par Koczy, Friedrich F.F.;Picciotto, Edgardo 
;Poulaert, Ginette ;Wilgain, S.
Référence Geochimica et cosmochimica acta, 11, 1-2, page (103-129)
Publication Publié, 1957
;Poulaert, Ginette ;Wilgain, S.Référence Geochimica et cosmochimica acta, 11, 1-2, page (103-129)
Publication Publié, 1957
Article révisé par les pairs
| Résumé : | In order to study the geochemistry of thorium isotopes in the hydrosphere, particularly in the ocean, a method has been worked out by which Th-232 (thorium)λ, Th-230 (ionium), Th-228 (radiothorium), and Th-227 (radioactinium) can be determined separately. Eight samples of 20 to 40 litres of sea-water, from 23·0% to 34·97%, salinity, were collected in November 1953, in the Skagerak and the Gullmarfjord (Sweden). Thorium was isolated by the following procedure: just after collection, the samples were brought to pH 2 and a given amount of Th-234 (UX1) was added as tracer. Thorium was first precipitated with Fe(OH)3 as carrier. Further purification was obtained by ion-exchange column chromatography followed by solvent extraction; the final fraction was obtained as the citric complex, a form suitable to incorporation in the photographic emulsion. The total yield varied from 8 to 23% according to the sample, as determined by the β-activity of the tracer. The various thorium isotopes were measured through their α-activity, using nuclear photographic emulsions, more precisely the double-emulsion technique. RdTh and RdAc both generate five-branched stars; more than 90% of these originated from RdTh, as indicated by the length of the tracks: while Io and Th only yield single tracks of range 18·8 μ and 15 μ respectively in the emulsion. Most samples showed a much lower activity than expected; this did not make it possible to discriminate between Io and Th through the range distribution of their tracks, thus we could only ascertain upper limits of Io and Th concentrations. Average concentrations corresponding to a total volume of 140 litres of water are as follows (in grams per ml): RdTh = (4.0 ± 1.4). 10-21 Th < 2.10-11 RdAc < 7.10-23 Io < 6.10-16. In one of these samples (salinity: 33·7%) we have found an Io concentration of 26.10-16 g/ml. This high value is attributed to a nonhomogeneous distribution of Io in the sea. Before the conclusions are drawn, we must point out the following restriction: 1. (1) Our water samples, including those in the oceanic range of salinity, were not collected in an oceanic environment, as all were taken in coastal waters. 2. (2) Our experimental results should correspond to the total thorium content of the samples. It must be pointed out, however, that a thorium fraction which both would not exchange with UX1 at pH 2 and would not coprecipitate with Fe(OH)3 would remain undetected with our procedure. We assume the following concentration for the other radioactive elements: U = 1·5. 10-9g/ml, Ra = 0·8.10-16 g/ml, Th < 6.10-12 g/ml. The state of radioactive equilibrium between two nuclides A and B shall be denned by their activity ratio: RA/B = λA. NA/λB. NB. The following conclusions can be drawn from the above data: 1. (1)RIo/U-238 < 0.02. More than 98% of the Io resulting from U-238 disintegration in the oceann cannot be accounted for. This lack of Io in the sea-water must be correlated with the presence of unsupported Io in the deep-sea sediments. These two corroborating facts definitely prove the hypothesis of ionium precipitation with the sediments. 2. (2) RIo/Ra < 0.15. Ra is in excess by a factor of 6 with respect to its equilibruim with Io. This could possibly result from the redissolution of part of the Ra originating from this Io of the sediments. 3. (3)The average RdTh concentration of 4.10-21g/ml should correspond to an equilibrium concentration of 2.6.10-11 g/ml of the Th-232. We have, however, shown that, in at least two samples, RdTh is far over its equilibrium value with Th. Indeed, if we assume Th < 6.10-12g/ml, this should lead to: RRdTh/Th > 4. We can only account for this surprising result by supposing that the excess RdTh results from an excess of its parent MsTh, (Ra-228) brought in by rivers or redissolved from the sediments. Owing to the short half-life of both these nuclides, such a RdTh excess should be found only in the vicinity of the shore or the bottom. 4. (4)RRdAc/U-235 < 0·1. More than 90% of the RdAc from U-235 in the ocean cannot be accounted for. Considering the short half-life of RdAc, this suggests that actinium or protactinium are precipitated with the sediments together with the Io. 5. (5)In both U-238 and Th-232 families, a radium isotope (Ra and MsTh) appears to be in excess over its parent thorium-isotope (Io and Th). The presence in the ocean of unsupported Ra(T = 1600 years) and MsTh (T = 6·7 years) is of great interest. A study of the distribution of these isotopes should yield valuable data on their diffusion rates and on deep currents. As far as radioactive geochemistry is concerned, the ocean is characterized by extremely low concentrations of nuclides of all three radioactive families and by the total disruption of the radioactive equilibrium in these families. A calculation of the geochemical balance of radioactive elements in the hydrosphere from the above data is given in the last part of the paper. © 1957. |
