Deep metamorphic carbon carried by hot spring controls carbon budget of mountainous rivers: Evidence from hydrochemistry and δ13C-DIC, δ13C-DOC isotopes in the south Tibetan Plateau

Southern Tibet includes Cona-Woka Rift, Yadong-Gulu Rift, and Shenzha-Dingjie Rift. This study investigates the effects of groundwater on riverine carbon dynamics in mountainous river systems. Hydrochemical compositions, hydrogen-oxygen isotopes, and δ 13 C-DIC and δ¹³C-DOC isotopes were analyzed for 20 hot springs and their associated upstream and downstream rivers. A mass balance approach, PHREEQC reactive transport models, and entropy-weighted water quality index (EWQI) calculations were employed to quantify the mixing of groundwater and evaluate its impacts on river water chemistry. The groundwater mixing ratio in rivers ranges from 0.2 % to 65.4 %, with an average of 14.5 %, and metamorphic deep-sourced carbon accounts for 82 % of the inorganic carbon carried by groundwater. Groundwater discharge significantly modifies riverine dissolved inorganic carbon (DIC) fluxes, contributing an estimated 10.8 ± 0.38 Tg C/year to rivers across the Qinghai-Tibet Plateau. The mixing of deep-sourced metamorphic carbon not only alters the riverine carbon budget and enhances CO₂ outgassing potential, but also deteriorates downstream water quality. These findings underscore the critical yet previously underestimated role of deep geological processes in regulating surface cycles within high-altitude orogenic systems, offering new perspectives for carbon flux estimations and aquatic ecosystem management in the Tibetan Plateau region. • Deep, weathering, and biodegradable carbon fuel riverine CO 2 on the Tibetan Plateau. • Metamorphic carbon source input regulates stream pCO 2 and carbon emissions. • Groundwater carbon flux to Tibetan rivers quantified at 10.8 ± 0.38 Tg C/year. • Groundwater inflow alters river water quality and downstream aquatic health. • Mountainous rivers act as key conduits linking deep carbon and surface processes.