Th-234 Scavenging in the Water Column off Southwestern Taiwan

台灣西南海域水體中T-234的清除作用

洪慶章(Chin-Chang Hung)；魏慶琳(Ching-Ling Wei)

Terrestrial, Atmospheric and Oceanic Sciences

3卷2期（1992 / 06 / 01）

183 - 197

英文

本研究於台灣西南海域選擇一測站(22°25'N 120°08'E)，採集海水以量測溶解相及顆粒相釷－234。自垂直剖面觀之，釷－234在透光層主要是受生物活動，深層則是被再懸浮顆粒所清除。於透光層中，由盒子模式計算溶解相與顆粒相釷－234之停留時間分別爲90-110天和12-28天；並據以推算釷－234在透光層底部的通量爲1220 dpm m^(-2)d^(-1)。從顆粒的外形鑑定，顯示深層及底層之顆粒體爲水平方向傳送過來之再懸浮顆粒組成，但難以定量。再懸浮顆粒體爲一有效之釷－234清除者，造成深層水中釷－234和鈾－238之不平衡。在深層水中，溶解相和顆粒相釷－234之停留時間分別爲112天和27天。我們估計釷－234進入沉積物中之最低通量爲1600 dpm m^(-2)d^(-1)。

A station off southwestern Taiwan (22°25'N 120°08'E) was selected to carry out detailed seawater sampling for the measurements of dissolved and particulate (superscript 234)Th. The profiles showed that (superscript 234)Th is subject to scavenging and removal processes by biological particles in the euphotic zone and by resuspended particles in the deep layer. Estimated by box-model, the residence times of dissolved and particulate (superscript 234)Th in the euphotic layer are in the ranges of 90-110 days and 12-28 days, respectively. A vertical (superscript 234)Th flux of 1220 dpm m^(-2) d^(-1) out of the euphotic layer is estimated. Horizontal transport of resuspended particles in the deep and bottom layer is evident based on morphological examination of particles but is difficult to quantify. Resuspended particles act as an effective scavenger for Th in the deep layer and cause a radioactive disequilibrium of (superscript 234)Th with respect to its progenitor, (superscript 238)U. Residence times of dissolved and particulate (superscript 234)Th in this layer are 112 days and 35 days, respectively. A lower limit of (superscript 234)Th flux of 1600 dpm m^(-2) d^(-1) into the bottom sediment can be estimated.