The Cretaceous is an interval of great interest for understanding the ecological evolution in terrestrial areas under hothouse condition. The reconstruction and ecological interpretation of the Cretaceous vegetation were performed by paleobotanical and palynological investigations, and recently, by organic geochemical analysis using terrestrial plant-derived biomarker. Records based on plant fossils and biomarkers are dominated by those of vascular plants, especially gymnosperm and angiosperm. In our works, we focus on the other members of terrestrial ecosystem, such as lichens and fungi to cover better scope of paleoecosystem on land. In addition, such organisms include pioneer species, which can grow in extreme environments. Hence retrieving paleontological information on such organisms has potential to piece out paleoecosystem reconstruction especially at the timing after severe environmental disturbance and paleoecology under extreme environments. Thus, we analyzed terrestrial biomarkers related to non-vascular plant, thallophyte, in sediments across the Cenomanian/Turonian boundary (CTB) of the Yezo Group distributed in the Tomamae area, northern Hokkaido, to reconstruct paleoenvironment and paleovegetation, and to examine the applicability for these indicators.
The CTB is known as an interval of the oceanic anoxic event (OAE) 2. In the study area, the OAE2 level reached about 700 m. Each phase of OAE2 was determined by δ13C of wood. By the sterane and hopane analyses, it shows immature of organic matter in the sediments. Perylene and dibenzofurans could be detected from all samples across the CTB. The perylene is thought to be mainly originated from terrestrial fungi. The dibenzofurans are known to be produced by plants, fungi and lichens, while the dibenzofurans having alkyl groups, especially at positions 1 and 9, are thought to be derived from lichens. Thus, the 1-methyl dibenzofuran ratio (1-MDBF ratio) was proposed as an indicator of lichen vegetation. The 1-MDBF ratios vary in the sediment across the CTB; the ratios increase during the 1st build up stage, consistently low values during the Trough, and tend to increase from the 2nd build up to Plateau stages. It is presumed that the variations are attributed to environmental and climatic changes in paleo-Hokkaido across the CTB.