| dc.description.abstract | This study investigates the behavior of molybdenum (Mo) geochemistry and the mechanism that controls Mo isotopic fractionation via weathering and erosion in a tectonically active high-stand island, 25 major river catchments in Taiwan was measured for Mo concentration and its isotopic composition. For source identification, bedrock, and riverine bedload sediments were also examined. Riverine Mo concentration ranges from 1.94 to 45.09 nM, and the δ98/95Mo ranges from -0.28‰ to +1.60‰, with an average of +0.95 ± 0.31‰ (1SD, n = 42) for the wet season. In the dry season, Mo concentration ranges from 2.15 to 58.27 nM and the δ98/95Mo from -0.23‰ to +1.34‰, with an average of +0.99 ± 0.28‰ (1SD, n = 43). The δ98/95Mo composition in the Taiwan river catchment shows little seasonal variation. Riverine δ98/95Mo signals are heavier than the bedrock. The observed Mo isotopic fractionation is challenging to explain because these rivers drain across different lithologies. Moreover, the δ98/95Mo isotopic composition in riverine bedload sediments negatively correlates with trace element (e.g., Nb, Mn, and Ti) concentrations. These trace elements are enriched in fine-grain residual Fe-Ti oxides such as titanite, rutile, ilmenite, and niobite. Thus, riverine bedload sediments may act as a light δ98/95Mo sink that makes the riverine dissolved load heavier. Furthermore, Taiwanese rivers discharge a significant Mo flux and a heavier mean δ98/95Mo (+1.00‰) to the oceans than world rivers (~ +0.8‰).
Additionally, this study also focused on Mo isotopes as a source tracer, for which Danshuei (polluted) and Liwu (non-polluted) rivers in Taiwan were chosen to analyze Mo isotopes during weathering and riverine transport. The δ98/95Mo in the Danshuei river ranges from +0.83‰ to +1.50‰ in the wet season and +0.54‰ to +1.25‰ in the dry season, respectively. The wet season is lighter, while the dry season is slightly heavier, with some outliers. The δ98/95Mo ranges from +0.54‰ to +1.30‰ in the Liwu river, becoming heavier downstream. Three Mo sources are identified in the Danshuei river using the MixSIAR model: seawater intrusion, rock–water interaction, and anthropogenic inputs. The heavy δ98/95Mo signature in the Liwu river is likely driven by pyrite oxidation and carbonate weathering, while light δ98/95Mo is sequestered into secondary mineral phases (Fe-Mn oxides) in bedload sediments.
Furthermore, these findings have significant implications for weathering-controlled riverine Mo and anthropogenic Mo sources delivered to the ocean and will help better constrain the global Mo cycling in modern seawaters, while short-term seasonal changes tend to show an insignificant effect.
Overall this thesis highlights the application of Mo-isotope systematics as a promising proxy to understand the Mo isotopic fractionation mechanism during continental weathering in small mountainous rivers (SMRs) of Taiwan. In addition, it also emphasizes source tracing applications of Mo in environmental geochemistry. | en_US |