Geosteroid and its derivatives detected in Oligocene marine mammal bones from Hokkaido: Insights into early diagenesis of fossil bone

Abstract

Lipids are a ubiquitous class of biomolecules whose evolution and diagenesis in sedimentary rocks has been extensively studied, particularly from plant, bacterial and algal sources. However, the diagenetic pathways that operate on steroids from bone tissue have yet to be elucidated. In this study, we perform biomarker analysis on three marine mammal bone fossils from the Oligocene Morawan Formation (Ashoro, Hokkaido) - two desmostylian and one whale bone. We find that desmostylian bones yielded a spectrum of C27-C29 steranes and diasteranes, while the whale bone was distinctive in displaying purely cholestane. However, the whale bone displayed a remarkable preservation of several polycyclic aromatic hydrocarbons (PAHs) that were indicative of early diagenesis. These results indicate that while the cholesterol in desmostylian bones could possibly have had a microbial input and had mostly altered to PAHs, those in the whale bone were likely indigenous, still in early diagenesis, with diagenetic continuum preserved. The whale bone, thus, displays an as-yet undetermined preservation mechanism, or is possibly allochthonous to the locality of the desmostylians.