Climate warming in the Arctic results in thawing of permafrost deposits. This promotes the accessibility of freeze-locked old organic matter (OM) accumulated during the past. Characterizing OM of different depositional ages, we were able to show that OM from last glacial Yedoma deposits possess the highest potential to provide organic substrates such as acetate for microbial greenhouse gas production and therefore to accelerate the carbon–climate feedback cycle during ongoing global warming.