[B-PT02] Biotic History

[BPT02-01] Reconstruction of paleovegetation changes by pollen, palynomorph, and kerogen analyses of the Late Cretaceous sediments from the Kotanbetsu area, Hokkaido, Japan

*Maho Hayakawa¹, Ken Sawada², Masashi A. Ikeda¹, Reishi Takashima³, Hiroshi Nishi⁴ (1.Graduate School of Science, Hokkaido University, 2.Department 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 Prefectural University)

Keywords:paleovegetation, Yezo Group, pollen, palynomorph, Cretaceous

The early Late Cretaceous (–Turonian) was characterized by one of the warmest paleoclimates, and subsequently the middle - late Late Cretaceous (Coniacian–Maastrichtian) is considered an interval of global cooling, possibly attributed by declining atmospheric CO₂concentration (e.g. Tabor et al., 2016). In the Cretaceous cooling condition, it is important to clarify the variation and evolution of paleovegetation adding angiosperm flora for understanding terrestrial paleoenvironment and paleoclimate. However, there have few reports for reconstructing long-time scale variation in paleovegetation during the Cretaceous. In the present study, we performed analyses of palynomorph including pollen and palynofacies (visual kerogen) of the sediments from the Upper Yezo Group (Hokkaido, Japan) to reconstruct the long-time scale variations in paleovegetation during the Late Cretaceous.

The mudstones of Saku Formation and Haborogawa Formation, Yezo Group, were sampled in the Kotanbetsu area, Hokkaido, Japan. In the Cretaceous Yezo Group, integrated bio- and carbon isotope stratigraphy have been well established in the studied section. We analyzed the Turonian–Santonian mudstone samples. We performed the treatments for separating pollen from sediments by 1) the common method for the Quaternary sediments, and 2) kerogen separation method. The palynomorphs including pollen and spore as well as kerogen were observed under transmitted and fluorescent light microscopes.

In the Coniacian–Santonian sediments, we identified trilete (fern origin), bisaccate (gymnosperm, especially Pinaceae), non-saccate gymnosperm pollen, and polyplicate (gymnosperm Ephedraceae). As angiosperm pollen, tricolpate, triporate, and tricolporate were ideintified. Interestingly, we found triprojectate, which is characteristically observed in the Late Cretaceous sediments from the Asia and North America. The angiosperm pollen/bisaccate ratios tended to decrease during the Coniacian, and subsequently increase during the Santonian. The bisaccate pollens are the most abundant, and could be divided to several types based on their sizes. The compositions of these bisaccate types varied, suggesting that the variations in coniferous vegetations can be reconstructed by using these bisaccate pollens. In addition, dinocysts were commonly observed. Microscopic observation of kerogen (palynofacies analysis) from sediments in the sediments from the Coniacian–Santonian Haborogawa Foramtion indicates that the kerogens mainly compose of wood and amorphous organic matter (AOM) such as fluorescent AOM (FA; plant cuticle origin) and non-fluorescent AOM (NFA; woody tissue origin), as well as sporomorphs (spore and pollen) and cuticle as minor component. The (Cuticle+FA)/(Wood+NFA) ratios, which is used as the proxy for herbaceous / woody vegetation ratios, tended to decrease from the early to middle Santonian, and slightly increased during late Santonian. However, the kerogen compositions by palynofacies analysis is thought to vary depending on sedimentological processes such as transport from land area, so that the sedimentological factors should be paid attention in the paleovegetation reconstruction by kerogen composition.