南海東沙島小潟湖內放射性核種（釷-234、鉛-210及釙-210）與顆粒體關係之探討

Abstract

本研究首度於珊瑚礁潟湖系統測量234Th 、210Pb及 210Po三種放射性核種的放射活度及其在溶解態與顆粒型態之分佈，並探討其變化，本研究於2013年6、12月及2014年5、9月至東沙島採樣，採樣規劃分為空間分布採樣、日夜時間序列採樣及顆粒粒徑差異採樣。 研究結果顯示，東沙島小潟湖內的234Th、210Pb及210Po皆主要富集於粒徑為0.45-10 µm的顆粒上，此外， 234Th於粒徑為153-335 µm的顆粒上亦出現高值，此部分可能與水體中的膠體聚合物或生物糞粒有關。 Kd值的高低顯示放射核種與顆粒的親和程度，234Th、210Pb及 210Po三核種的Kd變化範圍分別為102.74-106.53、104.02-105.27及104.51-105.30，於珊瑚礁系統中，放射性核種與顆粒體結合的順序為：210Po > 210Pb > 234Th，顯示210Po相較其他兩核種易受顆粒體清除而由表水移除；210Po/210Pb顯示顆粒體為生物性或無機性的相對比例，顆粒態210Po/210Pb介於0.12至2.06之間，顯示小潟湖內顆粒體之組成可能以無機性顆粒為主，此外，210Po/210Pb在10-335 µm的粒徑範圍內隨粒徑增加而降低。 本研究藉由Box-model估算234Th及210Po於小潟湖內的滯留時間 (τ) ，234Th的滯留時間小於1天，顯示234Th於小潟湖內反應性相當強且迅速；210Po的滯留時間約為16天，與近岸或河口環境的滯留時間相近。 東沙島小潟湖為半封閉系統，小潟湖底床易累積210Pb及 210Po，因此，本研究在空間分布上測量到東沙島小潟湖210Pb及 210Po的放射活度高於東沙環礁以及南海表水約8至10倍；小潟湖內的顆粒態234Th 、210Pb及 210Po與TSM皆有較大且規律性的變化，可能為潮汐或底棲生物擾動所產生的再懸浮現象所致。

This study reports the results of geochemical behavior of particle-reactive radionuclides, 234Th, 210Pb, and 210Po in Dongsha coral reef system. The sampling was carried out during June 2013 - September 2014 in the lagoon water. Temporal variation and partitioning in different particle sizes of the three radionuclides were investigated by a sampling scheme of filtration and size fractioning. The results showed that 234Th, 210Pb, and 210Po tend to concentrate in particles of the size ranging from 0.45 to 10 µm. It seems that 234Th also concentrates in particles of 153-335 µm, indicating the effect of colloid aggregation and/or the dominance of fecal pellets. The distribution coefficient (Kd), an indicator of particle affinity of elements, of 234Th, 210Pb, and 210Po are 102.7-106.5, 104.0-105.3 and 104.5-105.3 mL/g, respectively, which suggests the binding order of 210Po > 210Pb > 234Th in the lagoon. The 210Po/210Pb ratio in the particles ranged from 0.12 to 2.06, suggesting that the particulate pool might mainly be composed of inorganic materials. In addition, the 210Po/210Pb ratio decreases with particle size within the size range of 10-335 µm. Estimated by the irreversible scavenging model, the residence times of 234Th (tTh) and 210Po (tPo) are < 1 day and 16 day, respectively, similar to the values reported in coastal waters. The total 210Po activity in the lagoon was 8 to 10 fold higher than that in the vicinity of the South China Sea, which may be the result of accumulation of atmospheric deposition of 210Pb in the semi-closed atoll. Moreover, daily variation of 234Th, 210Pb and 210Po in the particulate phase is caused by resuspension of the underlying sediments in phase of tidal wave, which is consistent with the daily variation of particle concentration.