[MIS17-P12] Methane seep event recognized from microfossil assemblages of the Torigakubi Spur
Keywords:Torigakubi Spur, foraminifera, methane seep
Among them, unique benthic foraminiferal fossils (Rutherfordoides sp.) associated with methane concentration were confirmed with the abundance of suboxic species, from the core depth of 340 cm of PC 1706 collected in the eastern part of the chimney center on the methane sheep mound.The 14C age value of planktonic foraminiferal shells extracted from the same sample was estimated to be 23.28 ka.
Furthermore, over the upper 320 to 240 cmbsf, Rutherfordoides sp. continues to be distributed at a low frequency among poor number of benthic foraminifera assemblages characteristic of the Last Glacial Maximum (LGM) stage. These benthic foraminifera mostly have tiny specimens with a long diameter of less than 100μm. Rutherfordoides section recognized in PC 1706 corresponds to 24-20 ka from the 14C age and is consistent with the distribution of the cold diatom zone (C zone). Therefore, the methane seep area is estimated to spread by several hundred meters with the sea level drop on the survey chimney mound of the Torigakubi Spur. These occurrences are similar to the results of the Umitaka Spur where the relationship between negative anomalies with carbon isotope and Rutherfordoides species is discussed.
On the other hand, PC 1704 collected from the mound slope where crumbling cliffs are frequently observed in the periphery gave some different sedimentation ages from PC 1706 on the mound top. Especially, the sedimentation rate of the upper (younger) than 96 cmbsf where the 14C age (22.04 ka) was calculated is three times or more slower than the lower (older) one in PC 1704. Such an anomaly low sedimentation rate is expected to be due to the formation process of a steep slope accompanying the growth of hydrate mound during LGM.