Biological Versus Abiological Mechanisms Explaining Isotopically Heavy Isua Organic Matter

Abstract

The anomalously heavy organic carbon from Isua rocks of 3.8 billion years ago has been explained as an artifact of metamorphism (Schidlowski, 1988). The 613C value of -13.0+4.9%o does not fit within the a13C values 'characteristic' for biological debris (Hayes, 1996). The relatively small 13C content of organic matter considered 'typical' for biological CO2 fixation is based on the assumption that the Calvin cycle is the major CO2 fixation pathway and that other carbon fixation pathways with different carbon isotope fractionation effects have played a minor role in the Earth's history. However, assuming that the Isua organic matter is of a Calvin cycle origin the effect of metamorphism on the ~3C content of the organic matter must have been large, an increase of more than 10%o. Recently the effect of metamorphism on the ~3C content of organic matter is being discussed and an increase in 13C of 2-3~ due to metamorphism has been suggested (Watanabe et al., 1997; DesMarais, 1997). This implies that the 13C value of the original Isua organic matter of- 15 to - 16%0. Based on analyses of organic matter produced by green non-sulphur bacteria preserved in modern laminated microbial mat systems resembling stromatolites, a biological alternative for the heavy organic matter in the Isua formation is suggested. The organic matter produced by these bacteria, representing an early divergence from the tree of life, is substantially enriched in 13C relative to organic matter assimilated via the Calvin cycle. Chloroflexus aurantiacus, such a green non-sulphur bacterium, uses the 3-hydroxypropionate pathway for carbon fixation (Holo and Sirevag, 1986). This pathway