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Thorium-228 in marine plankton and sea-water
EARTH AND PLANETARY SCIENCE LETTERS 6 (1969) 451-456. NORTH-HOLLAND PUBLISHING COMP., AMSTERDAM
THORIUM-228
IN MARINE
PLANKTON
AND SEA-WATER
R.D.CHERRY, I.H.GERICKE Physics Department, University of Cape Town, Rondebosch, C.P., South Africa L.V.SHANNON Division of Sea Fisheries, Sea Point, C.P., South Africa Received 18 July 1969 Data are given for thorium-228 in zooplankton, phytoplankton and sea-water from seven different oceanographical water masses in the South Atlantic and around the Southern African coast. The levels in plankton are easily detectable, being up to 27 × 10-18 g thorium-228 per g wet zooplankton and up to 65 × 10-18 g thorium-228 per g wet phytoplankton. What appear to be real variations from one water mass to another ate observed, and the possible utility of thorium-228 as a natural oceanographic tracer is noted. The levels in sea-water are in general at or below the sensitivity level of the technique used, but for one set of samples a particularly high thorinm-228 content of 2.7 X 10-15 g thorium-228 per lithe sea-water is obtained. This set is from the Walvis Ridge area, and attention is drawn to the fact that this region is likely to be particularly interesting as far as thorium and certain other trace elements are concerned.
1. Introduction *
Recent studies o f thorium-series isotopes in sea water have raised intriguing problems. The situation can be summarized as follows: 1) Thorium-232 in solution in sea water has been quoted at levels ranging between 5 X 10 - 9 and 2 X I 0 - 1 0 g/1 for mid-ocean samples [1,2] and up to 2 X 10 - 7 g/1 for samples from inland seas and coastal waters [ 2 - 5 ] . Since uranium in solution in sea water is known to be about 3 × 10 - 6 g/l (e.g. [6,7] ), a thorium-uranium ratio very much less than the typical terrestrial value of about 3 results. 2) Moore and Sackett [1 ] and Somayajulu and Goldberg [2] have reported that the thorium-228 to
* Since submitting this manuscript two recently published papers have come to our attention. These are by W.S. Moore, "Measurement of Ra 228 and Th 228 in Sea Water", J. Geophys. Res. 74 (1969) 694 and A.Kaufman, "The Thorium-232 concentration of Surface Ocean Water", Geochim. et Cosmochim. Acta 33 (1969) 717. Both include interesting data on thorium-228 in sea-water, and should be referred to in our introductory paragraph.
thorium-232 activity ratio is between 10 and 25 for mid-ocean samples, and about unity for a single coastal water specimen. These levels correspond to a mid-oceanic thorium-228 concentration o f between 10 -19 and 10 -17 g/l. Moore and Sackett [1] predict radiums228 in coastal waters from measurements on shells, and quote values which imply an excess over thorium-228 o f one or two orders o f magnitude. Lazarev et al. [8] have found radium-228 in Black Sea waters at levels which give a radium-228 to thorium-232 activity ratio of 500. 3) In an attempt to explain some features of the above data, Somayajulu and Goldberg [2] have hypothesized that thorium in sea water might be associated with solid phases. Radium-228 could then be preferrentially leached from these phases, subsequently giving rise to the observed excess o f thorium-228 over thorium-232. The observed activity ratios imply that at least 10 to 25 times the amount o f thorium in solution is associated with the solid phase; if radium-228 is taken up by the solution with less than 100% efficiency, the amount o f thorium in the solid phase must be correspondingly greater. Support for the association o f thorium with
452
R.D.CHERRY, I.H.GERICKE and L.V.SHANNON
solid phases is provided by Dement'yev and Syromyatnikov [9], who have studied the mode of occurrence of thorium isotopes in ground waters using centrifuging and dialysis techniques. They f'md a tendency for thorium-232 to be associated with suspended matter and for thorium-228 to be found in ionic solution, and state, without giving data, that in an experiment on sea water "it was found that all thorium contained in sea water is retained on the f'dter and is, therefore, suspended in sea water". Nikolayev et al. [4] also favour this association, and attribute wide variations in thorium content in the Sea of Azov "to the presence of fairly considerable amounts of suspended matter". In contrast to the above results for sea water, data reporting thorium-series activity in marine biota are scarce. In particular, we are not familiar with any published figures for thorium-228 in marine plankton other than our own earlier work [10], which indicated a variable activity level averaging at about 30 X 10 -18 g thorium-228 per g of wet plankton for a selection of samples from mostly inshore localities. In the present paper we shall report results for thorium228 in marine plankton samples from different oceanic areas, and also additional data which show a level of thorium-228 in sea water from one of these areas which is considerably higher than the values quoted above [1,2].
