Variations in paleovegetation recorded by terrestrial plant-derived biomarkers in sediments from the northern Japan Sea over the last 4.3 Ma

The objectives of our study are to reconstruct long time-scale variations in paleovegetation and transport of terrigenous material to the northern Japan Sea during the Pliocene to Pleistocene (over the past 4.3 Ma). Sedimentary plant remains such as spore/pollen and plant mega-/mesofossils are commonly used for reconstructing paleovegetation, and recently, plant-derived organic molecules including biomakers are also applied. In the present study, we analyzed terrestrial plant-derived biomarkers such as terpenoids, and compared between the data for plant biomarkers and spore/pollen in the same samples.

The sediment cores studied were recovered at the U1423 site in the eastern part of the Japan Basin by IODP Expedition 346. Triterpenoids such as amyron and friedelin, and diterpenoids such as sugiol and dehydroabietic acid (deAbiA) were mainly identified in all samples. The plant-derived terpenoid concentrations were found to increase from ~1.3 Ma. The sugiol is consistently abundant, although the remarkable decreasing spikes are observed during 3.5 - 3.3 Ma and 1.3 - 1.1 Ma. The relative abundances of ferrugiol is higher during 3.5 - 3.0 Ma and 1.3 - 0.6 Ma. The sugiol and ferrugiol are known to be derived from Taxodiaceae and Cupressaceae. Thus, these families were possibly abundant in Hokkaido and/or the Japan Sea side of northern Honshu Island over the 4.3 Ma. The relative abundances of deAbiA, which is derived from conifer, especially Pinaceae, frequently varied and several increasing peaks are observed for 4.3 Ma. Interestingly, the increasing peaks of deAbiA are found to be correlated to those of the relative abundances of Taxodiaceae estimated by pollen analysis. Such increasing peaks of pollen-based Taxodiaceae abundances are likely to be related to the Asian monsoonal climate such as warm and humid conditions. From these results, it is suggested that the indicator using deAbiA, rather than sugiol, is more sensitively respond against the monsoonal climatic changes. Gymnosperm / angiosperm ratios estimated by terpenoid / diterpenoid ratios increase during 3.5 - 1.8 Ma, which is concordant with results of conifer / broad-leaf wood ratios based on pollen assemblage. Thus, gymnosperm-dominant paleovegetation in land areas around the U1423 site might be distributed during the Pliocene to early Pleistocene.