1. Scripps Institution of Oceanography, University of California, San Diego, La Jolla, California 92093-0236, USA
2. Geological Survey of Norway, Leiv Eirikssons vei 39, 7491 Trondheim, Norway
3. Institute of Geology, Tallinn Technical University, EE-0001 Tallinn, Estonia

Correspondence to: Correspondence and requests for materials should be addressed to M.A.v.Z. (e-mail: Email: mvanzuil@ucsd.edu).

Abstract

The isotopic composition of graphite is commonly used as a biomarker in the oldest (>3.5 Gyr ago) highly metamorphosed terrestrial rocks. Earlier studies on isotopic characteristics of graphite occurring in rocks of the approximately 3.8-Gyr-old Isua supracrustal belt (ISB) in southern West Greenland have suggested the presence of a vast microbial ecosystem in the early Archean^{1, 2, 3, 4}. This interpretation, however, has to be approached with extreme care⁵. Here we show that graphite occurs abundantly in secondary carbonate veins in the ISB that are formed at depth in the crust by injection of hot fluids reacting with older crustal rocks (metasomatism). During these reactions, graphite forms from the disproportionation of Fe(II)-bearing carbonates at high temperature. These metasomatic rocks, which clearly lack biological relevance, were earlier thought to be of sedimentary origin and their graphite association provided the basis for inferences about early life^{1, 2, 3, 4}. The new observations thus call for a reassessment of previously presented evidence for ancient traces of life in the highly metamorphosed Early Archaean rock record.