The
hydrocarbons preserved in an Archean rock were extracted, and their composition and distribution in consecutive slices from the outside to the inside of the rock were examined. The 2.7 Ga rock was collected from the Fortescue Group in the Pilbara region, Western Australia. The
bitumen I (
solvent-extracted rock) and
bitumen II (
solvent-extracted
hydrochloric acid-treated rock) fractions have different
hydrocarbon compositions.
Bitumen I contains only trace amounts of aliphatic
hydrocarbons and virtually no
aromatic hydrocarbons. In contrast,
bitumen II contains abundant aliphatic and
aromatic hydrocarbons. The difference seems to reflect the weathering history and preservational environment of the investigated rock. Aliphatic
hydrocarbons in
bitumen I are considered to be mainly from later
hydrocarbon inputs, after initial deposition and burial, and are therefore not indigenous. The lack of
aromatic hydrocarbons in
bitumen I suggests a severe weathering environment since uplift and exposure of the rock at the Earth's surface in the Cenozoic. On the other hand, the high abundance of
aromatic hydrocarbons in
bitumen II suggests that
bitumen II
hydrocarbons have been physically isolated from removal by their encapsulation within
carbonate minerals. The richness of
aromatic hydrocarbons and the relative scarcity of aliphatic
hydrocarbons may reflect the original compositions of organic materials biosynthesised in ancient organisms in the Archean era, or the high thermal maturity of the rock. Cyanobacterial
biomarkers were observed in the surficial slices of the rock, which may indicate that endolithic cyanobacteria inhabited the surface outcrop. The distribution of aliphatic and
aromatic hydrocarbons implies a high thermal maturity, which is consistent with the lack of any specific
biomarkers, such as
hopanes and steranes, and the
prehnite-pumpellyite
facies metamorphic grade.