[B-PT03] Biomineralization and Geochemistry of Proxies

[BPT03-04] Reconstruction of Late Cretaceous paleovegetation based on terrestrial palynomorphs and biomarker analysis of the forearc basin sediments from the Haborogawa Formation, Yezo Group, Hokkaido, Japan

*Maho Hayakawa¹, Masashi A. Ikeda¹, Ken Sawada², Reishi Takashima³, Hiroshi Nishi⁴, Hideto Nakamura² (1.Graduate School of Science, Hokkaido University, 2.Dpartment of Earth and Planetary Sciences, Faculty of Science, Hokkaido University, 3.The Center for Academic Resources and Archives, The Tohoku University Museum, Tohoku University, 4.Institute of Dinosaur Research, Fukui Prefectual University)

Keywords:paleovegetation, angiosperm, gymnosperm, palynomorph, northeast Asia, Yezo Group

The Late Cretaceous was important period in the evolutionary history of terrestrial ecosystems, marked by the transition from Mesozoic flora dominated by ferns and gymnosperms to Cenozoic flora characterized by diverse angiosperms. Assemblages of pollen and spore fossils gives clue for terrestrial paleoclimate and paleoenvironment, although such applications of palynological analysis have been mainly conducted in the Quaternary strata (Kurita et al., 1997), and some Cretaceous palynological studies have attempted to reconstruct paleovegetation and terrestrial paleoenvironments from palynological assemblages linking plant taxa at the family level to the characteristics of their habitats (Carvalho et al., 2019; Kujau et al., 2013). Compositions of palynological assemblages also suffer from differences in production and dispersal/floating distances, and biases due to the effects of fractionation during sedimentation (Kurita et al., 1997). Therefore, it is desirable to combine information of the depositional environment and other paleovegetation indicators to interpret paleovegetation (Creal et al., 2021). However, few studies have compared multiple paleovegetation proxies to reconstruct paleovegetation, and virtually none in the Cretaceous marine sediments. In this study, we present sporopollen records, kerogen compositions and biomarker indices from the Upper Cretaceous sediments from Yezo forearc basin to reconstruct paleovegetation in the hinterland at margin of northeastern Asia.
We analyzed Turonian–Santonian mudstones of the Haborogawa Formation, Yezo Group, sampled along the Kotanbetsu river sections of the Tomamae area, Hokkaido, Japan; where integrated bio- and carbon isotope stratigraphy have been well established (Takashima et al., 2019). Visual kerogen analysis was conducted by fluorescence microscopy of kerogen separated by acid treatment of the sediments. Pollen and spore fossils were identified based on comparison with literature (e.g. Takahashi, 1988; Takahashi and Sugiyama, 1991). Biomarker analysis was carried out by GC-MS analysis of aliphatic and aromatic lipid fraction of extractable organic matter (OM).
Visual kerogen analysis revealed that plant-derived OM generally accounted for more than ca. 30% of kerogen and its contribution increased towards the top of the section; indicating significant input of terrigenous OM throughout the late Cretaceous. Palynological analysis found fern spores, pollens of coniferous gymnosperms (Cupressaceae, Araucariaceae, Cheirolepidiaceae, Pinaceae, Podocarpaceae), non-conifer gymnosperms (Ephedraceae, Cycadaceae and seed fern), and angiosperms. Gymnosperm pollens were dominant in all samples, accounting for 68–80 % of the assemblages, and represented by Pinaceae and Cupressaceae pollens and Araucariaceae, Cycadaceae and seed fern pollen grains were scarce. Angiosperm pollens were characterized by low abundances (0–19 %). Comparing this result with Turonian-Santonian sporopollen records from other regions, we infer that there was significant extent of coniferous vegetation in the hinterland of the Yezo Group potentially due to relatively humid environment during the Late Cretaceous. Based on the lithology and biomarker indices, changes in sedimentary environments are not likely to be the primary factor for the variability of the sporopollen records observed in studied samples. Therefore, stratigraphic variation in sporopollen records was considered to reflect changes in vegetation. Biomarker paleovegetation indices HPP (Higher Plant Parameter; van Aarssen et al., 2000) and ar-AGI (aromatic Angiosperm/Gymnosperm Index; Nakamura et al., 2010) were compared with sporopollen compositions. Despite varying degrees of differences are obvious between sporopollen records and biomarker indices, both proxies considered to represent paleovegetation in the hinterland of the Late Cretaceous forearc basin and suggested to have sensitivity to detect large-scale vegetation change.