A light carbon reservoir recorded in zircon-hosted diamond from the Jack Hills

An early carbon reservoir The recent discovery of diamond and graphite inclusions in zircon grains formed over 4 billion years ago in the Jack Hills meta-sedimentary belt in Western Australia has given geologists a glimpse of Earth's earliest known carbon reservoir. New ion microprobe analyses of the carbon isotope composition of these inclusions reveal low carbon isotopic ratios, which could reflect deep subduction of biogenic surface carbon. Though this is not unambiguous evidence for life on Earth as early as 4,250 million years ago, as low carbon isotope values can also be produced by certain inorganic chemical reactions. The recent discovery of diamond graphite inclusions in the Earth's oldest zircon grains from the Jack Hills metasediments in Western Australia provides a unique opportunity to investigate Earth's earliest known carbon reservoir. This paper reports ion microprobe analyses of the carbon isotope composition of these diamond-graphite inclusions and finds low carbon isotopic ratios, which may reflect deep subduction of biogenic surface carbon. But such carbon isotope values may also be produced by inorganic chemical reactions. 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 1 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 δ 13 C PDB values (expressed using the PeeDee Belemnite standard) range between -5 per mil and -58 per mil with a median of -31 per mil. This extends beyond typical mantle values of around -6 per mil to values observed in metamorphic and some eclogitic diamonds that are interpreted to reflect deep subduction of low-δ 13 C PDB biogenic surface carbon. Low δ 13 C PDB values may also be produced by inorganic chemical reactions 2 , and therefore are not unambiguous evidence for life on Earth as early as 4,250 Myr ago. Regardless, our results suggest that a low-δ 13 C PDB reservoir may have existed on the early Earth.