南海深層短半衰期釷放射核種的沉降通量時序變化

Abstract

本研究於南海SEATS測站 (18°N, 116°E) 2000 m與3500 m，佈放兩組錨碇式沉降顆粒收集器，進行多年期 (2008~2009年, 2013~2015年) 收集沉降顆粒之實驗，測量沉降顆粒總質量通量 (FMass)、234Th通量 (FTh-234) 與228Th通量 (FTh-228) 的時序變化。 兩個深度的FMass時序變化，呈現季節性冬高夏低的循環，與過去在南海的基礎生產力研究結果契合，推測深海收集的顆粒體反應表層海洋基礎生產力的強弱與季節性變化；另外本研究也觀察到FMass有年際之間的差異，各年FMass出現最高值的時間不同。由於部分收集器故障未能收集到沉降顆粒樣品，加上234Th半衰期 (24.1天) 較短，本研究取得之可利用的FTh-234資料點少，有限的資料下顯示FTh-234在兩個深度的變化不大，與同地點測量之透光層中的234Th - 238U不平衡程度，及漂浮式沉降顆粒收集器量得之FTh-234比較，可看出顆粒體在沉降的過程中受分解作用影響，使顆粒體上234Th溶回水體，造成FTh-234隨深度減少之結果。相較於234Th，228Th的半衰期 (1.91年) 和深海顆粒體循環尺度相當，更適合做為探討海洋深層的示蹤劑，由研究前後近三年資料顯示，兩個深度間的FTh-228隨時間變化大，和FMass有些相似，南海沉降顆粒中228Th活度明顯高於其他海域，應受南海西南部寬廣陸棚影響，提供228Th的母核種228Ra來源有關，此外，長期觀測下亦發現南海夏天有低FMass高FTh-228的現象。透過FTh-234、FTh-228與FMass的相關性觀察，234Th與228Th的變化受顆粒垂直通量影響甚大，是釷核種受顆粒體清除作用的明顯證據。 本研究亦探討顆粒成分與分佈係數 (Kd) 之關係，與同測站懸浮顆粒，和透光層中利用漂浮式沉降顆粒收集器收集的沉降顆粒比較，本研究量測深海沉降顆粒的Log Kd變化範圍較小，推測可能與深海顆粒組成成分較穩定有關。最後，我們透過234Th/228Th比值關係，減少半衰期時間長度不同造成的影響，以及忽略水體間物理性的傳輸因子，估算南海顆粒的沉降速率，約為168 m d-1，相較於其他海域，顯示南海海域顆粒沉降速率快。本研究於南海深部進行FMass、FTh-234與FTh-228的時序觀測，為深海顆粒通量提供寶貴資料。

Fluxes of total mass (FMass), 234Th (FTh-234) and 228Th (FTh-228) were determined by time-series sediment traps deployed at two depths, 2000 m and 3500 m, at the SEATS (South East Asian Time-series Study, 18°N, 116°E) station in the northern South China Sea (SCS) during June 2008-July 2009 and September 2013-May 2015. The FMass at the two depths showed a synchronized pattern, reflecting the seasonal variability of the primary productivity in the surface water. The FMass also revealed inter-annual variability, which peaked at different time of each year. Due to malfunction of the sediment trap and the short half-life of 234Th (t1/2 = 24.1 days), the FTh-234 data was limited. Combining with the results of 234Th-238U disequilibrium and FTh-234 measured by the floating trap in the euphotic zone, the FTh-234 showed decreasing trend with depth, indicating the decomposition of particles and radioactive decay of 234Th while particles settled through the water column. In contrast to 234Th, due to its longer half-life, 228Th (t1/2 = 1.91 years) is a more suitable tracer for particle scavenging processes in the deep layer. The temporal variation of the FTh-228 was high during the collection period, especially in the summer of 2014. The FTh-228 increases with depth, which indicates that sinking particles continually take up the radionuclide while settling through the water column. It is also found that the specific activity of 228Th in the sinking particles was higher in the SCS than other oceans, which may be caused by higher concentration of its parent radionuclide, 228Ra, released into the seawater from the broad continental shelf of the SCS. Linear correlations between the fluxes of the two radionuclides and the FMass indicate the removal of particle-reactive elements is controlled by the vertical flux of particulate materials in the SCS. Similar Kd values of 234Th of sinking particles in the euphotic zone and deep ocean imply that the composition of settling particles do not change significantly with depth. Sinking velocity of particles, 168 m d-1, was estimated based on the 234Th/228Th ratio. This study is the first data set that provides the temporal observation of fluxes of short-lived thorium isotopes in the deep basin of the SCS.