Glacier runoff is a source of old bioavailable dissolved organic carbon (DOC) to downstream ecosystems. The DOC pool is composed of material of various origins, chemical compositions, ages, and levels of bioavailability. Using bioincubation experiments, we show that glacier DOC respiration is driven by a young source, rather than by ancient material which comprises the majority of the glacier carbon pool. This young bioavailable fraction could currently be a critical carbon subsidy for recipient food webs.