Turbiditic and pelagic sedimentary rocks from the Isua supracrustal belt in west Greenland [more than 3700 million years ago (Ma)] contain reduced carbon that is Likely biogenic. The carbon is present as 2- to 5-micrometer graphite globules and has an isotopic composition of delta(13)C that is about -19 per mil (Pee Dee belemnite standard). These data and the mode of occurrence indicate that the reduced carbon represents biogenic detritus, which was perhaps derived from planktonic organisms.