The South China Sea (SCS) is a large semi-enclosed marginal sea to the west of the tropical Pacific, and connected to the western Pacific through the Luzon Strait. The surface water circulation and hydrography in the SCS are strongly influenced by the East Asian monsoon system. The seasonal changes of the monsoon system induce changes in the mixed layer thickness, upwelling, primary production, and associated biogeochemistry. In order to understand the characteristics of the Kuroshio intrusion and the source strength of radium isotopes from the coastal zone, we carried out surface water and water column samplings for Ra-228 and Ra-226 measurements in the northern SCS and the Luzon Strait areas.
The Ra isotopes were much higher in the SCS surface waters than in the open ocean surface waters because the SCS was enclosed mostly by landmasses which are known as sources of these nuclides. Higher surface water activities are seen in the northern (shelf and slope) area; lower values are observed in the southern (deep basin) area; the lowest values appear in the eastern (Luzon Strait) area. Large temporal and spatial variations were also observed probably due to the source strength of radium isotopes from the coastal zone and intrusion of the Kuroshio Current.
The vertical 228Ra profiles are remarkably similar, showing high values in the surface layer and fairly uniform below about 500m depth but with an increase toward the bottom due to input from the underlying sediments. The shallow water profile on the shelf shows higher 228Ra values due to both vertical and horizontal mixing of the shelf water and additional source from the shore zone. The 226Ra profiles in the northern SCS are quite similar to those in the northwest Pacific both in pattern and magnitude, showing lowest values at the surface and an increase with depth although more scattered. 226Ra activities in the shallow water (less than 1000m depth) are higher in the northern SCS than in the northernwest Pacific Ocean, but they are quite comparable below this depth.
The Ra-228/Ra-226 activity ratios of the surface water decrease from the coastal zone and estuaries toward the basin and the Luzon Strait (from 4.11 to 1.03), indicating the former as a main source area for Ra-228. The vertical 228Ra/226Ra activity ratios decrease rapidly from the surface at 2.3 to a depth of 1500m at 0.5 below which the values are less than 0.5. This pattern is similar to that of the open ocean but these values are much higher than those in the open oceans where the surface water values are 0.5 or less and the deep water values are less than 0.1. This suggests a strong input of Ra-228 relative to Ra-226 from the bottom sediments into the deep water of the semi-enclosed SCS.
| Identifer | oai:union.ndltd.org:NSYSU/oai:NSYSU:etd-0913105-225914 |
| Date | 13 September 2005 |
| Creators | Lin, Hsiu-chuan |
| Contributors | Yu-chia Chung, none, H.-L. Lin |
| Publisher | NSYSU |
| Source Sets | NSYSU Electronic Thesis and Dissertation Archive |
| Language | Cholon |
| Detected Language | English |
| Type | text |
| Format | application/pdf |
| Source | http://etd.lib.nsysu.edu.tw/ETD-db/ETD-search/view_etd?URN=etd-0913105-225914 |
| Rights | off_campus_withheld, Copyright information available at source archive |
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