2. Methodology The technique used is based on that of Turner et al. [11]. Plankton samples are dried at 105°C prior to fine grinding for alpha-counting. Typically about 1 g of dried plankton is obtained and concentration factors from wet to dry plankton vary widely in the vicinity of 13 from less than 3 to more than 30. Seawater is collected in 200 ml polythene bottles, immediately frozen and subsequently freezedried in a 1.5 1 round-bottomed flask. This process results in the removal of water from the sample, and a fine homogeneous powder remains. The concentration factor from sea-water to salt powder is generally just under 30. This powder is packed immediately on zinc sulphide phosphor in the manner described by Turner et al. [11] and used previously in this laboratory [10, 12-14]. This method provides an individual sealed
sample phosphor system. No chemical treatment is involved, and the process as a whole involves minimum likelihood of contamination. Samples are stored in a dessicator over phosphorus pentoxide; failure to do this results in moisture being adsorbed on the zinc sulphide grains and consequent errors in the interpretation of data. After storage for a minimum period of three weeks to allow a degree of radioactive equilibrium to be attained, the samples are counted on a conventional 5-inch diameter photo-multiplier tube set-up. The thorium-228 contribution to the total activity is determined by counting electronically the pairs of alpha-particles emitted in rapid siccession by the nuclides thoron-220 and 0.16 sec half-life polonium-216. The ratio of the pairs count rate to the total count is a measure of the percentage of the total alpha-activity which is due to thorium-228 and daughters, and since ~ e thorium-series after thorium228 comes into equilibrium in less than three weeks one can obtain a value for the thorium-228 concentration in the sample. The theoretical formulae involved are simple and well-known [ 11,12,15], but cannot be used for exact calibration purposes because of lack of precision in alpha-particle range data and because the granular zinc sulphide phosphor does not provide a strictly 2rt-geometry system. Absolute levels of accuracy are not likely to be an issue in the present work, but we have calibrated our technique by freezedrying and counting samples of different composition loaded with traces of thorium sulphate known to be in equilibrium. The results agree with those predicted by standard alpharange formulae and 27r-geometry to better than 20%; we think that this figure is likely to be a reasonable estimate of the contribution of calibration error to our final results. The technique described above provides a non-contaminatory method of measuring thorium-series activity in sea-water and plankton samples, and is so straightforward that it permits a relatively large number of samples to be analyzed. The following limitations and practical considerations should, however, be noted: i) Because of the low activities and long counting times involved, the sensitivity of the technique is limited. Measurement on 4-inch diameter "blanks" show that the pairs rate background is certainly less than 0.05 pairs per h, and we have used this con-
THORIUM-228 IN MARINE PLANKTONAND SEA WATER servative upper estimate as our pairs background value. Using typical values for the concentration factors involved shows that this value corresponds to 3.4 X 10-16 g thorium-228 per I sea-water. The corresponding figure for plankton, where the quantity of material available usually limits us to 3-inch diameter discs, is about 1.0 × 10 -18 g thorium-228 perg wet plankton if we assume a wet to dry concentration factor of 13. These figures can be taken as indicative of the ultimate sensitivity of the technique. It is immediately plain that this sensitivity is inadequate to cope with the thorium-228 levels in sea water given refs. [1] and [2], but that the levels in plankton [10] should be detectable. ii) Interpretation of thick-source alpha-counting experiments, such as these, requires the assumption of homogeneous distribution of alpha-emitters within the sample. Since the freezedried salt powder is very time (of the order of micron size if kept dry) and since the dried plankton can be finely ground this assumption does not in our experience cause trouble, and unpublished theoretical calculations of the effect of inhomogeneities show that gross errors are very unlikely. The inhomogeneity problem should nonetheless be kept in mind as a possible difficulty when the stage is reached where the absolute accuracy is of importance. iii) As far as the sea-water data are concerned, it should be emphasized that, since the freeze-fried salt powder is counted without further processing, suspended matter in the sea-water will contribute to the final result as well as dissolved constituents. For previously published data such as in refs. [1] and [2] it is not entirely certain that this is the case. iv) The small 200 ml size of the sea-water samples makes for great convenience and ease of sampling. However, it does also aggravate the problem of genuine sample to sample scatter. Wide variations in trace elementeoncentrations in sea water have been reported on occasions when samU sample sizes are used, and this is particularly acute when particulate matter is involved [ 16]. Similar considerations, enhanced if anything because of species variability from sample to sample, are likely to apply to the plankton data. For this reason we feel that it is essential that no great credence be attached to individual samples as representative values, and we follow the practice of quoting mean values for as large a number of samples as possible for each oceanographic area.
453
3. Results Remits from 33 sea-water, 45 zooplankton and 7 phytoplankton samples are quoted in table 1. Mean values are quoted for seven different oceanographic water masses, as indicated approximately in the map in fig. 1. The discussion of the various water masses concerned is given in Shannon [ 17], as are their definition in terms of temperature and salinity ranges. The Agulhas Surface Water Samples were collected during July 1967, the Walvis Ridge Water Samples during November, 1968, and the remainder during March, 1968. All plankton samples were collected within the upper 200 metre surface layer. Species identification of the plankton showed little except that a wide variety of species were involved; only in the case of the phytoplankton samples from the Agulhas Surface Water was one category (viz. various Chaetoceros spp.) found to make up a large fraction of the sample material. All means have of course been corrected for the background blank values mentioned earlier. The figures in brackets in the table show the number of samples used in calculating each mean. The following conclusions emerge: i) For the sea-water samples all except one of the means are less than the value of 3.4 X 10-16 g thorium-228 per 1 sea-water which we have used for our background blank. Although five of these seven mean values are in fact positive, we are adopting the conservative standpoint that they are so close to the sensitivity limit of our technique that it would be dangerous to regard them as significantly different from zero. For the eight mean, viz. sea-water from the Walvis Ridge area, the level observed is approximately eight times our background level. The mean of eight samples gives a value of 27 X 10 -16 g thorium-228 per 1 for this water, and the range of values observed extends from 8 to 48 X 10 -16 g thorium-228 per 1. We regard these as quite definitely significant, particularly because they are confirmed by an additional set of sea-water samples (unfortunately without contingent plankton samples) obtained more than a year previously; this earlier set gave a mean values of 22 × 10 -16 g thorium-228 per 1. ii) All of the plankton mean values are above the sensitivity level of approximately 1.0 × 10 -18 g thorium-228 per g wet plankton. Real variations appear to exist between the means from the different
454
R.D.CHERRY, I.H.GERICKE and L.V.SHANNON
Table 1 Mean thorium-228 contents of plankton and sea water, in units of 10-18 g thorium-228 per g wet plankton of sea water. The number of samples used for determining each mean is given in brackets. Water mass
Sample station Nos.
