)
IRV (MCi)
173.9 60 32.8
91.4 208.9 154.1
11 7 4
Atlantic 519.5 Pacific 674.5 Indian Ocean 54.2
Total (MCi)
795.8 950.4 245.1 1991.3
Tritium observations for the Atlantic and the Pacific Oceans made during the GEOSECS field program allow to estimate the oceanic tritium inventories of these oceans for the time 1 9 7 1 - 1 9 7 3 . These estimates can be used as a constraint to the tritium input that we have derived above, insofar as the integrated model input has to agree with the oceanic inventories. Time-integrated inputs are obtained by replacing in equations (4) to (7) the tritium concentrations by the corresponding time-integrated concentrations, S, according to equation (1). Numerical values for S in 50°N and 50°S, respectively, are given in Table 2. Total time4ntegrated inputs (integration up to 1972) are presented in Tables 3 - 7 . The hemispheric totals of the open-ocean tritium input, DEp, into the individual ocean basins are summarized in Table 8, together with the amounts of the tritium added from the continents. For the North Atlantic, where apparently the continent-derived tritium amounts to about 25% of the total input, the situation has been discussed elsewhere [8]. The relatively high fraction of continental tritium is a result o f the small ocean surface area of the North Atlantic as compared to the size of, and the length o f the border lines to, the adjacent continents. The North Atlantic in this respect certainly is an extreme case compared to most parts of the world oceans. The other special case is the northern Indian Ocean where the continental tritium input is estimated to be comparable to that o f the direct input. This results from high river and vapor runoff in this
area and the small surface area of the northern Indian Ocean. The estimate of the continental tritium for the northern Indian Ocean is believed to be the most uncertain value in Table 8. This is because the hydrological regime of rivers like the Ganges is quite different to that of rivers in Europe or North America, so that the applicability of the river runoff delay model remains questionable. We believe that the uncertainty of the ocean basin input totals does not exceed -+20% in the northern hemisphere and -+30% in the southern hemisphere [ 19]. The largest source of uncertainty is the humidity, h (equation (3)). The uncertainty of the continental contributions may well exceed +30%. Table 9 compares the input totals for the North Atlantic and the Pacific with the corresponding oceanic tritium inventories. The Atlantic inventory has been reported previously [8]. Local Pacific inventories were obtained by vertical integration of available tritium versus depth profiles [5,14]. Multiplying local-inventory averages in 10 ° latitude bands by the band surface areas, and summing up latitudinally, gave the total tritium inventories in Table 9. The quoted uncertainty of +-6% includes the variability of the local inventories within the 10 ° latitude bands and the blank uncertainty of the tritium measurements [8]. For both oceans, the total inputs and the inventories agree within the limits o f uncertainty. The uncertainties of the inventory are distinctly
444
TABLE 9 Comparison of the total tritium input and the tritium inventories of the North Atlantic and the Pacific Ocean. North Atlantic data are taken from Weiss et al. [8] ; procedure to obtain the inventories see Weiss et al. [8]. Of the Pacific input 734.5 MCi reside in the North Pacific
5000 Q
~ o O ]
TU.m
~000
O D
North Atlantic (0-80 ° N) Pacific Ocean
Total tritium input * (MCi)
Tritium inventory (MCi)
693.4
665 -+ 8%
950.4
741 -+ 6%
~000
O
O
"[--112o
°<
2000
1000
* Estimated uncertainty -+20%. I
60
smaller than those o f the inputs; one might therefore tentatively correct the input rates down accordingly by a common factor. Such adjustment, however, presumably is within the uncertainty o f the input rates as we have defined them. In summary, the comparison o f the ocean basin time-integrated tritium inputs and oceanic tritium inventories, strongly supports our assessment of the tritium input. The comparison naturally can only be made on the totals. However, a comparison for the Pacific by latitude is presented in the next section, together with a general discussion.
