4:45 PM - 5:00 PM
[MIS02-10] Reconstruction of paleovegetation changes using plant terpenoids in sediments from Lake Suwa (central Japan) since the last glacial period
Keywords:biomarker, plant terpenoid, paleovegetation, Lake Suwa, mountain lake, the last glacier
Sediments collected from and around lake frequently record the change of various depositional environments such as floodplain and wetland depending on scaling of the lake water areas. Such various and unsteady sediments are important sources to interpret paleoclimatic and paleoenvironmental characteristics. Especially in inland area, the environmental conditions can vary sensitively with responses to climate changes. However, there have been few reports for paleoenvironmental and paleoclimatic reconstructions with the glacial-interglacial time scale in Japanese inland areas, especially sediments from lakes around mountain areas. Lake Suwa (Nagano Prefecture, central Japan), where is located in mountain area, has been changed the scales of the water areas, and the sediment core collected from the shore has various sedimentary facies. The authors have mainly reported the results for paleoenvironmental changes of the lake area associated with the scaling by biomarker analysis of the sediment core (Fukuchi et al., 2022). In this presentation, we will focus paleovegetational reconstructions using plant terpenoids in the sediment core from Lake Suwa, especially paleovegetational changes with the glacial-interglacial climate changes, comparing with previous pollen research.
In our study, we used a sediment core (ST2020) collected from the shore of Lake Suwa by boring during 2020. The ages were determined by AMS 14C dating, and the lowermost horizon was estimated to be about 2.7 cal yrs BP. The sediments (1-2 cm thick) were extracted by solvents and then separated into fractions by silicagel column. The apolar and polar fractions were analyzed by GC-MS. Sedimentary facies evaluated by sedimentary investigation indicated that the depositional environments such as floodplain, pond, lacustrine, and delta plain varied in the core ST2020 (Hatano et al., 2022).
In the ST2020 sediment samples, terrestrial plant-derived diterpenoids (DT; gymnosperm origin) and triterpenoids (TT; angiosperm origin) were mainly identified. We established the ratios of Sugiol/DT, Totarol/DT, Dehydroabietic acid (DAA)/DT, and Lupeol/TT (excluding β-amyrin and α-amyrin) as vegetation proxies. Considering the sugiol and totarol are mainly derived from the Taxodiaceae and Cupressaceae, the Sugiol/DT and Totarol/DT indicate the contributions of the Taxodiaceae and Cupressaceae to total gymnosperm vegetation. The DAA/DT ratio indicates mainly the Pinaceae contribution in the core, although the DAA is thought to be widely distributed among various gymnosperm, because the pollen records in the sediments from Lake Suwa suggested that Pinaceae was dominant in coniferous vegetation (Anma et al., 1990). The DAA/DT ratios were higher in the glacial period, but the Sugiol/DT and Totarol/DT ratios were higher in the deglacial stage. These results suggest that the vegetation around Lake Suwa converted from subalpine coniferous forest to temperate coniferous forest with deglacial warming. The lupane derivatives have been considered to be originated from Betulaceae, and also herbaceous plants. Thus, the Lupeol/TT ratios were lower in the glacial period, subsequently increased, and decrease during deglaciation. These trends are almost synchronous to the maximal peak of Betulaceae abundances based on the pollen records in bog in the Kirigamine highland, where is catchment of Lake Suwa (Yoshida et al., 2016), although these do not match the Betulaceae pollen records in Lake Suwa sediments (Anma et al., 1990). These results suggests that biomarker data had paleovegetational records at wider areas than pollen data. In addition, the lupeol proxy may reflect the contributions of angiosperm including herbaceous plants, and the transport and depositional processes were possibly different between the pollen and biomarkers.
In our study, we used a sediment core (ST2020) collected from the shore of Lake Suwa by boring during 2020. The ages were determined by AMS 14C dating, and the lowermost horizon was estimated to be about 2.7 cal yrs BP. The sediments (1-2 cm thick) were extracted by solvents and then separated into fractions by silicagel column. The apolar and polar fractions were analyzed by GC-MS. Sedimentary facies evaluated by sedimentary investigation indicated that the depositional environments such as floodplain, pond, lacustrine, and delta plain varied in the core ST2020 (Hatano et al., 2022).
In the ST2020 sediment samples, terrestrial plant-derived diterpenoids (DT; gymnosperm origin) and triterpenoids (TT; angiosperm origin) were mainly identified. We established the ratios of Sugiol/DT, Totarol/DT, Dehydroabietic acid (DAA)/DT, and Lupeol/TT (excluding β-amyrin and α-amyrin) as vegetation proxies. Considering the sugiol and totarol are mainly derived from the Taxodiaceae and Cupressaceae, the Sugiol/DT and Totarol/DT indicate the contributions of the Taxodiaceae and Cupressaceae to total gymnosperm vegetation. The DAA/DT ratio indicates mainly the Pinaceae contribution in the core, although the DAA is thought to be widely distributed among various gymnosperm, because the pollen records in the sediments from Lake Suwa suggested that Pinaceae was dominant in coniferous vegetation (Anma et al., 1990). The DAA/DT ratios were higher in the glacial period, but the Sugiol/DT and Totarol/DT ratios were higher in the deglacial stage. These results suggest that the vegetation around Lake Suwa converted from subalpine coniferous forest to temperate coniferous forest with deglacial warming. The lupane derivatives have been considered to be originated from Betulaceae, and also herbaceous plants. Thus, the Lupeol/TT ratios were lower in the glacial period, subsequently increased, and decrease during deglaciation. These trends are almost synchronous to the maximal peak of Betulaceae abundances based on the pollen records in bog in the Kirigamine highland, where is catchment of Lake Suwa (Yoshida et al., 2016), although these do not match the Betulaceae pollen records in Lake Suwa sediments (Anma et al., 1990). These results suggests that biomarker data had paleovegetational records at wider areas than pollen data. In addition, the lupeol proxy may reflect the contributions of angiosperm including herbaceous plants, and the transport and depositional processes were possibly different between the pollen and biomarkers.