Zooplankton
Phytoplankton
Sea-water
Agulhas surface water
1-7
9(8)
65(3)
< 0.34 (7)
Benguela current
8-9
3(3)
40(1)
< 0.34 (2)
7(7)
19(1)
<0.34 (5)
Subtropical surface water
10-12, 24-25
Confused area near Tristan da Cunha
13-17
15(7)
-(0)
< 0.34 (5)
Converge between sub-tropical and sub-antarctic surface waters
18-20
3(6)
-(0)
< 0.34 (3)
South side of convergence merging into sub-antarctic surface water
21-23
2(10)
9(2)
<0.34 (3)
Walvisridge water
26-29
-(0)
< 2.7 (8)
27(4)
s 20°~! O/~2|
wMt~
ATLANTIC OCEAN
2.5"
30"
g
35"
,;
,;
,.5
".
,2
-'
,"
o c e ,. N
SORGO2"CA"
I~ 1RISlAN DACUN~IA
,; 40"
'2
SUBTROPICAL 20
CONVERGENCE
2; SUBANTARCTIC REGION
~;~ 45"
,5 o
,o •
~o
0o
~o
,0 o
W
;~°
So:-
250
So*
s5 °
E
Fig. 1. Sample collection stations and approximate sub-division of oceanographic water masses. water masses. It is particularly noteworthy that the three lowest thorium-228 values are associated with cold waters such as the Benguela Current and the waters
at the convergence between sub-tropical and sub-antarctic surface waters. The water of Benguela Current is moreover "Central Water", and "Central Water" is
THORIUM-228 IN MARINE PLANKTONAND SEA WATER formed in the sub-tropical convergence region by a complex cellular mixing process [18]. We see therefore that the low thorium-228 levels all appear to be associated with water which has its origin in the region of the sub-tropical convergence; the possible utility of thorium-228 as a natural oceanographic tracer is obvious. Also noteworthy is the fact that the zooplankton from the Walvis Ridge area tops the list with amean of 27 X 10 -18 g thorium-228 per g. This mean was obtained from four samples, with a range from 23 to 36 X 10-18 g. iii) Data for phytoplankton in table 1 are sparse, but the four mean value which are available are all considerably higher than the corresponding zooplankton means. In view of the calcium-rich nature of phyto- as opposed to zooplankton, and the fact that previous data [19] suggest that diatoms select radium preferentially to calcium, it is probable that these higher values indicate that the source of 1.9 yr thorium228, in phytoplankton at least, is governed to an appreciable extent by decay from 6.7 year radium228. The implied [1] and observed [8] excess of radium-228 over thorium-232 activity in sea-water support this point of view. iv) If we assume that the published levels [ 1,2] of thorium-228 at about 10-18 g per 1 sea water are typical, and take our lowest plankton value of 2 X 10 -18 g thorium-228 per g wet plankton, we obtain a concentration factor from sea-water to plankton of about 2000. Use of any of the other mean values from table 1 for thorium-228 in plankton will Of course raise this factor, in some cases by an order of magnitide. On the other hand, direct use of the table 1 data for both plankton and sea-water from the Walvis Ridge area gives a concentration factor of just less than 10. If we can assume that both the previously published data and our own table 1 data are essentially correct, then it is plain that considerable real variation in this concentration factor must exist. It is not unlikely that such variations are connected with varying degrees of association of thorium-228 with the solid phase; Somayajulu and Golberg's hypothesis that thorium in sea-water might be associated with the solid phase should be investigated further, despite its apparent lack of quantitative attraction [20]. The high concentration factors of the order of 104 which appear to hold in some circumstances should be compared with the data of Szabo [19] for
455
radium-226. These indicated a radium concentration factor of much less than 103 (the exact figure depending on the value assumed for the wet-to-dry mass concentration factor for plankton). Direct biological removal of thorium-228 from sea-water to plankton would appear to be indicated as a process which competes with radium-228 decay as a source of thorium-228 in plankton. It is clear that data on both thorium-232 and thorium-228 in both sea-water and plankton will be required before the relative importance of biological removal of thorium isotopes can be assessed quantitatively. v) The Walvis Ridge area is obviously a region of considerable oceanographical interest. Shannon and Van Rijswijck [21] have noted an uplift of lower water masses over the Walvis Ridge region, and the South African West Coast fishing grounds are generally known to be areas of strong upwelling and high productivity [22]. Goldberg and Arrhenius [23] have associated high barium content in sediments with areas of high surface productivity, and Turekian and Tausch [24] have reported exceptionally high barium in the tops of sediment core from the general area of the southwest African coast. Furthermore Somayajulu and Goldberg [2] have drawn attention to a possible association between barium and thorium. It is tempting to postulate that upwelling over the Walvis Ridge introduces thorium-rich suspended matter from sediment tops into the overlying waters, but much additional data for thorium and other trace elements, preferably involving depth profiles, is necessary before finn conclusions can be drawn.
