Carbon isotope signatures of bacterial 28-norhopanoic acids in Miocene-Pliocene diatomaceous and phosphatic sediments

A series of C29-C31 28-norhopanoic acids occur in Neogene marine sediments of the Onnagawa, Funakawa and Tentokuji Formations, northeastern Japan, and in the Monterey Formation of California, USA. The 28-norhopanoic acids exist mainly as free acids but are partly bound to kerogen and the polar fraction of the solvent extract via an ester bond. The 13C depletions of C29 and C31 28-norhopanoic acids compared with those of C30 and C32 regular hopanoic acids indicate that these 28-demethylated and regular hopanoic acids are derived from different precursor organisms. The Upper Onnagawa Formation contains abundant 28-norhopanoic acids and is characterized by the occurrence of Calyptogena sp. (Bivalvia), which lives in a specific chemoautrophic ecosystem that utilizes chemical substrates from hydrothermal vents or cold seeps. The depletion of 28-norhopanoic acids in 13C compared with kerogen suggests that the 28-norhopanoic acids have been derived from bacteria that utilized 13C-depleted CO2 as their carbon source.