Abstract
A station off southwestern Taiwan (22°25'N 120°08'E) was selected to carry out detailed seawater sampling for the measurements of dissolved and particulate (superscript 234)Th. The profiles showed that (superscript 234)Th 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 (superscript 234)Th in the euphotic layer are in the ranges of 90-110 days and 12-28 days, respectively. A vertical (superscript 234)Th flux of 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 (superscript 234)Th with respect to its progenitor, (superscript 238)U. Residence times of dissolved and particulate (superscript 234)Th in this layer are 112 days and 35 days, respectively. A lower limit of (superscript 234)Th flux of 1600 dpm m^(-2) d^(-1) into the bottom sediment can be estimated.
Highlights
A lower limit of 234Th flux of 1600 dpm m-2 d-1 ihto the bottom sediment can be estimated. It is well-known that 'particle-reactive' elements are subject to scavenging onto parti cle surface and removed from the ocean via particle settling (Goldberg, 1954). · As radionuclides are .measured in both dissolved and particulate phases, we are able to estimate how fast the elements are scavenged and the settling velocity of particulate matter in the water column
The water column under investigation has a stratified two-layer euphotic layer and a deep layer subjected to the influence of active horizontal input of resuspended sediments
We adopted a box-model to estimate the residence times of dissolved and particulate 234Th. vertical fluxes of 234Th out of five compart ments were estimated
Summary
It is well-known that 'particle-reactive' elements are subject to scavenging onto parti cle surface and removed from the ocean via particle settling (Goldberg, 1954). · As radionuclides are .measured in both dissolved and particulate phases, we are able to estimate how fast the elements are scavenged and the settling velocity of particulate matter in the water column. It is well-known that 'particle-reactive' elements are subject to scavenging onto parti cle surface and removed from the ocean via particle settling (Goldberg, 1954). · As radionuclides are .measured in both dissolved and particulate phases, we are able to estimate how fast the elements are scavenged and the settling velocity of particulate matter in the water column. The power of radionuclides originates from its definite decay rate which provides a 'clock' for the processes which govern the distribu tion of elements in the ocean. Compared to the open ocean, coastal waters have enhanced scavenging properties and more dynamic 'charact risrics of:particle cycling,. Scavenging and particle removal rates will · be estimated using a box model calculation
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