| Summary: | 碩士 === 國立臺灣師範大學 === 海洋環境科技研究所 === 102 === Recent developments in analytical technique, particularly the advantages of Multi-Collector Inductively Coupled Plasma Mass Spectrometry (MC-ICP-MS), have allowed the precise determination of differences in the isotope compositions of Mo between samples and reference standards. Previously studies about Mo concentration and isotope composition included water samples from lake, river, estuary, open ocean and restricted basins. Other studies such as Mo concentration and isotope of manganese nodules, pelagic sediments, igneous rock, fluids of hydrothermal vent and glacials are also reported as well. However, about Mo and δ98/95Mo of a marginal sea like the Okhotsk Sea has yet been studied. The sea of Okhotsk is a marginal sea of the northwest Pacific Ocean, it accompanies with terrigenous and open ocean signals. This thesis study is hoped to add more understanding about the geochemistry of molybdenum and its isotope fractionation in the marginal sea.
The experiments included a sequential leaching technique to extract the authigenic molybdenum coating on the sedimentary Fe-Mn oxyhydroxides, and a robust chromatography technique to purify molybdenum for isotope composition analyses. This procedure is different from previous studies which apply whole bulk sediment for Mo concentration and isotope analyses. In addition, the double spike technique is used to analyze Mo isotope compositions.
Previous studies have shown that materials sources, redox conditions and early diagenesis largely influence Mo isotopic compositions. Under oxic condition, molybdate (MoO3) coating with authigenic ferromanganese crusts shows an average value of δ98/95Mo =–0.7‰. On the other hand, under anoxic condition which shows the analyses of tetrathiomolybdate (MoS42-) with a much heavier isotope value of δ98/95Mo =2.3‰. Compared to previous studies, Mo isotope compositions present in this study show much lighter values.
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