Study on the distributions of Th-234 and biological production in the open ocean

在四国盆地中纬度某站的重复观测结果表明，在海洋层结过程中，海水中的~(234)Th与其母核素~(238)&t；U接近平衡。这意味着该地区位于亚热带地区，属于少雨区，颗粒物通量很少。另一方面，在1999年12月的冬季巡航中，发现了^&lt；234&gt；Th对^&lt；238&gt；U的明显不平衡。通过这些观测发现，即使在夏季，叶绿素α最大值的次表层也始终存在显著的Th亏损。因此，这表明微粒去除过程确实在那里起作用。一些持续的生物活动通常存在于真光带底部，那里的营养物质是从深水中补充上来的。然而，叶绿素α库存并不总是与^&lt；234Th；Th缺乏有关。例如，在2000年6月综合叶绿素α含量较高时，Th的缺乏量较小，而在1999年12月，当6月份的叶绿素α含量约为一半时，Th缺乏率较大。虽然这种脱钩的问题与海水在两个参数之间的滞留时间的差异并不无关，但似乎该地区的低生产力依赖于一些脉动事件的增加而导致了这种差异的放大。冬季，上游水体普遍存在较高的叶绿素α，表现为Th缺乏型。由于这一模式与夏季出现次表层最大峰值的情况有很大不同，因此认为冬季水体的物理混合所导致的生产力结构影响了Th亏缺的分布。因此，这些都意味着，一次观测并不总是有效的，在每年的生物生产的少龙虾海洋。

Observation results repeatedly done at a station of the Shikoku Basin mid latitude showed that ^<234>Th in seawaters were nearly equilibrium with its parent nuclide ^<238>U during the ocean stratified. This means that this area is located in the subtropical regions and belongs to the oligotorophic regimes having few particulate fluxes. On the other hand, a profile of distinct disequilibrium of ^<234>Th against ^<238>U was found during the winter cruise in December 1999. Through these observations, the significant deficiency of ^<234>Th was always found in the subsurface layer of chlorophyll α maximum even in summer. Therefore, it shows that the particulate removal process does surely work there. Some continuous biological activities are considered to usually exist in the bottom of euphotic zone where nutrients are fueled up from deep waters.However, chlorophyll α inventories were not always related to ^<234>Th deficiency. For instance, ^<234>Th deficiency was small in June 2000 when the high amount of integrated chlorophyll α was observed, and it was large in December 1999 when the approximately half amount of chlorophyll α of June. Though the issue of this decoupling is not irrelevant to the difference of retention time in seawaters between two parameters, it seems that the low productivity in this region dependent on the enhanced production by some pulsant events causes to amplify that. In case of winter, relative high chlorophyll α was widely distributed in the upper waters, and such a pattern looked like that of ^<234>Th deficiency. As this pattern is largely different from that in summer having the peaked subsurface maximum, it is suggested that ^<234>Th deficiency profiles are influenced by the productive structure ascribed to the physical mixing in winter waters. Therefore, these things mean that one shot observation is not always effective on the annual biological production of oligotorophic oceans.