Riverine molybdenum isotopic fractionation in small mountainous rivers of Taiwan: The effect of chemical weathering and lithology

The molybdenum (Mo) concentration and its isotopic composition from 25 major river catchments throughout Taiwan have been measured to better understand the geochemical behavior of Mo and the mechanism that controls Mo isotopic fractionation via weathering and erosion processes, in a tectonically active high-stand is -land. In addition, the Mo isotopic composition and its abundance in bedrock and riverine bedload sediments were also studied for source identification. The concentration of Mo in river water varies between 1.94 and 45.09 nM, and the 898/95Mo ranges from-0.53%o to +1.35%o, with an average of +0.70 +/- 0.31%o (1SD, n = 42) for the wet season. In the dry season, Mo concentration varies between 2.15 and 58.27 nM, and the 898/95Mo ranges from-0.48%o to +1.09%o, with an average of +0.74 +/- 0.28%o (1SD, n = 43). The seasonal variability of 898/95Mo composition is very small in Taiwan river catchment.The dissolved riverine 898/95Mo are heavier than those of the bedrock (--0.72to +1.03%o). These rivers flow through quite different lithologies and are thus difficult to identify a common cause responsible for the observed Mo isotopic fractionations. However, 898/95Mo of riverine bedload sediments from three rivers show a negative trend with trace elements, e.g., Nb, Mn, and Ti, that are enriched in fine-grain residual Fe-Ti oxides, such as titanite, rutile, ilmenite, and niobite, important hosts for Mo. Consequently, the riverine bedload sediments may be the sink for light 898/95Mo, which drives the riverine dissolved load towards a heavier 898/95Mo isotopic signature through adsorption processes. In general, the rivers of Taiwan discharge a significant flux and a heavier mean 898/95Mo (+0.75%o) to the oceans than that of the mean global rivers (-+0.55%o). While the results will help better constrain the global Mo cycle, short-term seasonal variations, e.g., precipitations, tend to have an insignificant effect on the Mo cycle in the oceans.