PETROGRAPHIC ANALYSIS AND CHEMICAL WEATHERING SOURCES IN THE CHOSHUI RIVER WATERSHED ON THE HIGH-STANDING ISLAND OF TAIWAN

The Long-Term Carbon Cycle operates over geologic time and involves the exchange of carbon among rocks and the ocean, atmosphere, and soil. Carbon dioxide is removed from the atmosphere when calcium and magnesium silicate minerals undergo chemical weathering which results in the precipitation of calcium carbonate in the ocean. While the processes of erosion, sediment transport, and sediment deposition are well known, their importance to the carbon cycle are still poorly quantified. The focus of this research is to determine the source material for physical and chemical weathering in the Choshui River watershed of Taiwan. Taiwan experiences some of the highest uplift rates globally, averaging 5 mm/year, driven by the active collision between the Asian continental plate margin and the Luzon Arc. Taiwan is considered a High-Standing Island (HSI) because its stream headwaters are at an elevation of 1000 m above sea level. Studies have shown that while HSIs account for approximately 3% of continental land mass, they could contribute as much as 33% of the global sediment input to oceans, making them ideal locations to study weathering and the source materials for weathering. Multiple meta-sedimentary rock samples collected in the Choshui River watershed were analyzed to determine the mineral composition of the bedrock. A petrographic microscope was utilized to determine the preliminary mineralogy of the rock samples and X-ray Diffraction (XRD) was then performed on powdered rock samples. Examination of the rock samples reveals that quartz is the dominate mineral in the Choshui watershed, along with abundant feldspars and phyllosilicates. Despite previously published work stating that no potassium feldspar is found in this region, XRD patterns show that the mineral exists in multiple rock samples in this study. Sericite and iron oxides and hydroxides are present in most of the rock samples which suggest that hydrothermal alteration is a common process in this region and may have a heavier influence on weathering patterns than previously predicted. Source rock studies are consistent with weathering product determination and weathering rates calculated by our research group.