CHEMICAL WEATHERING IN THE RED RIVER IN NORTHERN VIETNAM AND SOUTHERN CHINA

We investigated the major ion and Sr isotopic composition of the Red River in south China and north Vietnam to better understand the relationship between chemical weathering, geology, and discharge (with the aid of GlS). The Red River flows from eastern Tibetan Plateau to the Gulf of Tonkin in the South China Sea. It formed after the collision of Eurasia and India in the Cenozoic, and lies in an area that is still tectonically unstable. The effect of geology is examined by comparing the tributaries—Da, Lo, and Red which drain different lithologies. The overall water chemistry of all three channels is largely affected by carbonate weathering. However, the main channel flows through the Red River fault zone which has undergone various tectonic activities—intrusion of magma, exposure of old rocks, and influx of thermal waters. Though Da, Red, and Lo are geographically close to each other, each tributary has distinctive features in their geology. As a result, their water chemistry shows different aspects. Unlike other tributaries, in the Red main channel, there are higher concentration of Na, Cl, K, and SO₄. In addition, we found that ⁸⁷Sr/⁸⁶Sr ratios are higher than 0.710 with a maximum of 0.7217—except one sample (0.7085). The effect of discharge is examined by comparing samples collected in the summer (high-stage) versus winter (low stage). TDS (total dissolved solutes) of the Red River varies 70-244 mg/L (mean 172 mg/L) in summer and 76-338 mg/L (mean 179 mg/L) in winter. Though the discharge is higher in summer, the TDS does not vary significantly. Compared to global rivers, the Red River is a carbonate-dominated river. Based on these data, we determine weathering rates of Red River using an inversion model. In the Red River, carbonate rocks have a significant role in weathering (70-99 %), whereas weathering of silicate is less important (1-27 %).