9:45 AM - 10:00 AM
[BCG07-04] Paleovegetation changes reconstructed by terrestrial plant biomarker analyses in the sediments deposited across the Cenomanian/Turonian boundary in the Tomamae area, Hokkaido, Japan
Keywords:Paleovegetation, lichen, biomarker, Yezo Group, C/T boundary, Cretaceous
We analyzed sedimentary rocks across the CTB in the Saku Formation,Yezo Group, were collected from the Omagari-zawa sections of the Tomamae area, northern Hokkaido, Japan. The sediments used correspond to the OAE2 intervals (1st build-up, Trough, 2nd build-up and Plateau phases), determined by δ13C stratigraphy (Omatsu et al., 2014). For the biomarker analysis, the extractions of freeze-dried sediments were fractionated using silica-gel column and analyzed by GC-MS.
By sterane and hopane ratios, we confirmed the high contribution of terrigenous matter input and low maturity of organic matter (vitrinite reflectance ca. 0.4% level; lignite to subbituminous coal rank) in the Omagari-zawa section. These results agree with those reported by Ando et al. (2017). Higher Plant Parameter (HPP) as the vegetation index of higher plants increased in the 1st build-up and 2nd build-up stages, and declined in the Trough and Plateau stages. We interpret that the coniferous vegetation has expanded under arid condition. Moreover, we analyzed the proportion of 1-methyl dibenzofuran in total methyl dibenzofurans as a lichen indicator (1- MDBF ratio), and perylene/ pyrene ratio as a fungal assemblage index. Variations in lichen and fungi vegetation tend to decrease from the 1st build-up to the 2nd build-up stage, which are resemble to those of coniferous vegetation. Thus, it is considered that the vegetation overall declined against the mid-Cretaceous environmental disturbance, especially affected by arid/humid fluctuation.