Abstract
Vast stores of arctic permafrost carbon that have remained frozen for millennia are thawing, releasing ancient dissolved organic carbon (DOC) to arctic inland waters. Once in arctic waters, DOC can be converted to CO (sub 2) and emitted to the atmosphere, accelerating climate change. Microbial utilization and sunlight-driven photoreactions oxidize DOC, converting a portion to CO (sub 2) and leaving behind a modified pool of dissolved organic matter (DOM). Samples from the Kolyma River, its tributaries, and streams draining thawing yedoma permafrost were collected. Microbial incubations, solar irradiation experiments, and radiocarbon dating were employed to assess the lability of ancient permafrost-DOC in natural and laboratory generated samples containing a mix of modern and ancient DOC. Photolabile DOC was always modern, with no measurable photochemical loss of ancient permafrost-DOC. By contrast, permafrost-derived DOC was highly biolabile. Optical and ultrahigh resolution mass spectrometric measurements revealed that photomodification resulted in a loss of aromatic compounds, whereas biomodification resulted in a loss of high H/C compounds. These findings indicate that both photo- and bio-modification may influence the chemistry and fate of permafrost-derived DOC, but that microbial processes are by far the more efficient at removing this ancient DOC from inland waters.