南海深層鉛-210與釙-210通量的時序變化

Abstract

本研究於南海 SEATS (South East Asian Time-series Study) 時序測站(116°00°E, 18°00°N) 從2008年六月至2009年七月，施放兩梯次的錨碇式沉降顆粒收集器進行顆粒通量觀測工作，取得一年期間南海2000 m 及3500 m 兩深度的210Pb與210Po通量的時序變化數據。 由2000 m 量得的沉降顆粒總通量 (FMass) 介於12.2與551.1 mg/m2/d 之間，呈現夏秋低冬春高的時序變化並有年間的變化，其變化範圍和表層基礎生產力變化幅度相當；於3500 m 的FMass則介於89.3與250.8 mg/m2/d 之間，兩深度的FMass並無明顯的差異，顆粒體橫向傳輸的現象不明顯。210Pb通量 (FPb-210) 介於4.5與105.2 dpm/m2/d 之間，兩深度的FPb-210顯示同步性的時序變化，3500 m 的FPb-210高於2000 m，且與FMass呈良好線性關係，表示南海水體中210Pb的清除速率受顆粒垂直通量控制。210Po通量 (FPo-210) 介於2.6與309.3 dpm/m2/d 之間，其季節變化不如FPb-210明顯，3500 m 的FPo-210高於2000 m，FPo-210與FMass相關性不高；與水體中母子核種間放射性不平衡所估算之通量相比，實測之FPb-210與模式估算值相當，但實測之FPo-210則遠低於模式估算值，應為模式所假設之210Po分布呈穩定狀態不成立使然。 本研究嘗試以210Pb作為顆粒有機碳通量的示縱劑，以估算顆粒有機碳通量(FPOC)，得到FPOC和210Pb的通量與來源通量比值 (F/P) 間的相關公式為FPOC = 0.2 + 472.9 (F/P)Pb-210，公式中的來源通量必須考慮由呂宋海峽流入的深層西菲律賓海水輸入的210Pb。

Vertical fluxes of total mass (FMass), particulate organic carbon (FPOC), particulate inorganic carbon (FPIC), 210Pb (FPb-210) and 210Po (FPo-210) were determined by sediment traps deployed at two depths, 2000 m and 3500 m, at SEATS (South East Asian Time-series Study, 116°00°E, 18°00°N) in the northern South China Sea during June 2008~ July 2009. The FMass ranges from 12.2 to 551.1 mg/m2/d and from 89.3 to 250.8 mg/m2/d, at 2000 m and 3500 m, respectively, and shows seasonal and inter-annul variations. Insignificant difference of the FMass between the two depths was found, indicating the minor role of horizontal process in particulate transport. The FPb-210 ranges from 4.5 to 105.2 dpm/m2/d and is in phase with the FMass, implying the scavenging of 210Pb is controlled by the vertical flux of particulate materials. The FPo-210 ranges from 2.6 to 309.3 dpm/m2/d and, unlike 210Pb, shows a poor correlation with FMass. Compared with the removal fluxes predicted from the disequilibria of parent-daughter radionuclides, the FPb-210 is comparable with whereas the FPo-210 is significantly lower than the modeled values. The discrepancy of the FPo-210 is attributed to the invalid assumption of steady-state in the mass balance of the scavenging model. A strong correlation of the flux and source ratio of 210Pb and the particulate organic carbon flux was found, which gives a relationship of FPOC (ug/cm2/y) = 0.2 + 472.9 (F/P)Pb-210. The advective transport from the Western Philippine Sea should be considered in 210Pb budgeting, which contributes ~60 % of 210Pb in the deep basin of the South China Sea.