Dissolved constituents and Sr isotopes in river waters from a mountainous island – The Danshuei drainage system in northern Taiwan

Abstract

Taiwan is a typical active orogenic belt situated at the collision boundary between the Eurasian Continental Plate and the Philippine Sea Plate. Dissolved major and trace constituents, as well as Sr and Sr isotopes in river waters collected from the Danshuei River basin in northern Taiwan have been studied to evaluate chemical weathering processes. The results of principal component analysis show that the ion sources in these river waters can be categorized into 3 major components: chemical weathering, seasalt contribution and local anthropogenic input. The chemical weathering is the most dominant factor that contributes about 85% of total variances. Significantly increased Na/Cl and Ca/Cl, as well 87Sr/ 86Sr, were observed in most upper stream samples. The Na/Cl and Ca/Cl ratios in the Dahan Stream, however, are much higher than the Shindien Stream. Even though average rainfall is stronger in the Shindien drainage basin, chemical evidence from river waters supports less intense weathering in the region. Selective dissolution of secondary calcites explains the observed high Ca/Cl, Sr/Cl and Ca/Na in the Dahan Stream. These results highlight the potential importance of tectonic factors, such as uplift and physical erosion in studying chemical weathering in an active orogenic belt. The variations of 87Sr/ 86Sr in the Danshuei River are quite large, reflecting some strata that released Sr. Most of the upstream waters exhibit more radiogenic 87Sr/ 86Sr, 0.713243–0.714338, due to weathering of ambient low-grade metamorphic rocks, 0.71678–0.72216. The distributions of Cl, Sr and 87Sr/ 86Sr in the main stream were affected by somewhat conservative mixing with sources varing between ambient rocks and seasalt. In the upper Dahan Stream, heavy 87Sr/ 86Sr ratios were coincident with large deviations of Na/Cl from the average seawater value, as high as 40. The degree of chemical weathering in ambient rocks plays a dominant role in affecting the distribution of Sr and 87Sr/ 86Sr in the Danshuei River. This isotopic characteristic makes 87Sr/ 86Sr an invaluable tracer for studying source mixing, migration pathways and chemical weathering in an active orogenic belt.