4. CONCLUSION We feel that the data in this paper show clearly: (a) That thorium-228 is present in marine plankton at easily detectable levels ranging up to 27 X 10 -18 g thorium-228 per g wet plankton for zooplankton and to 65 X 10 -18 g per g for phytoplankton. (b) That real variations in the thorium-228 content of plankton occur, and that these variations show interesting trends from one water mass to another. The possible utility of thorium-228 as a natural oceanographic indicator is apparent. (c) That in at least one region of the ocean, viz. the Walvis Ridge area, level the of thorium-228 in sea-
456
R.D.CHERRY, I.H.GERICKE and L.V.SHANNON
water is considerably higher than the commonly accepted values [1,2]. The possibility that this is due to the association of thorium with the solid phase should not be overlooked. (d) That the Walvis Ridge area is a region of particular oceanographical interest, at least insofar as certain trace element concentrations are concerned. (e) That intriguing problems concerning the biogeochemical balance o f thorium-232, thorium-228 and radium-228 in sea-water and in marine biota still remain to be solved. Thorium-232 to thorium-228 ratios must be measured in plankton as well as in sea-water and an attempt should be made to differentiate between thorium isotopes in solution and those in the solid phase. The very difficult problem o f radium228 measurement is also worthy of serious consideration.
5. Acknowledgements We are grateful for financial assistance from the Staff Research Fund, University of Cape Town, the C.S.I.R. Oceanographic Research Unit at the University o f Cape Town, and the South African Atomic Energy Board. We are particularly appreciative of the extensive sample collection facilities provided b y the Division o f Sea Fisheries, Sea Point, and we must thank Dr. A.De Decker and Mrs. A.Coghlan for carrying out the species identification of the plankton.
References [1] W.S.Moore and W.M.Sackett, Uranium and thorium series inequiHbrium in sea water, J. Geophys. Res. 69 (1964) 5401. [ 2] B.L.K.Somayajulu and E.D.Goldberg, Thorium and uranium isotopes in sea water and sediments, Earth and Planetary Science Letters 1 (1966) 102. [3] K.F.Lazarev, D.S.Nikolayev and S.M.Grashchenko, Concentration of the thorium isotopes of sea water, Radiokkhimiya 3 (1961) 623. [4] D.S.Nikolayev, K.F.Lazarev and S.M.Grashchenko, The concentration of thorium isotopes in the water of the Sea of Azov, Doklady of the Academy of Sciences of the U.S.S.R., Earth Sciences Sections, 138 (1962) 489. [5] Y.Miyake, K.Saruhashi, Y.Katsuragi, T.Kanazawa and Y. Sugimura, Uranium, radium, thorium, ionium, strontium 90 and cesium 137 in coastal waters of Japan, in: "Recent Researches in the Fields of Hydrosphere, Atmosphere and Nuclear Geochemistry", Sugawara Festival Volume, Tokyo (1964) 127.
[6] E.Rona, L.O.Gilpatrick and L.M.Jeffrey, Uranium determination in sea water, Trans. Am. Geophys. Union. 37 (1956) 697. [7] Y.Miyake, Y.Sugimura and T.Uchida, Ratio U234/U 238 and the uranium concentration in sea water in the Western North Pacific, J. Geophys. Res. 71 (1966) 3083. [8] K.F.Lazarev, S.M.Grashchenko, D.S.Nikolayev and V.M.Drozhshin, Concentration of mesothorium 1 in Black Sea water, Doklady of the Academy of Sciences U.S.S.R., Earth Sciences Sections 164 (1965) 189. [9] V.S.Dement'yev and N.G.Syromyatnikov, Mode of occurrence of thorium isotopes in ground waters, Geochemistry International 2 (1965) 141. [ 10] R.D.Cherry, Alpha-radioactivityof plankton, Nature 203 (1964) 139. I11 [ R.C.Turner, J.M.Radley and W.V.Mayneord, The alpharay activity of human tissues, British Journal of Radiology XXXI (1958) 397. [12] R.D.Cherry, The determination of thorium and uranium in geological samples by an alpha-counting technique, Geochim. et Cosmochim. Acta 27 (1963) 183. [ 13] R.D.Cherry, Thorium and uranium contents of australites, Nature 195 (1962) 1184. [ 14] V.Hasson and R.D.Cherry, Alpha-radioactivityof human blood, Nature 210 (1966) 591. [15] G.D.Finney and R.D.Evans, The radioactivity of solids determined by alpha-ray counting, Phys. Rev. 48 (1935) 503. [16] M.B.Schaefer,and Y.M.M.Bishop, Particulate iron in offshore waters of the Panama Bight in the Gulf of Panama, Limnology and Oceanography 3 (1958) 137. [17] L.V.Shannon, The Oceanic Circulation Pattern off South Africa, Fish. Bull No. 6 (in press). [ 18] M.J.Orren, Hydrology of the Southwest Indian Ocean, Investl. Rep. Div. Sea Fish. S. Aft. No. 55 (1966). [ 19] B.J.Szabo, Radium content in plankton and sea water in the Bahamas, Geochim. et Cosmochim. Acta 31 (1967) 1321. [20] M.Bernat and E.D.Goldberg, Thorium Isotopes in the Marine Environment, Earth and Planetary Science Letters 5 0969) 308. [21] L.V,Shannon and M.van Rijswijck, Physical Oceanography of the Walvis Ridge Region, Investl. Rep. Div. Sea Fish. S. Aft. No. 73 (1968). [22] L.V.Shannon, Hydrology of the Sotuh and West Coasts of South Africa, Investl. Rep. Div. Sea Fish. S. Aft., No. 58 (1966). [23] E.D.Goldberg and G.O.S.Arrhenius, Chemistry of Pacific pelagic sediments, Geochim. et Cosmoehim. Acta 13 (1958) 153. [ 24] K.K.Turekian and E.M.Tausch, Barium in deep-sea sediments of the Atlantic Ocean, Nature 201 (1964) 696.
