臺灣海峽表層水體清除作用探討：釷-234/鈾-238不平衡應用

Abstract

本研究利用釷-234/鈾-238不平衡探討臺灣海峽表層水體的顆粒清除現象，於2006年5月20~27日聯合水文探測期間，首度在臺灣海峽海域內大範圍地進行234Th採樣工作，船上同時測量基本的水文參數（溫度、鹽度、螢光和透光度）、總懸浮顆粒濃度和顆粒有機碳濃度，利用不可逆清除模式與周遭物理流速資料來估算顆粒的清除、沉降速率和滯留時間。 依據水文特徵和234Th分佈等資料，臺灣海峽的清除環境可劃分為五個區域：（Ⅰ）臺灣海峽南部；（Ⅱ）臺灣灘一帶；（Ⅲ）濁水溪口；（Ⅳ）臺灣海峽中部；（Ⅴ）閩江口。由模式計算結果，清除速率介於57~520 dpm/m3/d，以濁水溪口最高，其次依高至低順序為臺灣海峽中、南部、臺灣灘一帶、閩江口；顆粒沉降速率介於24~381 dpm/m3/d，以濁水溪口最高，依序為臺灣海峽中、南部、閩江口、臺灣灘一帶。大致上，全區的溶解態、顆粒態234Th滯留時間分別為0~6和1~3天，只有臺灣海峽南部的溶解態234Th滯留時間及臺灣灘一帶的顆粒態234Th滯留時間例外，其值高達16及18天。另一方面，濁水溪口、臺灣海峽中部及閩江口的顆粒有機碳平均移除速率分別為7.2、6.6及5.0 mmole/m3/d；平均滯留時間分別為1.3、1.5及2.7天。

This study aims at the investigation of scavenging phenomenon by 234Th/238U disequilibria in the surface water of Taiwan Strait. Spatial sampling of 234Th was carried out by the Joint Hydrographic Survey during May 20~27, 2006. Along with 234Th measurement, basic hydrographic parameters (temperature, salinity, fluorescence and light transmission), total suspended matter concentration and particulate organic carbon concentration were also measured. The scavenging model incorporating the physical transport effect was used to estimate 234Th scavenging and removal rates in the region. According to the distributions of hydrographic parameters and 234Th, five major domains in the Taiwan Strait can be identified: (Ⅰ) the Southern Taiwan Strait; (Ⅱ) the Taiwan Bank; (Ⅲ) the Cho-shuei River mouth; (Ⅳ) the Middle Taiwan Strait; and (Ⅴ) the Ming River mouth. The 234Th scavenging rates ranged from 57 to 520 dpm/m3/d. The Cho-shuei River mouth showed the highest scavenging rate, and the lowest scavenging rate was found in the Taiwan Bank. The 234Th removal rates ranged from 24 to 381 dpm/m3/d. The highest removal rate appeared in the Cho-shuei River mouth, and the lowest removal rate was found in the Ming River mouth. The residence times of dissolved and particulate 234Th in the Taiwan Strait were short, 0~6 and 1~3 days, respectively. Using 234Th as a proxy of particulate organic carbon, average removal rates of particulate organic carbon in the Cho-shuei River mouth, the Middle Taiwan Strait, and the Ming River mouth were 7.2, 6.6, and 5.0 mmole/m3/d, respectively.