Sinking Fluxes of Short-lived Thorium Radionuclides in the Deep Basin of the South China Sea
Date Issued
2016
Date
2016
Author(s)
Chen, Ying-Ying
Abstract
Fluxes of total mass (FMass), 234Th (FTh-234) and 228Th (FTh-228) were determined by time-series sediment traps deployed at two depths, 2000 m and 3500 m, at the SEATS (South East Asian Time-series Study, 18°N, 116°E) station in the northern South China Sea (SCS) during June 2008-July 2009 and September 2013-May 2015. The FMass at the two depths showed a synchronized pattern, reflecting the seasonal variability of the primary productivity in the surface water. The FMass also revealed inter-annual variability, which peaked at different time of each year. Due to malfunction of the sediment trap and the short half-life of 234Th (t1/2 = 24.1 days), the FTh-234 data was limited. Combining with the results of 234Th-238U disequilibrium and FTh-234 measured by the floating trap in the euphotic zone, the FTh-234 showed decreasing trend with depth, indicating the decomposition of particles and radioactive decay of 234Th while particles settled through the water column. In contrast to 234Th, due to its longer half-life, 228Th (t1/2 = 1.91 years) is a more suitable tracer for particle scavenging processes in the deep layer. The temporal variation of the FTh-228 was high during the collection period, especially in the summer of 2014. The FTh-228 increases with depth, which indicates that sinking particles continually take up the radionuclide while settling through the water column. It is also found that the specific activity of 228Th in the sinking particles was higher in the SCS than other oceans, which may be caused by higher concentration of its parent radionuclide, 228Ra, released into the seawater from the broad continental shelf of the SCS. Linear correlations between the fluxes of the two radionuclides and the FMass indicate the removal of particle-reactive elements is controlled by the vertical flux of particulate materials in the SCS. Similar Kd values of 234Th of sinking particles in the euphotic zone and deep ocean imply that the composition of settling particles do not change significantly with depth. Sinking velocity of particles, 168 m d-1, was estimated based on the 234Th/228Th ratio. This study is the first data set that provides the temporal observation of fluxes of short-lived thorium isotopes in the deep basin of the SCS.
Subjects
SEATS
234Th flux
228Th flux
particle scavenging
Type
thesis
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