[H-QR06] 第四紀：ヒトと環境系の時系列ダイナミクス

[HQR06-05] Compound-specific ¹⁴C analyses of fatty acids as potential dating tools for lake sediments: A case study from Lake Kawaguchi, central Japan

★Invited Papers

*山本 真也¹、西澤 文勝²、吉本 充宏¹、宮入 陽介³、横山 祐典³、菅 寿美⁴、大河内 直彦⁴ (1.山梨県富士山科学研究所、2.神奈川県立生命と星・地球博物館、3.東京大学大気海洋研究所、4.海洋研究開発機構)

キーワード：湖底堆積物、放射性炭素年代法、脂肪酸

Lake sediments contain high-resolution records of Quaternary climate and environmental changes. Terrestrial plant remains are generally considered ideal ¹⁴C-dating materials in lake sediments; however, the scarcity of such plant materials often makes it difficult to provide robust age estimates, especially in settings where vegetation cover is limited. Total organic carbon (TOC) may be used as an alternative, but this often contains uncertainties that arise from the reservoir effect, source organic matter heterogeneities, and the contribution of re-deposited old organic matter. In this study, we performed compound-specific radiocarbon analyses (CSRA) of fatty acids over the last 3 ka from a sediment core (KA-1A) obtained from Lake Kawaguchi, located near Mount Fuji, central Japan, to assess their potential for dating sediment in lake environments.The ¹⁴C ages of the C₁₆, C₂₄, and C₂₆fatty acids isolated from the core gradually decreased with increasing core depth. However, the ¹⁴C ages of the same samples showed a maximum difference of 781 years between the compounds, suggesting source heterogeneity and a contribution from reworked organic materials. On the other hand, the ¹⁴C age of the C₁₆fatty acid (983 ± 56 yr BP) was almost identical to the lake reservoir age (937 ± 22 yr BP), suggesting that this compound was likely derived from primary producers (mainly diatoms) in the lake. To check the variability of the lake reservoir age, we compared the ¹⁴C ages of C₁₆fatty acid and that of plant (leaf) materials contained in the Kawagodaira tephra (Kg) layer at 396.5 to 401.5 cm depth. The differences between the ¹⁴C ages of the C₁₆fatty acid and the plant material was 1002 ± 73 yr BP, which is consistent with the modern reservoir age of the lake (983 ± 56 yr BP) within an error, suggesting that the lake reservoir age did not changed significantly over the last 3 ka. Based on these results, we estimated the depositional age of the Omuro scoria layer that is derived from the largest flank eruption at the base of Mount Fuji over the last 3 ka. The age estimate for the Omuro scoria layer, based on the ¹⁴C ages of the C₁₆fatty acid, is 2835 ± 35 cal BP, which is in good agreement with ages derived from plant remains in the Lake Kawaguchi core (2931 ± 15 cal BP) and in sediment from Lake Motosu (2935 ± 137 cal BP). These results indicate that, by correcting the lake reservoir ages, we can estimate depositional ages using the plankton-derived C₁₆fatty acid. This method could be useful to improve chronologies in lake environments where plant remains are limited.