Vast quantities of ancient organic carbon (OC) are stored in Arctic permafrost soils. Arctic warming is thawing these frozen soils, releasing OC from its millennial deepfreeze into Arctic rivers as dissolved OC (DOC).  Respiration of this newly liberated fossil DOC is postulated to release the greenhouse gas carbon dioxide, fuelling a positive feedback between permafrost thaw and climate change, which has been described as a carbon bomb. Despite mounting evidence for the above feedback, no study has dated or molecularly characterized the permafrost DOC mineralized within rivers. Radiocarbon data showing that stream bacteria utilize fossil (>20,000 year old) permafrost-derived DOC in the aquatic environment will be presented, providing definitive proof that thawed permafrost DOC is efficiently converted to CO2. In addition, high resolution mass spectrometry (FTICR-MS) derived molecular signatures of the degraded DOM reveal that permafrost DOM starts out with a unique molecular signature, which becomes shifted toward more classical riverine DOM signatures during bacterial degradation. The quality of the degraded DOM is discussed, offering new insight into the fundamental chemical controls upon DOM biodegradation.