THORIUM-228
IN MARINE
PLANKTON
AND SEA-WATER
R.D.CHERRY, I.H.GERICKE Physics Department, University of Cape Town, Rondebosch, C.P., South Africa L.V.SHANNON Division of Sea Fisheries, Sea Point, C.P., South Africa Received 18 July 1969 Data are given for thorium-228 in zooplankton, phytoplankton and sea-water from seven different oceanographical water masses in the South Atlantic and around the Southern African coast. The levels in plankton are easily detectable, being up to 27 × 10-18 g thorium-228 per g wet zooplankton and up to 65 × 10-18 g thorium-228 per g wet phytoplankton. What appear to be real variations from one water mass to another ate observed, and the possible utility of thorium-228 as a natural oceanographic tracer is noted. The levels in sea-water are in general at or below the sensitivity level of the technique used, but for one set of samples a particularly high thorinm-228 content of 2.7 X 10-15 g thorium-228 per lithe sea-water is obtained. This set is from the Walvis Ridge area, and attention is drawn to the fact that this region is likely to be particularly interesting as far as thorium and certain other trace elements are concerned.
1. Introduction *
Recent studies o f thorium-series isotopes in sea water have raised intriguing problems. The situation can be summarized as follows: 1) Thorium-232 in solution in sea water has been quoted at levels ranging between 5 X 10 - 9 and 2 X I 0 - 1 0 g/1 for mid-ocean samples [1,2] and up to 2 X 10 - 7 g/1 for samples from inland seas and coastal waters [ 2 - 5 ] . Since uranium in solution in sea water is known to be about 3 × 10 - 6 g/l (e.g. [6,7] ), a thorium-uranium ratio very much less than the typical terrestrial value of about 3 results. 2) Moore and Sackett [1 ] and Somayajulu and Goldberg [2] have reported that the thorium-228 to
* Since submitting this manuscript two recently published papers have come to our attention. These are by W.S. Moore, "Measurement of Ra 228 and Th 228 in Sea Water", J. Geophys. Res. 74 (1969) 694 and A.Kaufman, "The Thorium-232 concentration of Surface Ocean Water", Geochim. et Cosmochim. Acta 33 (1969) 717. Both include interesting data on thorium-228 in sea-water, and should be referred to in our introductory paragraph.
thorium-232 activity ratio is between 10 and 25 for mid-ocean samples, and about unity for a single coastal water specimen. These levels correspond to a mid-oceanic thorium-228 concentration o f between 10 -19 and 10 -17 g/l. Moore and Sackett [1] predict radiums228 in coastal waters from measurements on shells, and quote values which imply an excess over thorium-228 o f one or two orders o f magnitude. Lazarev et al. [8] have found radium-228 in Black Sea waters at levels which give a radium-228 to thorium-232 activity ratio of 500. 3) In an attempt to explain some features of the above data, Somayajulu and Goldberg [2] have hypothesized that thorium in sea water might be associated with solid phases. Radium-228 could then be preferrentially leached from these phases, subsequently giving rise to the observed excess o f thorium-228 over thorium-232. The observed activity ratios imply that at least 10 to 25 times the amount o f thorium in solution is associated with the solid phase; if radium-228 is taken up by the solution with less than 100% efficiency, the amount o f thorium in the solid phase must be correspondingly greater. Support for the association o f thorium with
452
R.D.CHERRY, I.H.GERICKE and L.V.SHANNON
solid phases is provided by Dement'yev and Syromyatnikov [9], who have studied the mode of occurrence of thorium isotopes in ground waters using centrifuging and dialysis techniques. They f'md a tendency for thorium-232 to be associated with suspended matter and for thorium-228 to be found in ionic solution, and state, without giving data, that in an experiment on sea water "it was found that all thorium contained in sea water is retained on the f'dter and is, therefore, suspended in sea water". Nikolayev et al. [4] also favour this association, and attribute wide variations in thorium content in the Sea of Azov "to the presence of fairly considerable amounts of suspended matter". In contrast to the above results for sea water, data reporting thorium-series activity in marine biota are scarce. In particular, we are not familiar with any published figures for thorium-228 in marine plankton other than our own earlier work [10], which indicated a variable activity level averaging at about 30 X 10 -18 g thorium-228 per g of wet plankton for a selection of samples from mostly inshore localities. In the present paper we shall report results for thorium228 in marine plankton samples from different oceanic areas, and also additional data which show a level of thorium-228 in sea water from one of these areas which is considerably higher than the values quoted above [1,2].
