樹木年輪の酸素同位体を用いた過去1000年間の日本の降水量復元に関する研究

学位の種別: 課程博士審査委員会委員 : （主査）東京大学准教授 岡 顕, 東京大学教授 多田 隆治, 東京大学准教授 芳村 圭, 東京大学教授 植松 光夫, 東京大学教授 横山 祐典University of Tokyo(東京大学

Radiocesium mobility in different parts of the two major tree species in Fukushima

Abstract Radiocesium (137Cs) released in the Fukushima Dai-ichi Nuclear Power Plant accident is still cycling in the forest ecosystem. We examined the mobility of 137Cs in the external parts—leaves/needles, branches, and bark—of the two major tree species in Fukushima, Japanese cedar (Cryptomeria japonica) and konara oak (Quercus serrata). This variable mobility will likely lead to spatial heterogeneity of 137Cs and difficulty in predicting its dynamics for decades. We conducted leaching experiments on these samples by using ultrapure water and ammonium acetate. In Japanese cedar, the 137Cs percentage leached from current-year needles was 26–45% (ultrapure water) and 27–60% (ammonium acetate)—similar to those from old needles and branches. In konara oak, the 137Cs percentage leached from leaves was 47–72% (ultrapure water) and 70–100% (ammonium acetate)—comparable to those from current-year and old branches. Relatively poor 137Cs mobility was observed in the outer bark of Japanese cedar and in organic layer samples from both species. Comparison of the results from corresponding parts revealed greater 137Cs mobility in konara oak than in Japanese cedar. We suggest that more active cycling of 137Cs occurs in konara oak

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Radiocarbon dating of wetland sediment from the Konsen Plateau, eastern Hokkaido, Japan

Highlights • Study of wetland sediment in the easternmost Asian monsoon affected region. • Sediment on the Konsen Plateau has continuously deposited since ∼4.0 ka. • C/N and C/S ratios suggest environmental changes during the late Holocene. Abstract Wetland soils are among the largest carbon reservoirs in terrestrial ecosystems and are vulnerable to climate change. Here we elucidate the developmental history of a wetland soil located on the Konsen Plateau in Hokkaido, Japan, during the Holocene. Through radiocarbon dating of sediment samples, we revealed that continuous sedimentation persisted over the past 4000 years. This onset of peat sedimentation was simultaneous with those reported in other wetlands in eastern Hokkaido. We hypothesized that cooling associated with the end of climatic optimal may have contributed to peat preservation. Elemental analyses also confirmed the formation of a stable wetland condition during the late Holocene. The C/N ratios were approximately 15–25 throughout the core, indicating that the organic materials in the sediments were steadily derived from terrestrial plants. The C/S ratios of the core varied between 43 and 97, suggesting that the sediments are free from marine influences. Our results provided continuous and stable record of wetland sedimentary environment in eastern Hokkaido over the past 4000 years