Diatom biomarkers during the Eocene/Oligocene transition in the Ilʹpinskii Peninsula, Kamchatka, Russia

Marine strata of the Ilʹpinskii section, northeastern Kamchatka, Russia, expose the complete Eocene/Oligocene (E/O) succession. Although diatom fossils are very poorly preserved in these mudstones and carbonate concretions, Oligocene mudstones contain typical diatom biomarkers, the C25 highly branched isoprenoids (HBIs), in concentrations that rise markedly above the E/O boundary. The abundance of the dinoflagellate biomarkers known as dinosteranes decreases through the late Eocene to Oligocene interval. A rise in the abundance of HBIs through this time interval suggests that the post-E/O diatom community of the northwest Pacific region was dominated by the genus Rhizosolenia. HBIs in sedimentary rocks spanning the E/O transition may show the appearance of the Oligocene marine diatom Rhizosolenia oligocaenica, and may therefore be useful as biomarkers in regional biostratigraphic correlation of the northwest Pacific during this interval. An E/O boundary in the Ilʹpinskii section is proposed based on HBI distribution.