Photodegradation Unlocks Permafrost Carbon in Qinghai-Tibetan Plateau Rivers

High-altitude rivers on the Qinghai-Tibetan Plateau serve as distinctive photochemical reactors, where intense solar radiation interacts with ancient carbon reservoirs to fundamentally reshape the fate of dissolved organic matter (DOM). Based on isotopic tracers (δ2H, δ18O, δ3H, Δ14C, and δ13C) and molecular-level characterization with 7979 unique formulas identified along the Yarlung Tsangpo River, we reveal a synergistic control of sunlight and carbon source on DOM cycling. Isotopic signatures indicate considerable inputs of "old water" (i.e., glacial meltwater) and confirm permafrost-derived carbon as a dominant source of aged sedimentary organic matter. Molecular analyses show that aromatic DOM, mainly composed of terrestrial lignin- and tannin-like compounds, is highly photoreactive, and strong sunlight drives its dearomatization and oxidation. Photodegradation products of aromatic DOM share 26.8% of their molecular formulas with Fpre1953-related DOM and 44.8% with permafrost-derived DOM. This process unlocks previously stable and old carbon into labile fraction, enhancing fluvial carbon mineralization. The liberated ancient DOM shows higher bioavailability, potentially promoting CH4/CO2 emissions and mercury accumulation. Our study identifies high-altitude rivers as critical photochemical hubs that mobilize cryospheric organic carbon and imply a positive climate feedback in warming alpine regions.