U-rich Archaean sea-floor sediments from Greenland – indications of >3700 Ma oxygenic photosynthesis

>3700 Ma metamorphosed pelagic shale from West Greenland contains up to 0.4 wt% reduced carbon with δ13C values down to −25.6‰ [PDB, PeeDee Belemnite]. The isotopic signature and mode of occurrence suggest that the carbon derived from planktonic organisms. The Pb isotopic composition shows that the shale had high primary U/Th. This indicates that organic debris produced a local reducing environment which precipitated U transported to the site of sedimentation by oxidized ocean water. The existence of highly productive plankton that fractionated C isotopes strongly and set up oxidation contrast in the environment suggests that oxygenic photosynthesis evolved before 3700 Ma.