南海水體中鉛-210及釙-210的季節分佈與通量研究

Abstract

本研究利用SEATS (South East Asian Time-series Study) 航次從2007年一月至2008年六月期間，於時序測站 (18°N, 116°E) 進行海水及沉降顆粒採樣工作，獲得四個高密度垂直剖面生地化數據。本研究除分析溶解相與顆粒相的210Pb及210Po於水體中的垂直分佈外，首次以漂浮式沉降顆粒收集器及錨定式時間序列沉降顆粒收集器 (KK6, KK7) 收集沈降顆粒，直接量測210Pb及210Po顆粒通量。 相對於226Ra，210Pb的放射活度呈現明顯不足現象，研究結果顯示，210Pb/226Ra比值於南海的深層介於0.3至0.5之間，與阿拉伯海之比值相近。210Po相對於210Pb不足的程度則較小，且不同航次間有明顯變化。本研究210Po與210Pb活度與Obata et al. (2004) 於SEATS西南方約300 km測站之數據比較大致相符，但與Chung and Wu (2005) 之數據相比則有系統性差量。 在開放性大洋中，210Pb的滯留時間為50年以上；東海及日本海等邊緣海地區，因為靠近陸地顆粒體較多，使得清除速率較快，210Pb之滯留時間則可縮短至15年左右，本研究經模式推估210Pb於深層海水之滯留時間則介於7與29年之間 ，與文獻相符。210Po於南海的清除速率與同緯度的阿拉伯海相似，滯留時間平均約為2年左右。 由水體中210Pb/226Ra不平衡程度估算深海3500 m處，210Pb之移除通量介於39與46 dpm/m2/day之間，由210Po/210Pb不平衡程度估算210Po通量則介於430與1033 dpm/m2/day之間。錨定式時間序列沉降顆粒收集器於KK6及KK7兩次施放所採集的數個樣品中，於3500 m測得的210Pb、210Po之通量分別為31.5~51.1 dpm/m2/day及25.8~52.2 dpm/m2/day。將模式計算與實測結果相比對，兩種方式推估的210Pb移除通量大致相符，但210Po通量模式計算大於直接量測結果，應與南海海盆中210Po顆粒通量的季節變化大有關。 本研究使用漂浮式沉降顆粒收集器於30、100、160 m所量得的210Pb通量依序為9~36、8~34、14~37 dpm/m2/day，210Po之通量則依序為為31~127、11~108及12~42 dpm/m2/day。由水體中210Pb/226Ra不平衡程度估算於160 m之210Pb移除通量，為11~27 dpm/m2/day；由210Po/210Pb不平衡程度估算210Po於160 m通量則為5.8~31.2 dpm/m2/day。

Four vertical profiles of dissolved and particulate of 210Pb and 210Po were measured at the SouthEast Asian Time-series Study (SEATS) station in the northern South China Sea (18°N and 116°E) from October 2007 to June 2008. In this study, floating traps and moored traps were also deployed to directly measure the fluxes of 210Pb and 210Po at the site. All the profiles show a large 210Pb deficiency relative to 226Ra in the deep water of the South China Sea. According to this study, the 210Pb/226Ra ratio range between 0.3 and 0.5, similar to the ratio to the findings in the Arabian Sea. Compared with the 210Pb profiles, 210Po profiles display a small deficiency with respect to 210Pb. Significant variation of 210Po profiles were found among the four cruises. The data of this study is more consistent with Obata et al. (2004) than with Chung and Wu (2005). The residence time of 210Pb is ~50 years in the open ocean. Since the removal is more intense in the marginal seas, i.e., East China Sea and Japan Sea, in which more particulate matters was found, the residence time of 210Pb in these regions is shorter than 15 years. By using the irreversible scavenging model, we calculated the residence time of 210Pb in the deep water of the South China Sea, which range between 7 and 29 years. Shorter residence of 2 years for 210Po in the deep layer of the South China Sea was found. The scavenging rate of 210Po in the South China Sea is similar to the Arabian Sea. Based on the deficiencies of 210Pb and 210Po in the water column, the removal fluxes of 210Pb and 210Po at 3500 m are 39~46 dpm/m2/day and 430~1033 dpm/m2/day, respectively. Direct measurements of limited samples collected by the moored traps (KK6 and KK7) at same depth show that the 210Pb and 210Po fluxes are between 31.5~51.1 dpm/m2/day and between 25.8~52.2 dpm/m2/day, respectively. The 210Pb removal flux estimated from the scavenging model is comparable with the directly measured flux, whereas 210Po removal flux is much higher than that directly measured by the moored sediment traps. Large temporal variability of particle flux at the site is proposed to be the cause of large discrepancy between the modeled 210Po flux and the measured flux. We also deployed the floating traps to collect sinking particle at 30 m, 100 m and 160 m during the four cruises. The 210Pb fluxes range between 9 and 36, 8 and 34, 14 and 37 dpm/m2/day, respectively. The 210Po fluxes range from 31 to 127, from 11 to 108, from 12 to 42 dpm/m2/day at 30 m, 100 m and 160 m, respectively. The removal fluxes estimated from the 210Pb/226Ra and 210Po/210Pb disequilibrium at 160 m, are 11~27 dpm/m2/day and 5.8~31.2 dpm/m2/day, for 210Pb and 210Po, respectively.