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

臺灣東方及東北海域黑潮水中鉛-210及釙-210之分布及其放射性不平衡狀態

林育農(Y. N. Lin) ； 鍾玉嘉(Y. Chung)

《Terrestrial, Atmospheric and Oceanic Sciences》 2卷3期 (1991/09) Pp. 243-265

摘要

本文主要探討台灣東部外海及東北海域，黑潮與東海海水混合區域中之Pb-210和Po-210的分佈與其放射性不平衡狀態。在黑潮及其邊緣水體之水域中，四個垂直剖面及三十四個表水測站皆完成Po-210和Po-210總活性之偵測。
沿著黑潮北流路逕的三個剖面測站具有一些共同的特徵：Po-210最大值皆在濃度25~100m間；Po-210的最大值則在甚接近200m濃度處；二核種在200m以下皆隨濃度遞減且在500m以下達活性平衡。因此每一水柱可依據二核種之活性比區分成三層。在表層水(0~150m)中，Po-21/Po-210比值小於1，顯示Po-210可能較先由水中移走。於中層水(150~500m)中，此比值大於1，顯示Po-210可能因溶解或脫　附作用而再循環。在深層水(500m以下)中，此比值大約等於1，顯示此二核種幾乎已達活性平衡。
利用一簡單的盒子模式(box model)並假設其在穩定狀態下(steady-state)，可計算出表層水中Po-210和Po-210之駐留時間各爲0.9和1.5年。由於二核種對水中顆粒物質的反應性極高，臻使其在表層水中快速地被顆粒態特質所清除。然而由表層水去除之Po-210至少有40%在中層水中溶解而再循環。而只有少於5%的P-210以此路筩再循環。這些特徵在開放的大洋中是常見的，這表示黑潮本身並沒有影響到此二核種的分佈。在測站H16的深層水中Pb-210的平均駐留時間是12年，只有另一測站R23的十分之一，顯示在測站H16中，顆粒物質的邊界和現場清除作用可能較爲有效。
在東海陸棚水中，Po-210於表水以下25m處即呈超量狀態且延伸至海底(120m)。這與黑潮　水中所觀察到的現象截然不同，在黑潮水中超量Po-210出現在150m以下。東海陸棚水中之超量Po-210可能是由於因再循環作用而富含Po-210之黑潮中層水的侵入和湧昇所造成。此現象和由水文調查，觀察到之黑潮次表層水被擡昇約130m的結果一臻。
在表水測站中Po-210和Po-210之活性濃度由近陸水域或陸棚區朝深海增加，反應了顆粒物質濃度和邊界清除作用的影響。然而所有的表水樣品Po-210/Pb-210之活性比皆相當接近，爲0.5~0.6，顯示Po-210的平均駐留時間約爲0.5至0.8年。此結果和由盒子模式(box model)在穩定狀態(steady-state)下進行顆粒清除作用時，計算所得之表層水(0~150m)的平均駐留時間0.9年大臻吻合。

並列摘要

This paper presents the distributions of Pb-210 and Po-210 and the extent of their radioactive disequilibrium in areas off eastern and northeastern Taiwan where the Kuroshio flows northward and mixes with the East China Sea water. Four vertical profiles 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 along the Kuroshio track share some common features:
the Pb-210 maximum occurs at a depth of between 25 m and 100 m; the Po-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 inidd1e 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 is 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 from the upper layer by particle scavenging. However, at least 40% of the Po-210 removed from 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 distribution of these nuclides within its domain. The mean residence time for Pb-210 in the lower layer at one station (12 years, 1116) 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 (120 m). 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 shelf 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 has been eievated about 130 m.
Both Po-210 and Pb-210 in the surface water increase in concentration from land mass or shelf zone toward the deep sea, reflecting the influence of particluate concentrations and of the boundary scavenging processes. However, the Po-210/Pb-210 activity ratio remains fairly constant at 0.5 0.6 for 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 for the upper layer using a box model that involves particulate scavenging under a steady-state assumption.