Air-Mass Influence, Radiation, and Melt-Season Dynamics Jointly Shape Dissolved Organic Matter Composition and Radiocarbon Age in Glacier Outflows on the Tibetan Plateau.

Glacier melt increasingly delivers bioavailable yet radiocarbon-depleted dissolved organic matter (DOM) to headwaters, but controls on DOM molecular composition and radiocarbon signatures remain unclear for the Tibetan Plateau. We coupled ultrahigh-resolution mass spectrometry with Δ14C measurements for 30 samples from 16 glacier outflows, including three ablation-season time series across contrasting climatic settings. DOC concentrations were uniformly low, but DOM composition was highly heterogeneous. Multivariate analyses identified longitude and shortwave radiation as primary regional correlates, while marked differences among neighboring outlets indicated strong local controls. Seasonal DOM patterns varied by site: at the monsoon-influenced Rongbuk Glacier on the northern slope of Mount Everest, DOM shifted from lipid-rich to more lignin-, tannin-, and aromatic-enriched compositions as inorganic deposition indicators increased, consistent with enhanced allochthonous influence; however, Δ14C became more modern over the season. In contrast, the westerly-influenced, high-radiation Xiongcai Glacier showed comparatively stable DOM composition but elevated and dynamic peptide-like signals. Across the Δ14C subset (n = 23), Δ14C correlated negatively with lipid-like contributions, revealing an underappreciated "old-but-potentially labile" component. These results show that regional climate, local catchment setting, and melt-season progression jointly govern the export of mixtures of young and old, labile and compositionally complex DOM.