2. Methodology The technique used is based on that of Turner et al. [11]. Plankton samples are dried at 105°C prior to fine grinding for alpha-counting. Typically about 1 g of dried plankton is obtained and concentration factors from wet to dry plankton vary widely in the vicinity of 13 from less than 3 to more than 30. Seawater is collected in 200 ml polythene bottles, immediately frozen and subsequently freezedried in a 1.5 1 round-bottomed flask. This process results in the removal of water from the sample, and a fine homogeneous powder remains. The concentration factor from sea-water to salt powder is generally just under 30. This powder is packed immediately on zinc sulphide phosphor in the manner described by Turner et al. [11] and used previously in this laboratory [10, 12-14]. This method provides an individual sealed
sample phosphor system. No chemical treatment is involved, and the process as a whole involves minimum likelihood of contamination. Samples are stored in a dessicator over phosphorus pentoxide; failure to do this results in moisture being adsorbed on the zinc sulphide grains and consequent errors in the interpretation of data. After storage for a minimum period of three weeks to allow a degree of radioactive equilibrium to be attained, the samples are counted on a conventional 5-inch diameter photo-multiplier tube set-up. The thorium-228 contribution to the total activity is determined by counting electronically the pairs of alpha-particles emitted in rapid siccession by the nuclides thoron-220 and 0.16 sec half-life polonium-216. The ratio of the pairs count rate to the total count is a measure of the percentage of the total alpha-activity which is due to thorium-228 and daughters, and since ~ e thorium-series after thorium228 comes into equilibrium in less than three weeks one can obtain a value for the thorium-228 concentration in the sample. The theoretical formulae involved are simple and well-known [ 11,12,15], but cannot be used for exact calibration purposes because of lack of precision in alpha-particle range data and because the granular zinc sulphide phosphor does not provide a strictly 2rt-geometry system. Absolute levels of accuracy are not likely to be an issue in the present work, but we have calibrated our technique by freezedrying and counting samples of different composition loaded with traces of thorium sulphate known to be in equilibrium. The results agree with those predicted by standard alpharange formulae and 27r-geometry to better than 20%; we think that this figure is likely to be a reasonable estimate of the contribution of calibration error to our final results. The technique described above provides a non-contaminatory method of measuring thorium-series activity in sea-water and plankton samples, and is so straightforward that it permits a relatively large number of samples to be analyzed. The following limitations and practical considerations should, however, be noted: i) Because of the low activities and long counting times involved, the sensitivity of the technique is limited. Measurement on 4-inch diameter "blanks" show that the pairs rate background is certainly less than 0.05 pairs per h, and we have used this con-
THORIUM-228 IN MARINE PLANKTONAND SEA WATER servative upper estimate as our pairs background value. Using typical values for the concentration factors involved shows that this value corresponds to 3.4 X 10-16 g thorium-228 per I sea-water. The corresponding figure for plankton, where the quantity of material available usually limits us to 3-inch diameter discs, is about 1.0 × 10 -18 g thorium-228 perg wet plankton if we assume a wet to dry concentration factor of 13. These figures can be taken as indicative of the ultimate sensitivity of the technique. It is immediately plain that this sensitivity is inadequate to cope with the thorium-228 levels in sea water given refs. [1] and [2], but that the levels in plankton [10] should be detectable. ii) Interpretation of thick-source alpha-counting experiments, such as these, requires the assumption of homogeneous distribution of alpha-emitters within the sample. Since the freezedried salt powder is very time (of the order of micron size if kept dry) and since the dried plankton can be finely ground this assumption does not in our experience cause trouble, and unpublished theoretical calculations of the effect of inhomogeneities show that gross errors are very unlikely. The inhomogeneity problem should nonetheless be kept in mind as a possible difficulty when the stage is reached where the absolute accuracy is of importance. iii) As far as the sea-water data are concerned, it should be emphasized that, since the freeze-fried salt powder is counted without further processing, suspended matter in the sea-water will contribute to the final result as well as dissolved constituents. For previously published data such as in refs. [1] and [2] it is not entirely certain that this is the case. iv) The small 200 ml size of the sea-water samples makes for great convenience and ease of sampling. However, it does also aggravate the problem of genuine sample to sample scatter. Wide variations in trace elementeoncentrations in sea water have been reported on occasions when samU sample sizes are used, and this is particularly acute when particulate matter is involved [ 16]. Similar considerations, enhanced if anything because of species variability from sample to sample, are likely to apply to the plankton data. For this reason we feel that it is essential that no great credence be attached to individual samples as representative values, and we follow the practice of quoting mean values for as large a number of samples as possible for each oceanographic area.
453
3. Results Remits from 33 sea-water, 45 zooplankton and 7 phytoplankton samples are quoted in table 1. Mean values are quoted for seven different oceanographic water masses, as indicated approximately in the map in fig. 1. The discussion of the various water masses concerned is given in Shannon [ 17], as are their definition in terms of temperature and salinity ranges. The Agulhas Surface Water Samples were collected during July 1967, the Walvis Ridge Water Samples during November, 1968, and the remainder during March, 1968. All plankton samples were collected within the upper 200 metre surface layer. Species identification of the plankton showed little except that a wide variety of species were involved; only in the case of the phytoplankton samples from the Agulhas Surface Water was one category (viz. various Chaetoceros spp.) found to make up a large fraction of the sample material. All means have of course been corrected for the background blank values mentioned earlier. The figures in brackets in the table show the number of samples used in calculating each mean. The following conclusions emerge: i) For the sea-water samples all except one of the means are less than the value of 3.4 X 10-16 g thorium-228 per 1 sea-water which we have used for our background blank. Although five of these seven mean values are in fact positive, we are adopting the conservative standpoint that they are so close to the sensitivity limit of our technique that it would be dangerous to regard them as significantly different from zero. For the eight mean, viz. sea-water from the Walvis Ridge area, the level observed is approximately eight times our background level. The mean of eight samples gives a value of 27 X 10 -16 g thorium-228 per 1 for this water, and the range of values observed extends from 8 to 48 X 10 -16 g thorium-228 per 1. We regard these as quite definitely significant, particularly because they are confirmed by an additional set of sea-water samples (unfortunately without contingent plankton samples) obtained more than a year previously; this earlier set gave a mean values of 22 × 10 -16 g thorium-228 per 1. ii) All of the plankton mean values are above the sensitivity level of approximately 1.0 × 10 -18 g thorium-228 per g wet plankton. Real variations appear to exist between the means from the different
454
R.D.CHERRY, I.H.GERICKE and L.V.SHANNON
Table 1 Mean thorium-228 contents of plankton and sea water, in units of 10-18 g thorium-228 per g wet plankton of sea water. The number of samples used for determining each mean is given in brackets. Water mass
Sample station Nos.
