Abstract

Plant material that provides type III organic matter has an atomic O/C ratio of >0.6. On burial, at the beginning of diagenesis, it can be almost unaltered biochemically, or can be altered to varying degrees in peat; this can reduce the atomic O/C ratio to as low as 0.35. In the Bekapai-1 well, Kalimantan, the ratio for plant material, c. 0.6 at the time of burial, was not reduced to 0.35 until burial to a depth of c. 1400 m. The paleoenvironment of coalified plant material, and hence its probable initial atomic O/C ratio, need assessment before reaching conclusions on the significance of a particular ratio determined for a sedimentary sequence. “Peat” should not be defined by atomic O/C ratio or carbon content, but should be restricted to material with >75% (ash-free) in-ground moisture and/or buried <100 m. Huminite macerals in peat have a wide range of reflectance values, but reflectance data are lacking for dispersed huminite in modem surface marine sediments; this huminite is the precursor of vitrinite commonly used in estimation of diagenesis in marine sequences. Like atomic O/C ratios, the initial reflectance values may be influenced by the degree of alteration prior to burial, preventing unique interpretation of vitrinite reflectance values.