Abnormally abundant alkenone-derived C37 and C38 n-alkanes in Miocene Onnagawa siliceous mudstones, northeast Japan

Abnormally abundant n-C37 and n-C38 alkanes, together with a significant amount of the n-C39 alkane, were found in the Miocene Onnagawa siliceous mudstones. The relative abundance of these alkanes resembles that of ΣC37, ΣC38 and ΣC39 alkenones from Emiliania huxleyi and Gephyrocapsa oceanica. The stable carbon isotope ratio of the n-C37 alkane is −23.5‰, supporting their planktonic origin. The n-C37-C39 alkanes are likely to be derived from alkenones produced by the family Gephyrocapsaceae during the Middle to Late Miocene period. Total organic carbon concentration of the samples rich in the alkenone-derived n-alkanes is in the range of 1.5–2.3%, and the samples are characterized by a low pristane/phytane ratio and a high homohopane index, suggesting anoxic depositional conditions. These sediments are relatively immature (Ro=0.33–0.45%), suggesting that the generation of the long-chain n-alkanes from alkenones took place during early diagenesis. The mudstones rich in n-C37-C39 alkanes are generally poor in CaO (<1.9%) and rich in SiO2 (77.9–85.3%). There is no relationship between CaO concentration and abundance of the n-C37-C39 alkanes. Since the Onnagawa siliceous mudstones were formed in a deep-sea environment, the low concentration of CaO is most probably due to the removal of calcareous skeletons by dissolution during deposition below the carbonate compensation depth. These alkenone-derived n-alkanes are useful biomarkers for detecting contributions of calcareous nannoplankton to deep-sea sediments.