Zooplankton
Phytoplankton
Sea-water
Agulhas surface water
1-7
9(8)
65(3)
< 0.34 (7)
Benguela current
8-9
3(3)
40(1)
< 0.34 (2)
7(7)
19(1)
<0.34 (5)
Subtropical surface water
10-12, 24-25
Confused area near Tristan da Cunha
13-17
15(7)
-(0)
< 0.34 (5)
Converge between sub-tropical and sub-antarctic surface waters
18-20
3(6)
-(0)
< 0.34 (3)
South side of convergence merging into sub-antarctic surface water
21-23
2(10)
9(2)
<0.34 (3)
Walvisridge water
26-29
-(0)
< 2.7 (8)
27(4)
s 20°~! O/~2|
wMt~
ATLANTIC OCEAN
2.5"
30"
g
35"
,;
,;
,.5
".
,2
-'
,"
o c e ,. N
SORGO2"CA"
I~ 1RISlAN DACUN~IA
,; 40"
'2
SUBTROPICAL 20
CONVERGENCE
2; SUBANTARCTIC REGION
~;~ 45"
,5 o
,o •
~o
0o
~o
,0 o
W
;~°
So:-
250
So*
s5 °
E
Fig. 1. Sample collection stations and approximate sub-division of oceanographic water masses. water masses. It is particularly noteworthy that the three lowest thorium-228 values are associated with cold waters such as the Benguela Current and the waters
at the convergence between sub-tropical and sub-antarctic surface waters. The water of Benguela Current is moreover "Central Water", and "Central Water" is
THORIUM-228 IN MARINE PLANKTONAND SEA WATER formed in the sub-tropical convergence region by a complex cellular mixing process [18]. We see therefore that the low thorium-228 levels all appear to be associated with water which has its origin in the region of the sub-tropical convergence; the possible utility of thorium-228 as a natural oceanographic tracer is obvious. Also noteworthy is the fact that the zooplankton from the Walvis Ridge area tops the list with amean of 27 X 10 -18 g thorium-228 per g. This mean was obtained from four samples, with a range from 23 to 36 X 10-18 g. iii) Data for phytoplankton in table 1 are sparse, but the four mean value which are available are all considerably higher than the corresponding zooplankton means. In view of the calcium-rich nature of phyto- as opposed to zooplankton, and the fact that previous data [19] suggest that diatoms select radium preferentially to calcium, it is probable that these higher values indicate that the source of 1.9 yr thorium228, in phytoplankton at least, is governed to an appreciable extent by decay from 6.7 year radium228. The implied [1] and observed [8] excess of radium-228 over thorium-232 activity in sea-water support this point of view. iv) If we assume that the published levels [ 1,2] of thorium-228 at about 10-18 g per 1 sea water are typical, and take our lowest plankton value of 2 X 10 -18 g thorium-228 per g wet plankton, we obtain a concentration factor from sea-water to plankton of about 2000. Use of any of the other mean values from table 1 for thorium-228 in plankton will Of course raise this factor, in some cases by an order of magnitide. On the other hand, direct use of the table 1 data for both plankton and sea-water from the Walvis Ridge area gives a concentration factor of just less than 10. If we can assume that both the previously published data and our own table 1 data are essentially correct, then it is plain that considerable real variation in this concentration factor must exist. It is not unlikely that such variations are connected with varying degrees of association of thorium-228 with the solid phase; Somayajulu and Golberg's hypothesis that thorium in sea-water might be associated with the solid phase should be investigated further, despite its apparent lack of quantitative attraction [20]. The high concentration factors of the order of 104 which appear to hold in some circumstances should be compared with the data of Szabo [19] for
455
radium-226. These indicated a radium concentration factor of much less than 103 (the exact figure depending on the value assumed for the wet-to-dry mass concentration factor for plankton). Direct biological removal of thorium-228 from sea-water to plankton would appear to be indicated as a process which competes with radium-228 decay as a source of thorium-228 in plankton. It is clear that data on both thorium-232 and thorium-228 in both sea-water and plankton will be required before the relative importance of biological removal of thorium isotopes can be assessed quantitatively. v) The Walvis Ridge area is obviously a region of considerable oceanographical interest. Shannon and Van Rijswijck [21] have noted an uplift of lower water masses over the Walvis Ridge region, and the South African West Coast fishing grounds are generally known to be areas of strong upwelling and high productivity [22]. Goldberg and Arrhenius [23] have associated high barium content in sediments with areas of high surface productivity, and Turekian and Tausch [24] have reported exceptionally high barium in the tops of sediment core from the general area of the southwest African coast. Furthermore Somayajulu and Goldberg [2] have drawn attention to a possible association between barium and thorium. It is tempting to postulate that upwelling over the Walvis Ridge introduces thorium-rich suspended matter from sediment tops into the overlying waters, but much additional data for thorium and other trace elements, preferably involving depth profiles, is necessary before finn conclusions can be drawn.
