The carbon isotope ratio of grap hite; problems associated with its use as a biomarker in early archean rocks

The search for signatures of extinct life in the Early Archean is made difficult if not impossible due to poor preservation of the rock record. The very few exposed formations that date back beyond 3 5 Ga ago, have all been subject to high or. intermediate grade metamorphism. This would have transformed possible morphological microfossils beyond recognition, turned organic matter into kerogen and ultimately crystalline graphite. Therefore the search for life in these rocks depends entirely on chemical and isotopic indicators. It is widely assumed that carbon isotope signatures can be used to distinguish biogenic graphite from abiogenic varieties. The carbon isotope signature of graphite can be partly altered during metamorphism due to isotopic exchange with carbonates, or due to volatilization reactions. However, isotopically light graphite has been found in highly metamorphosed early Archean rocks, suggesting that some isotopic biosignatures can escape metamorphic resetting. For instance, earlier reports of isotopically light graphite occurring in carbonate-rich rocks from the 3.8 Ga old Isua Supracrustal Belt (ISB) in southern West Greenland have suggested the presence of a vast microbial ecosystem in the early Archean. Here we show that such interpretations need to be approached with extreme care. It can be shown that graphite-bearing carbonate deposits in the ISB, which were originally interpreted as primary deposits in a shallow marine environment, are actually secondary veins that resulted from extensive metasomatism. The occurrence of graphite, forming the basis for some earlier biogenic interpretations, is entirely restricted to these metasomatic carbonate deposits, while most sediments like banded iron formations (BIF's) and metacherts contain virtually no graphite. Furthermore, within the metacarbonates graphite appears to be specifically associated with siderite and magnetite. Thermal decomposition of siderite; 6 FeCO3 = 2Fe3 O4 + 5CO2 + C, is the process seemingly responsible for the graphite formation. This petrographic evidence therefore excludes a biogenic origin of this graphite. From this study of graphite in the ISB we conclude that the interpretation of life from carbon isotope ratios has to be assessed within the context of specific geologic circumstances requiring (1) control on recent contamination in the rock sample, (2) understanding of different graphite-producing mechanisms and related carbon isotopic systematics, and (3) reliable protolith interpretation and control of secondary, metasomatic processes.