FIGURE 2. Mineralogical and chemical characteristics of the 17-m palaeosol section at Schagen (see Methods for details).

The parental rock is carbonate-free serpentinite. The uppermost palaeosol (zone I) is overlain by the 2.6-Gyr Black Reef Formation. The solid bar in the ¹³C_org graph represents the ¹³C values of organic matter in shales from the Black Reef Formation at other localities; the open circle represents the ¹³C value of organic matter in a stromatolite from the Malmani subgroup at Schagen. ¹³C = [(¹³C/¹²C)_sample/(¹³C/¹²C)_standard] -1; where the standard is PDB (Belemnite from the Peedee Formation).