Natural and anthropogenic radionuclide distributions in the northwest Atlantic Ocean

Details Natural and anthropogenic radionuclide distributions in the northwest Atlantic Ocean Natural and anthropogenic radionuclide distributions in the northwest Atlantic Ocean

Jul 1987

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1987e%26psl..84..135c&link_type=abstract

Earth and Planetary Science Letters, Volume 84, Issue 2-3, p. 135-152.

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Scientific paper

We have used in-situ pumps which filter large volumes of sea water through a 1 μm cartridge prefilter and two MnO2-coated cartridges to obtain information on dissolved and particulate radionuclide distributions in the oceans. Two sites in the northwest Atlantic show subsurface maxima of the fallout radionuclides 137Cs, 239,240Pu and 241Am. Although the processes of scavenging onto sinking particles and release at depth may contribute to the tracer distributions, comparison of predicted and measured water column inventories suggests that at least 35-50% of the Pu and 241Am are supplied to the deep water by advection.
The depth distributions of the naturally occurring radionuclides 232Th, 228Th and 230Th reflect their sources to the oceans. 232Th shows high dissolved concentrations in surface waters, presumably as a result of atmospheric or riverine supply. Activities of 232Th decrease with depth to values <= 0.01 dpm/1000 l. 228Th shows high activities in near surface and near bottom water, due to the distribution of its parent, 228Ra. Dissolved 230Th, produced throughout the water column from 234U decay, increases with depth to ~ 3000 m. Values in the deep water (> 3000 m) are nearly constant (~ 0.6-0.7 dpm/1000 l), and the distribution of this tracer (and perhaps other long-lived particle-reactive tracers as well) may be affected by the advection inferred from Pu and 241Am data.
The ratio of particulate to dissolved activity for both 230Th and 228Th is ~ 0.15-0.20. This similarity precludes the calculation of sorption rate constants using a simple model of reversible sorption equilibrium. Moreover, in mid-depths 228Th tends to have a higher particulate/dissolved ratio than 230Th, suggesting uptake and release of 230Th and 228Th by different processes. This could occur if 228Th, produced in surface water, were incorporated into biogenic particles formed there and released as those particles dissolved or decomposed during sinking. 230Th, produced throughout the water column, may more closely approach a sorption equilibrium at all depths. 230Th, 241Am and 239,240Pu are partitioned onto particles in the sequence Th > Am > Pu with ~ 15% of the 230Th on particles compared with ~ 7% for Am and ~ 1% for Pu. Distribution coefficients (Kd) are 1.3-1.6 × 107 for Th, 5-6 × 106 for Am and 7-10 × 105 for Pu. The lower reactivity for Pu is consistent with analyses of Pu oxidation states which show ~ 85% oxidized (V + VI) Pu. However, the Kd value for Pu may be an upper limit because Pu, like 228Th, may be incorporated into particles in surface waters and released at depth only by destruction of the carrier phase.

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