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The geochemical and biological study of corals
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Monthly-resolved coral barium isotopic records in the South China Sea and its paleoceanographic implication
P-G3-04
Xiaohua Li* , Center of Deep Sea Research, Institute of Oceanology, Chinese Academy of Sciences, Qingdao, China
Zhen Zeng, CAS Key Laboratory of Crust-Mantle Material and Environment, School of Earth and Space Science, University of Science and Technology of China, Hefei, China
Yi Liu, Institute of Surface-Earth System Science, Tianjin University, Tianjin, China
Huimin Yu, CAS Key Laboratory of Crust-Mantle Material and Environment, School of Earth and Space Science, University of Science and Technology of China, Hefei, China
Fang Huang, CAS Key Laboratory of Crust-Mantle Material and Environment, School of Earth and Space Science, University of Science and Technology of China, Hefei, China
Chuan-Chou Shen, High-Precision Mass Spectrometry and Environment Change Laboratory (HISPEC), Department of Geosciences, National Taiwan University, Taipei, Taiwan
Presenter Email: lxh1990@mail.ustc.edu.cn |
| Marine barium (Ba) is an important proxy for the nutrient cycle, productivity and the mixing of water masses. However, the factors affected Ba cycle are complicated and not fully understood. Coral-based Ba/Ca proxy have been used to reconstruct the timing and variability of seawater Ba concentration. However, the variation of Ba/Ca in aragonitic coral skeletons remains difficult to be interpreted as environmental proxy. This is mainly due to the disturbance by internal (biomineralization) and multiple external (environmental) processes on Ba incorporation into coral skeletons, and these processes are hard to be constrained with Ba/Ca alone. Ba stable isotope measurements may improve the use of coral Ba as a tracer and better constrain the cycling of Ba. Here we use Multicollector-Inductively Coupled Plasma-Mass Spectrometry (MC-ICP-MS) with double spike correction to obtain the first records of the monthly resolution Ba isotopic composition (¦Ä137/134Ba) in shallow-water corals (Porites) collected alive in the Nanwan (NW), supplemented by the analysis of monthly Ba/Ca ratios. Monthly NW ¦Ä137/134Ba records express a seasonal cycle from 0.30¡ë in dry season to 0.24¡ë in rainy season, negative correlating with coral Ba concentration (R = -0.53, p < 0.05). The most striking feature of NW coral record is the significant increase in the Ba/Ca baseline that occurred from 1987 to 1996. The baseline ratios increase from 2.39 mmol/mol to 3.34 mmol/mol, an increase of about 40%, which are attributed to the terrestrial derived Ba input induced by human activities. However, with the increase of NW Ba/Ca, there is only a small decrease in Ba-isotopic compositions (~0.06¡ë our 2SD long term measurement precision is 0.03¡ë. The relatively lighter Ba from terrestrial input preferentially absorb on the suspended sediment before incorporation into the coral skeleton and these may be the the main cause. In addition, NW Ba/Ca ratios exhibit a double-peak phenomenon, with the first large peak in spring and second smaller peak in summer/fall. Terrestrial derived Ba input by the large precipitation in summer/fall is the main reason for the second peak. Other possible causes, such as SST, upwelling, phytoplankton bloom and coral spawn are also discussed, but none provide satisfactory explanations for the first peak, further study is needed. Clearly, Ba isotopes in shallow-water corals combined with Ba/Ca show great potential as a proxy for land-sea interactions and water mass mixing. |
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