The recent discovery of diamond-graphite inclusions in the Earth's oldest zircon grains (formed up to 4,252 Myr ago) from the Jack Hills metasediments in Western Australia provides a unique opportunity to investigate Earth's earliest known carbon reservoir. Here we report ion microprobe analyses of the carbon isotope composition of these diamond-graphite inclusions. The observed δ13CPDB values (expressed using the PeeDee Belemnite standard) range between -5per mil and -58per mil with a median of -31per mil. This extends beyond typical mantle values of around -6per mil to values observed in metamorphic and some eclogitic diamonds that are interpreted to reflect deep subduction of low-δ13CPDB biogenic surface carbon. Low δ13CPDB values may also be produced by inorganic chemical reactions, and therefore are not unambiguous evidence for life on Earth as early as 4,250 Myr ago. Regardless, our results suggest that a low-δ13CPDB reservoir may have existed on the early Earth.