Comparative geochemistry of 234Th, 210Pb, and 210po: A case study in the Hung-Tsai Trough off southwestern Taiwan

Abstract

Detailed profiles of dissolved and particulate 234Th, 210Pb, and 210Po activities at three stations in the Hung-Tsai Trough off southwestern Taiwan were determined. The total 234Th activity is 20-25% deficient from its secular equilibrium in the entire water column. Except for an evident excess of 210Po at some depths in the mixed layer and in the pycnocline layer, total 210Po activity is also lower than total 210Pb activity. As a result of atmospheric deposition, 210Pb is about 25% in excess of its parent, 226Ra, throughout the water column of the Hung-Tsai Trough. The ratios of the distribution coefficients of 234Th, 210Pb, and 210Po show that the order of particle affinity is Po > Pb > Po in the pycnocline layer of the Hung-Tsai Trough. Mass balance equations incorporating horizontal advection are established to estimate the scavenging and removal rates of 234Th, 210Pb, and 210Po. The vertical fluxes of 234Th, 210Pb, and 210Po within the euphotic layer for the Hung-Tsai Trough are 6760, 630, and 400 dpm m-2 d -1, respectively. The residence times of the three radionuclides in the euphotic zone are remarkably close to one another (within 2 weeks). By using 234Th and 210Po as proxies of particulate organic carbon and nitrogen, export flux from the euphotic layer of the Hung-Tsai is 40 mmol C m-2 d-1 based on the 234Th approach and 64 mmol C m-2 d-1 based on the 210Po approach.