Plutonium and americium concentrations in sea water and in sinking particles@en

Vertical fluxes of 239+240Pu and 241Am and temporal changes in their inventories in the northwestern Mediterranean Sea have been examined through high-resolution water column sampling coupled with direct measurements of the vertical flux of particle-bound transuranics using time-series sediment traps. Water column profiles of both radionuclides showed well-defined sub-surface maxima (2391240Pu between 100-400 m; 241Am at 100-200 m and 800 m), the depths of which are a result of the different biogeochemical scavenging behavior of the two radionuclides. Comparison of deep water column (0-2,000 m) transuranic inventories with those derived from earlier measurements demonstrate that the total 2391240Pu inventory had not substantially changed between 1976-1990 whereas 241Am had decreased by approximately 24%. Enhanced scavenging of 241Am and a resultant, more rapid removal from the water column relative to 239+240Pu was also supported by the observation of elevated Am/Pu activity ratios in sinking particles collected in sediment traps at depth. Direct measurements of the downward flux of particulate 239+240Pu and 241Am compared with transuranic removal rates derived from observed total water column inventory differences over time, show that particles sinking out of deep waters (1,000-2,000 m) could account for 26-72% of the computed total annual 239+240Pu loss and virtually all of the 241Am removal from the water column. Upper water column (0-200 m) residence times based on direct flux measurements ranged from 20-30 yr for 239+240Pu and 5-10 yr for 241Am. The observation that 241Am/239+240Pu activity ratios in unfiltered Mediterranean seawater are six times lower than those in the north Pacific suggests the existence of a specific mechanism for enhanced scavenging and removal of 241Am from the generally oligotrophic waters of the open Mediterranean. It is proposed that atmospheric inputs of aluminosilicate particles transported by Saharan dust events which frequently occur in the Mediterranean region could enhance the geochemical scavenging and resultant removal of 241Am to the sediments.本研究通过高分辨率水柱采样结合时间序列沉积物捕集器（time-series sediment traps）直接测量颗粒结合型超铀元素（particle-bound transuranics）的垂直通量，对地中海西北部海域中钚-239和钚-240（239+240Pu）、镅-241（241Am）的垂直通量及其水体储量的时间变化展开了分析。两种放射性核素的水柱剖面均呈现清晰的次表层峰值：钚-239+240的峰值位于100~400米水深，镅-241的峰值则分布于100~200米及800米水深，其出现深度差异源于两种放射性核素不同的生物地球化学清除行为。将深水层（0~2000米）超铀元素储量与早期测量结果进行对比后发现，1976年至1990年间钚-239+240的总储量未发生显著变化，而镅-241的储量下降了约24%。沉积物捕集器采集的深层沉降颗粒中镅/钚活度比（activity ratios）升高的观测结果，进一步证实了相较于钚-239+240，镅-241的清除作用更强，且从水体中移除的速度更快。将钚-239+240和镅-241的颗粒向下通量直接测量值，与基于水体总储量随时间变化计算得到的超铀元素移除速率进行对比后可知，深层水体（1000~2000米）的颗粒沉降可解释计算得到的钚-239+240年总流失量的26%~72%，同时几乎是镅-241从水体中移除的全部原因。基于直接通量测量结果计算得到的上层水体（0~200米）停留时间（residence times）显示，钚-239+240为20~30年，镅-241为5~10年。地中海未过滤海水中镅-241/钚-239+240活度比仅为北太平洋的六分之一，这一观测结果表明，在开放地中海普遍的寡营养水域（oligotrophic waters）中，存在增强镅-241清除并移除的特定机制。本研究提出，地中海地区频发的撒哈拉沙尘事件（Saharan dust events）所输送的铝硅酸盐颗粒（aluminosilicate particles）的大气输入，可增强镅-241的地球化学清除作用，进而使其更高效地沉降至沉积物中。