[U-03] 人新世・第四紀の気候および水循環

[U03-P03] Radiocarbon dating of fossil pollen to estimate the carbon sequestration rate of peat

*常岡 廉^1,2、坂下 渉³、太田 耕輔⁴、宮入 陽介¹、近藤 玲介¹、横山 祐典^1,2,5,6,7 (1.東京大学 大気海洋研究所、2.東京大学 大学院理学系研究科 地球惑星科学専攻、3.国立研究開発法人 森林研究・整備機構 森林総合研究所、4.国立研究開発法人 産業技術総合研究所 活断層・火山研究部門、5.東京大学 大学院総合文化研究科 国際環境学教育機構、6.国立研究開発法人 海洋研究開発機構 生物地球化学プログラム、7.オーストラリア国立大学 物理学専攻)

キーワード：放射性炭素年代測定、花粉化石、フローサイトメトリー、泥炭、炭素固定

Peatlands play a critical role in mitigating climate change, as they are one of the largest carbon sinks among terrestrial ecosystems. A number of attempts have been made to determine the long-term carbon sequestration rate of peat using radiocarbon (¹⁴C) dating. However, the bulk peats generally contain various organic materials, which can make bulk ¹⁴C dating unreliable. ¹⁴C dating on macrofossils (e.g., leaves, twigs) provide accurate ages, but cannot always be found from peat sequences, and therefore cannot determine all ages at targeted depths. In this study, we focused on pollen fossils, which are generally ubiquitous in terrestrial sediments, and suitable for ¹⁴C dating. It is necessary to collect hundreds of thousands of pollen grains for ¹⁴C analysis, and conventional pollen extraction methods from sediment (i.e., physicochemical treatments, hand-picking) have certain drawbacks, such as the possibility of incomplete removal of impurities and excessively long processing times. In recent years, however, several studies have attempted to apply flow cytometry techniques to extract pollen fossils from lake sediments for ¹⁴C analysis. This technique allows for more efficient pollen screening compared to conventional methods. In this study, we hypothesized that flow cytometry can also be applied to peat soils. The objective of this study is 1) to verify the validity of the pollen fossil ¹⁴C ages obtained using flow cytometry and 2) to estimate the carbon sequestration rate based on those ages.
¹⁴C analyses of the pollen fossils were performed at four depths throughout a peat core obtained from a wetland in eastern Hokkaido. Comparisons between the tephra and pollen fossils ¹⁴C ages revealed that the ¹⁴C ages of pollen fossils are consistent with the age of tephra. Additionally, our results showed that the pollen fossils ¹⁴C ages were about 500-1200 years older than the bulk ¹⁴C ages of the adjacent layers. This offset between bulk and pollen ¹⁴C ages may be due to the contribution of younger ¹⁴C from overlying layers to the lower sediments (e.g., fine roots turnover). In this presentation, we will also report on the carbon sequestration rate calculated from the preliminary results of pollen fossils ¹⁴C ages.