4. CONCLUSION We feel that the data in this paper show clearly: (a) That thorium-228 is present in marine plankton at easily detectable levels ranging up to 27 X 10 -18 g thorium-228 per g wet plankton for zooplankton and to 65 X 10 -18 g per g for phytoplankton. (b) That real variations in the thorium-228 content of plankton occur, and that these variations show interesting trends from one water mass to another. The possible utility of thorium-228 as a natural oceanographic indicator is apparent. (c) That in at least one region of the ocean, viz. the Walvis Ridge area, level the of thorium-228 in sea-
456
R.D.CHERRY, I.H.GERICKE and L.V.SHANNON
water is considerably higher than the commonly accepted values [1,2]. The possibility that this is due to the association of thorium with the solid phase should not be overlooked. (d) That the Walvis Ridge area is a region of particular oceanographical interest, at least insofar as certain trace element concentrations are concerned. (e) That intriguing problems concerning the biogeochemical balance o f thorium-232, thorium-228 and radium-228 in sea-water and in marine biota still remain to be solved. Thorium-232 to thorium-228 ratios must be measured in plankton as well as in sea-water and an attempt should be made to differentiate between thorium isotopes in solution and those in the solid phase. The very difficult problem o f radium228 measurement is also worthy of serious consideration.
5. Acknowledgements We are grateful for financial assistance from the Staff Research Fund, University of Cape Town, the C.S.I.R. Oceanographic Research Unit at the University o f Cape Town, and the South African Atomic Energy Board. We are particularly appreciative of the extensive sample collection facilities provided b y the Division o f Sea Fisheries, Sea Point, and we must thank Dr. A.De Decker and Mrs. A.Coghlan for carrying out the species identification of the plankton.
References [1] W.S.Moore and W.M.Sackett, Uranium and thorium series inequiHbrium in sea water, J. Geophys. Res. 69 (1964) 5401. [ 2] B.L.K.Somayajulu and E.D.Goldberg, Thorium and uranium isotopes in sea water and sediments, Earth and Planetary Science Letters 1 (1966) 102. [3] K.F.Lazarev, D.S.Nikolayev and S.M.Grashchenko, Concentration of the thorium isotopes of sea water, Radiokkhimiya 3 (1961) 623. [4] D.S.Nikolayev, K.F.Lazarev and S.M.Grashchenko, The concentration of thorium isotopes in the water of the Sea of Azov, Doklady of the Academy of Sciences of the U.S.S.R., Earth Sciences Sections, 138 (1962) 489. [5] Y.Miyake, K.Saruhashi, Y.Katsuragi, T.Kanazawa and Y. Sugimura, Uranium, radium, thorium, ionium, strontium 90 and cesium 137 in coastal waters of Japan, in: "Recent Researches in the Fields of Hydrosphere, Atmosphere and Nuclear Geochemistry", Sugawara Festival Volume, Tokyo (1964) 127.
[6] E.Rona, L.O.Gilpatrick and L.M.Jeffrey, Uranium determination in sea water, Trans. Am. Geophys. Union. 37 (1956) 697. [7] Y.Miyake, Y.Sugimura and T.Uchida, Ratio U234/U 238 and the uranium concentration in sea water in the Western North Pacific, J. Geophys. Res. 71 (1966) 3083. [8] K.F.Lazarev, S.M.Grashchenko, D.S.Nikolayev and V.M.Drozhshin, Concentration of mesothorium 1 in Black Sea water, Doklady of the Academy of Sciences U.S.S.R., Earth Sciences Sections 164 (1965) 189. [9] V.S.Dement'yev and N.G.Syromyatnikov, Mode of occurrence of thorium isotopes in ground waters, Geochemistry International 2 (1965) 141. [ 10] R.D.Cherry, Alpha-radioactivityof plankton, Nature 203 (1964) 139. I11 [ R.C.Turner, J.M.Radley and W.V.Mayneord, The alpharay activity of human tissues, British Journal of Radiology XXXI (1958) 397. [12] R.D.Cherry, The determination of thorium and uranium in geological samples by an alpha-counting technique, Geochim. et Cosmochim. Acta 27 (1963) 183. [ 13] R.D.Cherry, Thorium and uranium contents of australites, Nature 195 (1962) 1184. [ 14] V.Hasson and R.D.Cherry, Alpha-radioactivityof human blood, Nature 210 (1966) 591. [15] G.D.Finney and R.D.Evans, The radioactivity of solids determined by alpha-ray counting, Phys. Rev. 48 (1935) 503. [16] M.B.Schaefer,and Y.M.M.Bishop, Particulate iron in offshore waters of the Panama Bight in the Gulf of Panama, Limnology and Oceanography 3 (1958) 137. [17] L.V.Shannon, The Oceanic Circulation Pattern off South Africa, Fish. Bull No. 6 (in press). [ 18] M.J.Orren, Hydrology of the Southwest Indian Ocean, Investl. Rep. Div. Sea Fish. S. Aft. No. 55 (1966). [ 19] B.J.Szabo, Radium content in plankton and sea water in the Bahamas, Geochim. et Cosmochim. Acta 31 (1967) 1321. [20] M.Bernat and E.D.Goldberg, Thorium Isotopes in the Marine Environment, Earth and Planetary Science Letters 5 0969) 308. [21] L.V,Shannon and M.van Rijswijck, Physical Oceanography of the Walvis Ridge Region, Investl. Rep. Div. Sea Fish. S. Aft. No. 73 (1968). [22] L.V.Shannon, Hydrology of the Sotuh and West Coasts of South Africa, Investl. Rep. Div. Sea Fish. S. Aft., No. 58 (1966). [23] E.D.Goldberg and G.O.S.Arrhenius, Chemistry of Pacific pelagic sediments, Geochim. et Cosmoehim. Acta 13 (1958) 153. [ 24] K.K.Turekian and E.M.Tausch, Barium in deep-sea sediments of the Atlantic Ocean, Nature 201 (1964) 696.