6. Discussion As an illustration, Fig. 3 shows the comparison of the time-integrated input and the observed, local inventories for the Pacific by latitude. In general the correspondance is gratifying, although it can neither prove nor disprove the latitude dependence o f our tritium input. In detail, the local inventories appear to be highest in 2 0 - 4 0 ° N , whereas the input maximum is found at higher latitudes. Because the bulk of the tritium has had about 10 years travel time in the ocean between input and observation, a latitudinal displacement between the input and the inventories is to be expected. In fact, a general net southward transport o f tritium for the northern North Pacific has been postulated previously [27]. South of about 40°S the model input seems to be systematically
i
t
,~0
i
L
20*N
i
i
0
i
I
20*'3
I
I
gO
I
i
6O
i
I
8O
Fig. 3. Local tritium inventories, in TU . m (103 TU - m = 3.24 ~Ci/m2), for the Pacific Ocean, obtained by vertical integration of tritium profiles reported in Ostlund et al. [5] (circles) and in Michel [14] (squares). The data have been time-corrected to represent the conditions in 1972. The histogram gives the input of tritium per 5 ° latitudinal band (numerical values cf. Table 3). A corresponding comparison for the North Atlantic is given in Weiss et al. [8].
higher than the local inventories. Part of the latter discrepancy may be caused by the uncertainty o f the input in these areas. It is to be noted that no latitudinal shift between input and inventories is apparent for the North Atlantic, which agreement, however, is regarded as fortuitous [8, fig. 1 ]. For the Pacific tritium totals, the contribution o f the higher latitudes is smaller than may appear at first glance from Fig. 3, because the areas involved are relatively small. The latitudinal variation of the tritium input (Fig. 3 and [8, fig. 1]) has its northern hemispheric maxima polewards of those of the 9°Sr deposition [28]. This is believed to be caused by the net deposition rates in the case o f tritium being very different for land and sea areas, contrary to the situation for 9°Sr: The principal marine deposition mechanism for tritium is vapor exchange (driven by evaporation, see equation (3)), whereas on the continent tritium is reevaporated from the ground. This results in the marine net deposition rates being roughly one order of magnitude larger than continental ones. For 9°Sr
445 such differences are minor, because the dominant deposition mechanism is rainout. Therefore tropospheric tritium appreciably accumulates over continental areas. Because the atmospheric circulation is primarily zonal, relatively more tritium than 9°Sr is available in the troposphere for oceanic deposition, in latitudes of a larger land to sea surface area ratio * The observed shift results because the land/sea area ratio considerably increases with latitude north of ~40°N. We can, of course, not exclude further causes such as differences in stratosphere-troposphere transfer. If our proposed mechanism is effective, it is further to be expected that the latitudinal shift in the tropospheric concentrations should still be larger, because the evaporation drops off faster with latitude than does precipitation. The predicted values of total tritium input are 734 MCi for the North Pacific and 693 MCi for the North Atlantic. Considering that the surface area of the North Pacific exceeds that of the North Atlantic by almost a factor of two, the results of comparable totals of tritium input surprises. This discrepancy is mainly a result of differences in evaporation and precipitation in both oceans, north of about 30°N, and of the limited northward extension of the North Pacific. A third, though minor, aspect is the difference in the continental contributions, which are almost a factor 3 higher in the North Atlantic. From the data given in Table 8 the global oceanic tritium inventory can be estimated: our input total at the end of 1972 gives 2.0 GCi of tritium to be found in the oceans; 75% of this amount has been added to the northern, 25% to the southern ocean. As Table 9 indicates the input calculations to be on the high side, we correct the 2.0 GCi tentatively down by 10%, In other words, we recalibrate our input totals by comparison to the North Atlantic and Pacific inventories. Furthermore, the tritium remaining in the continental hydrosphere can be estimated, by using the abovementioned river-runoff delay model, to be about 5%. Neglecting any atmospheric contributions, a global tritium inventory of 1.9 GCi (for end-1972) results. The uncertainty of this global inventory is believed to
* Apart from the tritium excess considered in our calculations, the continent-derived tritium accumulation presumably also penetrates into higher tropospheric layers, which have much longer transfer times into the ocean.
be no larger than +_20%. Similar inventory figures have been published previously [6,14,30]. Our estimate corresponds to an average tritium yield in nuclear fusion of 0.9 kg/Mt fusion (see Miskel [31 ]iAcknowledgements We are grateful to H. G6te 0stlund, Miami, for letting us use his GEOSECS Pacific tritium results. This work was supported by the Deutsche Forschungsgemeinschaft and in part also by the Heidelberger Akademie der Wissenschaften.
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