9:30 AM - 9:45 AM
[MIS18-13] Reconstruction of paleoenvironmental changes in inland area by biomarker analysis of sediments from Lake Suwa (Japan) since the last glacial period
Keywords:Paleovegetation, Biomarker, Lake Suwa, the Last Glacial period, Lakes around mountain areas
The climate and vegetation in inland area of Japan are likely to fluctuate drastically affected by rapid cooling/warming during the Last Glacial period including deglacial stage and the Younger Dryas event, and the Holocene. However, there have been few reports for paleoenvironmental and paleoclimatic reconstruction with the glacial-interglacial time scale in Japanese inland areas, especially sediments from lakes around mountain areas, although the glacial-interglacial scale paleoenvironmental researches have been extensively performed in marine sediment and ice cores. In the present study, we analyzed biomarkers in a sediment core from Lake Suwa (Nagano Prefecture) to reconstruct the paleoenvironmental variations in inland area around central Japan, and examine the applicability of biomarker proxies in inland areas by the preliminary results.
In our study, we used a sediment core (ST2020) collected from the coast of Lake Suwa by boring during 2020. The ages were determined by 14C dating, and the lowermost horizon was estimated to be about 2.7 cal yrs BP. Biomarker analysis was performed as follows; freeze-dried sediments were extracted by solvents and then separated into fractions by silica-gel column chromatography. Resulted apolar and polar fractions were analyzed by GC-MS. Sedimentary facies evaluated by sedimentary investigation indicated that the depositional environments such as lacustrine, pond, and floodplain varied in the core ST2020 (Hatano et al., 2021).
In the ST2020 sediment samples, n-alkanes, hopanoids, and steroids were mainly identified as biomarkers. We analyzed carbon preference index (CPI), average chain length (ACL), and aquatic plant index (Paq) as alkane proxies, as well as ββ-hopane ratio, C27/C29 sterol ratios, and stanol/sterol ratios (Sta/Ste), as hopanoid and steroid proxies in the core. The CPI and ββ-hopane ratio, which are diagenetic indicators in modern sediment, showed reverse trends; the CPI results suggest that the input of diagenetic compounds increase in lacustrine layers, but the ββ-hopane results showed fresh biological compounds in most of layers.The discrepancy may be caused by the differences between n-alkane (terrestrial plant leaf wax) and hopanoid (bacteria). Also, in the high CPI layers, n-alkane concentrations clearly increase, suggesting that large amount of older and more diagenetic materials were transported from land area and deposited. Paleovegetation reconstructed by ACL indicated that grass vegetation was dominant in Last Glacial Period, forest might expand during deglaciation, and grass were dominant again during the Holecene. The general trends of paleovegetation are similar to those estimated by pollen analysis around Lake Suwa as reported by Anma et al. (1990). Redox conditions were evaluated by C27-C29 Sta/Ste ratios, and remarkable increasing spike were observed at ca 20,000 and ca 9,000-12,000 cal yr BP. It is postulated that these spikes suggest higher contribution of the reduced materials such as older soils from land areas into the core site.
In our study, we used a sediment core (ST2020) collected from the coast of Lake Suwa by boring during 2020. The ages were determined by 14C dating, and the lowermost horizon was estimated to be about 2.7 cal yrs BP. Biomarker analysis was performed as follows; freeze-dried sediments were extracted by solvents and then separated into fractions by silica-gel column chromatography. Resulted apolar and polar fractions were analyzed by GC-MS. Sedimentary facies evaluated by sedimentary investigation indicated that the depositional environments such as lacustrine, pond, and floodplain varied in the core ST2020 (Hatano et al., 2021).
In the ST2020 sediment samples, n-alkanes, hopanoids, and steroids were mainly identified as biomarkers. We analyzed carbon preference index (CPI), average chain length (ACL), and aquatic plant index (Paq) as alkane proxies, as well as ββ-hopane ratio, C27/C29 sterol ratios, and stanol/sterol ratios (Sta/Ste), as hopanoid and steroid proxies in the core. The CPI and ββ-hopane ratio, which are diagenetic indicators in modern sediment, showed reverse trends; the CPI results suggest that the input of diagenetic compounds increase in lacustrine layers, but the ββ-hopane results showed fresh biological compounds in most of layers.The discrepancy may be caused by the differences between n-alkane (terrestrial plant leaf wax) and hopanoid (bacteria). Also, in the high CPI layers, n-alkane concentrations clearly increase, suggesting that large amount of older and more diagenetic materials were transported from land area and deposited. Paleovegetation reconstructed by ACL indicated that grass vegetation was dominant in Last Glacial Period, forest might expand during deglaciation, and grass were dominant again during the Holecene. The general trends of paleovegetation are similar to those estimated by pollen analysis around Lake Suwa as reported by Anma et al. (1990). Redox conditions were evaluated by C27-C29 Sta/Ste ratios, and remarkable increasing spike were observed at ca 20,000 and ca 9,000-12,000 cal yr BP. It is postulated that these spikes suggest higher contribution of the reduced materials such as older soils from land areas into the core site.