| Summary: | 碩士 === 國立成功大學 === 地球科學系碩博士班 === 97 === The Ca isotope composition of synthetic carbonates and corals have been measured in order to evaluate the Ca isotope fractionation during the carbonate formation; and the δ44/42Ca of a seawater profile in South China Sea (SCS) has been performed as well to realize the oceanic Ca cycle in preliminary. A fast and high-precision analytical technique for the Ca isotopes has been developed using high-resolution multi-collector ICP-MS (Neptune, Thermo-Fisher) coupled with a desolvation system (APEX-IR, ESI). Two-steps separation technique has been successfully applied to separate matrix component from the analyzed compound to avoid potential spectral interferences. The external reproducibility for δ44/42Ca analysis is better than 0.11‰ (2σ) using a standard-sample-standard bracketing method. Calcium carbonates obtained from controlled inorganic precipitation were used for evaluating the Ca isotope fractionation during precipitation and display a rather small temperature-dependent fractionation at 5-40°C (0.002 and 0.001‰/°C for aragonite and calcite, respectively). The Ca isotope fractionation factor (α) in synthetic carbonates reveals a significant difference compared with previous studies, which was attributed to the difference in growth rate under various precipitation techniques. Furthermore, Ca isotopes in cultured and field corals, Porites sp., show also only minor temperature-dependent fractionation. Kinetic rather than the temperature effects would strongly affect the Ca isotope fractionation in carbonates. On the other hand, seawaters collected from SCS displays a small but significant δ44/42Ca vertical variation. The lowest value of δ44/42Ca in the SCS is detected in the bottom water which could be partly attributed to the intense partial dissolution on marine carbonates. Further studies are necessary to improve analytical precision and to understand the main controlling factors on seawater Ca isotopes.
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