台灣周圍海域表水鉛-210與釙-210的分佈

Abstract

本研究藉海科中心2007年聯合探測 (Joint Hydrographic Survey) 計畫，自2007年5月31日至6月11日利用四船聯合探測任務，以OR1-833、OR2-1444、OR3-1226、FR1-9605四個航次於台灣周圍海域採85個海洋表層水樣，進行溶解態與顆粒態鉛-210、釙-210活度測量。本論文研究成果首次顯現鉛-210及釙-210於台灣四周海域大範圍的空間分佈。由鉛-210、釙-210的分佈及水文化學等各項參數可看出兩放射性核種的分佈主要受控於台灣鄰近海域的洋流系統及不同顆粒的清除能力。 於黑潮流經台灣東邊的海域，有一低活度之溶解態鉛-210舌狀構造由南向北延伸。整體來說，台灣西部及大陸沿岸海域，溶解態鉛-210的活度最低，台灣東部開放海洋區最高。北南海表水的鉛-210活度則介於台灣海峽和台灣東部海域之間，乃因黑潮支流與北南海表層水混合後，使得溶解態釙-210活度略微上升，北南海水沿澎湖水道向北輸送，形成溶解態鉛-210一清晰的舌狀鋒面。顆粒態鉛-210活度明顯小於溶解態鉛-210，其活度值的空間分佈較無系統性變化。 同樣地受黑潮的傳輸影響，溶解態及顆粒態釙-210的活度等值線在台灣東部海域呈現蜿蜒曲線分佈構造，釙-210活度有向外洋遞增的趨勢。與水文資料比對後，6 dpm/100L等值線大致與鹽度34.2 (psu) 的等值線相符，意味台灣東部海域釙-210的清除速率受到黑潮與沿岸海水的交互作用而多變。於台灣東北部海域，因地形效應的湧升流提供高營養鹽，致使基礎生產力提高，生物性顆粒加速釙-210清除，而顆粒態釙-210的活度在此處同步升高。 北南海海域西部的鉛-210和釙-210活度值低於台灣東部黑潮區域，意味著在黑潮水進入南海後加速兩核種的清除作用。另外，北南海水沿著澎湖水道形成的鉛-210及釙-210的舌狀鋒面 ，顯示水平傳輸對於此二親顆粒核種的分佈亦扮演重要角色。 本論文以不可逆清除模式推算釙-210及鉛-210在台灣周圍海域表層水中的滯留時間。在沿岸區域，釙-210的滯留時間可達600天，而外洋區則為200天。鉛-210的滯留時間主要受控於鉛-210的大氣沉降通量，若我們假設台灣四周海域的鉛-210大氣通量為0.6 dpm/cm2/y，所得出的滯留時間，大陸沿岸與台灣西部沿岸大約50天，而外洋區可達200天；若假設大氣通量為1.6 dpm/cm2/y，滯留時間明顯變短，沿岸區域則約25天，外洋區約100天。

A total of 85 surface seawater samples were collected for the determination of dissolved and particulate 210Pb and 210Po as part of the Joint Hydrographic Survey project from 31 May to 11 June 2007. Sampling tasks were carried out on board R/V Ocean Researcher I (OR1-833), II and III (OR2-1444 and OR3-1226), and Fishery Researcher I (FR1-9605). The results provide the first synoptic picture of the spatial distributions of 210Pb and 210Po in the surface waters surrounding Taiwan. About with the hydrographic data, it is evident that the spatial distributions of the two radionuclides are controlled by the current system and particle scavenging force in the seas surrounding Taiwan. A tongue structure with relatively low dissolved 210Pb extends northwardly in the region off the eastern Taiwan, where the Kuroshio pathway is found. The lowest dissolved 210Pb was found in the coastal water of China and the coastal water of western Taiwan whereas the highest value was found in the region off eastern Taiwan. Along the Peng-Hu channel, the mixture of Kuroshio Branch Water and the northern South China Sea surface water formed a tongue of relatively high dissolved 210Pb, which extends northwardly into the Taiwan Strait. The dissolved 210Pb in this water lie between the value in the Taiwan Strait and that in the region off eastern Taiwan. The particulate 210Pb was significantly lower than dissolved 210Pb, and its spatial distribution is not as systematic as that of dissolved 210Pb. The contour lines of dissolved and particulate 210Po off eastern Taiwan revealed a meandering feature, where the Kuroshio flows through. Compared with hydrographic data, the contour of 6 dpm/100L dissolved 210Po matches with the S=34.2 contour, indicating 210Po scavenging rate is affected by the interaction of the Kuroshio and the coastal water off western Taiwan. An elevation of particulate 210Po in the region is due to the increased primary production off northeastern Taiwan, in which the scavenging of 210Po by the biological particles is enhanced. Both 210Pb and 210Po in the region to the west of the northern South China Sea were lower than that in the Kuroshio, indicating enhanced removal of the two radionuclides after the Kuroshio intrusion through the Luzon Strait. Moreover, the tongues of high 210Pb and 210Po originated from the northern South China Sea extend along the Peng-Hu Channel, depicting the important of lateral transport plays an important role in the distributions of the two radionuclides. The residence times of 210Pb and 210Po in the surface water surrounding Taiwan were estimated by the irreversible scavenging model. The residence time of 210Po was 600 days in the shelf area and 200 days in the Kuroshio. The residence time of 210Pb was highly related to the atmospheric 210Pb flux. If 0.6 dpm/cm2/y of atmospheric 210Pb flux was assumed, the residence times of 210Pb were 50 days and 200 days in the coastal region and in the open ocean, respectively. If a higher atmospheric 210Pb flux of 1.6 dpm/cm2/y was assumed, then the residence times would be 25 days and 100 days in the coastal and in the open ocean, respectively.