Th-234 Scavenging in the Water Column off Southwestern Taiwan


A station off southwestern Taiwan (22¢X 25'N 120¢X 08'E) was selected to carry out detailed seawater sampling for the measurements of dissolved and particulate 234Th. The profiles showed that 234Th 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 234Th in the euphotic layer are in the ranges of 90-110 days and 12-28 days, respectively. A vertical 234Th fluxof 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 234Th with respect to its progenitor, 238U. Residence times of dissolved and particulate 234Th in this layer are 112 days and 35 days, respectively. A lower limit of 234Th flux of 1600 dpm m^-2 d^-1 into the bottom sediment can be estimated.

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