Abstract
Tertiary sedimentary sequences containing the Paleogene/Neogene boundary are well exposed in Sakhalin, Russia. Sequences of Oligocene to Miocene mudstones from Korsakov, Makarov, and Shmidt Peninsula areas were studied to reveal the palaeoceanographic environment at the time of the Paleogene/Neogene transition. Late Oligocene rocksare characterized by siliceous mudstones. The SiO2 contents and total organic carbon concentra tions range from 70 to 95% and from 0.5 to 2.7%, respectively. Siliceous rocks from the Shmidt Peninsula are rich in organic carbon, and were deposited in an anoxic environment as indicated by the abundance of C35 homohopane, and low C/S ratio. Open system pyrolysis and pyrolysis gas chromatography show that siliceous rocks from the Shmidt Peninsula are rich in typical marine Type II kerogen with hydrogen indices ranging from 250 to 400mgHC/gC. The siliceous rocks from the Makarov section are characterized by low hydrogen indices of less than 200mgHC/gC. Their maximum temperatures experienced during burial diagenesis are estimated to be less than 100°C based on silica mineral transformation and biomarker isomer ratios.
Late Oligocene siliceous mudstones are rich in biomarkers derived from diatoms and dinoflagellates, showing enhanced primary productivity of the Late Oligocene ocean in the Sakhalin region. Invasion of nutrient-rich proto-AABW (Antarctic Bottom Water) and its upwelling might be responsible for the high primary production in the Late Oligocene. Benthic water in the Paleogene / Neogene transition was abruptly oxidized, which is attributable to a decline of proto-AABW. This palaeoceanographic event seems to be related to global warming in the Paleogene/Neogene transition.
