Elements of Eoarchean life trapped in mineral inclusions

In situ infrared spectroscopy maps the occurrences of chemical bonds within tiny inclusions in 3,700-million-year-old metasedimentary rocks from West Greenland, finding greater evidence for organic life at this early date. Garnets of evidence for early organic life From stromatolites in the sunlit surface waters of oceans to hydrothermal vents in the ocean depths, the evidence for organic life having existed 3.7 billion years ago is growing. Here, the researchers describe very tiny inclusions of lightly metamorphosed organic material inside garnets from 3.7-billion-year-old rocks in Greenland. They use a relatively new technique that combines atomic force microscopy and infrared spectroscopy to map the occurrences of various chemical bonds within these inclusions including carbonyl, anhydride, phosphate and nitrile. This corroborates earlier work on carbon isotopes in the same rocks but goes further by identifying these specific chemical bonds and finding much greater evidence for organic life at this early date. Metasedimentary rocks from Isua, West Greenland (over 3,700 million years old) contain 13 C-depleted carbonaceous compounds, with isotopic ratios that are compatible with a biogenic origin 1 , 2 , 3 . Metamorphic garnet crystals in these rocks contain trails of carbonaceous inclusions that are contiguous with carbon-rich sedimentary beds in the host rock, where carbon is fully graphitized. Previous studies 4 , 5 have not been able to document other elements of life (mainly hydrogen, oxygen, nitrogen and phosphorus) structurally bound to this carbonaceous material. Here we study carbonaceous inclusions armoured within garnet porphyroblasts, by in situ infrared absorption on approximately 10 −21 m 3 domains within these inclusions. We show that the absorption spectra are consistent with carbon bonded to nitrogen and oxygen, and probably also to phosphate. The levels of C–H or O–H bonds were found to be low. These results are consistent with biogenic organic material isolated for billions of years and thermally matured at temperatures of around 500 °C. They therefore provide spatial characterization for potentially the oldest biogenic carbon relics in Earth’s geological record. The preservation of Eoarchean organic residues within sedimentary material corroborates earlier claims 2 , 6 for the biogenic origins of carbon in Isua metasediments.