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

点击翻译按钮获取中文摘要

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.