Highly Negative δ13C Values in Organic Carbon in the Mt. Roe #2 Palaeosol: Terrestrial Life at 2.765 Ga?

Abstract

The ~13C value of organic carbon at the top of the Archaean Mt. Roe #2 palaeosol is l km. Near the sonthern end of the palaeosol exposure there is a thin (c. 50 cm) lens (c. 20 m x 20 m) of black material interposed between the normal white palaeosol and the overlying basalt. Petrographic microscopy and EPMA show that the white palaeosol layer and the black palaeosol lens both are nearly pure sericite with a small amount of quartz. In thin section the black palaeosol appears to be made up of palaeosol fragments c. 1 mm in diameter. The fragments have no preferred orientation, and probably represent the fragile upper surface of the soil. Some of the palaeosol fragments in the black palaeosol contain sheaves and layers of black material which have the optical properties of graphite. These sheaves and layers are reminiscent of algal/bacterial mat fragments. The 513C values of organic carbon in three samples from the black layer are -41.1%o, -40.8%o, and -40.3%0 PDB. These values are consistent with values found by Hayes (1994) for organic carbon in some late Archaean sediments and suggest that methanotrophic microorganisms colonized the Mt. Roe #2 palaeosol 2.765__.0.010 Ga. The methane these organisms needed could have been generated at the base of the mats during the bacterial degradation of organic matter. It might also have been derived from the ambient atmosphere. The isotopic and morphological observations presented above suggest that methanogens and methanotrophs lived on or near the surface of soils during the Archaean. If there was a methanotrophic