Pb-210 and Po-210 Distributions and Their Radioactive Disequilibria in the Kuroshio Waters off Eastern and Northeastern Taiwan

  • Author(s): Y. N. Lin and Y. Chung
  • DOI: 10.3319/TAO.1991.2.3.243(O)
  • Keywords:
  • Citation: Lin, Y. N. and Y. Chung, 1991: Pb-210 and Po-210 Distributions and Their Radioactive Disequilibria in the Kuroshio Waters off Eastern and Northeastern Taiwan. Terr. Atmos. Ocean. Sci., 2, 243-265, doi: 10.3319/TAO.1991.2.3.243(O)
Abstract

This paper presents the distributions of Pb-210 and Po-210 and the extent of their radioactive disequilibrium in areas o eastern and northeastern Taiwan where the Kuroshio flows northward and mixes with the East China Sea water. Four vertical pro l s of total Pb-210 and Po-210 activities have been measured together with 34 surface water stations within the domain of the Kuroshio and its edge waters.
Three of the profiles long the Kuroshio track share some common features: the Pb-210 minimum occurs at a depth of between 25 m and 100 m; the P 210 maximum occurs fairly close to the 200 m depth; both nuclides decrease with depth below 200 m and reach radioactive equilibrium below 500 m. Thus, each water column may be divided into three layers based on the activity ratio of these two nuclides. In the upper layer (0 - 150 m), the Po-210/Pb-210 ratio is less than unity, suggesting a preferential removal of Po-210. In the miiddle layer (150 500 m), the ratio is greater than unity, suggesting a recycling of Po-210 by dissoultion or desorption. In the lower layer (below 500 m), the ratio about unity, indicating a radioactive equilibrium between the two nuclides.
Using a simple box model with some reasonable assumptions for key param­eters in a steady state, one calculates a mean residence time of 0.9 and 1.5 years respectively for Po-210 and Pb-210 in the upper layer. As both nuclides are highly reactive they are rapidly removed om the upper layer by particle scavenging. How­ ever, at le st 40% of the Po-210 removed om the upper layer is recycled within the middle layer, but less than 5% of the Pb-210 is recycled this way. These features are common in an open ocean water column and suggest that the Kuroshio has no influence on the distribut1on of these nuclides within its domain. The mean residence time for Pb-210 in the lower layer at one station (12 years, H16) is only one tenth of that at another (120 years, R23) , suggesting that both boundary scavenging and in-situ scavenging by particulate matter are very effective at H16.
In the East China Sea shelf water, the Po-210 excess occurs about 25 m below the surface and extends to the bottom (120m). This is quite different from the Po-210 excess observed in the Kuroshio water where the excess occurs below the · 150 m depth. The Po-210 excess in the she water may result from intrusion and upwelling of water from the Kuroshio middle layer in which Po-210 is enriched due to recycling. This is consistent with hydrographic observations which suggest that the subsurface water in this area h been e1evated about 130 m.
Both Po-210 and Pb-210 in the surface water increase in concentration om land m s or shelf zone toward the deep sea, reflecting the influence of parti­culate concentrations and of the bound y scavenging processes. However, the Po-210/Pb-210 activity ratio remains fairly constant at 0.5 ~ 0.6 r all surface water samples, suggesting a mean Po-210 residence time of about 0.5 to 0.8 years.. This is quite consistent with a value of 0.9 years calculated r the upper layer using a box model that involves particulate scavenging under a steady-state assumption.

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