Modeling of 226Ra in the North Pacific Ocean

• Main Authors: Shih-Jen Wu, 吳世仁
• Other Authors: Yu-chia Chung
• Format: Others
• Language: zh-TW
• Published: 2002
• Online Access: http://ndltd.ncl.edu.tw/handle/44517851928439657944

碩士 === 國立中山大學 === 海洋地質及化學研究所 === 90 === A total of 10 vertical 226Ra profiles from the North Pacific was published as part of the GEOSECS program. These profiles are located on an east-west section along ~300N. The purpose of this thesis is to use one-dimensional and two-dimensional diffusion-advection models to estimate some important geochemical parameters. There are some discrepancies between the modeling results and the observed tracer distributions due to restrictions on end-members in the models. Fitting the observed data to the model, one obtains values of the Z* between 0.8 and 1.0 km from potential temperature and that of JBa/w between 0.0020 and 0.0064 nM/kg/m from Ba. Where Ba profiles are available, the corresponding 226Ra profiles are similar from the surface to the deep water, showing a linear correlation with a slope at 0.22 dpm/100nM or 4.4×10-9 for the Ra/Ba molal ratio. The slope has an intercept of 0.48 nM/kg on the Ba axis. That 226Ra is linearly correlated with Ba from surface water to deep water is probably because algebraic sum of the production term and the radioactive decay is nearly constant. Thus the revised model has a form equivalent to that for Ba. Where Ba profiles are available, one can fit the observed data to the model to obtain the needed parameters with (JRa-λCRa)/JBa as a constant. The upwelling velocity, w, obtained from model calculations on the Ba profiles is: 3.8 m/yr at 204; 4.8 m/yr at 212; 3.5 m/yr at 226, 1.2 m/yr at 224. These values are consistent with those (1 to 12m/yr) reported for the Pacific deep water based on other methods. Fitting 226Ra profile data to the vertical one-dimensional diffusion-advection model for the in-situ production rate, J, and the associated upwelling velocity, w, one finds that J and w are coupled with a negative correlation. This is consistent with that reported in the literature. Profiles from the northeast Pacific show a deep Ra maximum, with an “excess” which extends westward while decreasing and finally vanishing in the northwest Pacific near Japan. A horizontal diffusivity of 5 ×105 cm2/s was obtained based on this “excess” 226Ra with a horizontal 1-D model by Chung and Craig (1980). A numerical finite-difference method is applied to the 2-D model fitting on the potential temperature data for the horizontal diffusivity, Kx. A Kx of 4.58 ×105 cm2/s is obtained within the depth range of 1000m to 3000m. The value is 4.82 ×105 cm2/s for the depth range from 1000m to 4000m. These two approaches yield nearly identical results.