226Ra, 210Pb, 210Po Distributions at the Sea off Southern Taiwan: Radioactive Disequilibria and Temporal Variations

Abstract

210Pb and 210Po in both particulate and dissolved phases were measured at the sea off southern Taiwan, where the Kuroshio intrudes as a branch in winter and mixes with the South China Sea water and coastal waters. Although the effects of mixing and circulation, based on hydrographic data were significant, the removal rates of these two nuclides due to scavenging by sinking particles were evaluated.

Dissolved and total 210Pb and 210Po in surface water increased but particulate 210Pb decreased away from the coast in September, 1994, as well as in January, 1995. On the other hand, both particulate and dissolved 210Po in surface water varied temporally and were higher in January, 1995. The mean residence time calculated for the surface water 210Pb with respect to particulate scavenging was generally longer in January, 1995 (0.57-0.71 yr) than in September, 1994 (0.25-0.41 yr) if a constant atmospheric flux of 2 dpm/cm2/y in the area was assumed. Similarly, for 210Po it was also longer in January, 1995 (0.56-7.7 yr) than in September, 1994 (0.23-2.0 yr).

The 210Pb and 210Po profiles measured from three stations in different periods showed that their concentrations varied temporally and spatially, therefore probably reflecting changes in hydrography and particulate scavenging. In the upper layer where 210Pb was in excess of 226Ra, the mean residence time for 210Pb with respect to the removal by sinking particulates was about 1.3 - 3 yrs. In the lower layer where 210Pb was deficient relative to 226Ra , the mean residence time ranged from 26 to 44 yrs resulting from variations in 210Pb and 226Ra inventories or varying 210Pb deficits.

The mean residence time calculated for 210Po in the upper, intermediate and lower layers as defined by 210Po deficit or excess relative to 210Pb also varied in the three sampling periods. These variations were due to the fact that both 210Po and 210Pb profiles varied in response to particulate scavenging and the mixing of several water masses in variable proportions at different times. 210Pb was not recycled, but at two stations in January, 1995, 210Po was recycled over 100%, requiring a lateral input. This feature was also observed at the sea off northeastern Taiwan.

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