[H-QR04] Quaternary, Diachronic dynamics of human-environment interactions

[HQR04-P06] High-resolution dating of surface peat based on ¹³⁷Cs and ²¹⁰Pb radioactivity concentration

*Ren Tsuneoka¹, Yusuke Yokoyama¹, Kosuke Ota¹, Yosuke Miyairi¹, Reisuke Kondo¹, Minoru Yokochi², Kazuhiro Kaneko³, Takashi Inoue⁴, Norio Kito⁵, Kyota Uemura⁶, Mari Sumita⁷, Arata Momohara⁸, Hiroko Fujita⁹ (1.Atmosphere and Ocean Research Institute, The University of Tokyo, 2.Graduate School of Global Food Resources, Hokkaido University, 3.Graduate School of Agriculture, Hokkaido University, 4.Research Faculty of Agriculture, Hokkaido University, 5.International and Regional Studies, Hokkaido University of Education, 6.College of Humanities and Sciences, Nihon University, 7.GEOMAR Helmholtz Centre for Ocean Research Kiel, 8.Faculty of Horticulture, Chiba University, 9.The Field Science Center for Northern Biosphere, Hokkaido University)

Keywords:wetland, peat, Cs-137 dating, Pb-210 dating, γ-ray detector, carbon reservoir

Wetlands are one of the largest carbon reservoirs in terrestrial ecosystems (e.g. Yu et al., 2010; Loisel et al., 2014). The carbon sequestration of peat is mainly controlled by the degree of decomposition, which is climate-dependent especially in subarctic wetlands where the low temperature inhibits the decomposition of organic materials. However, recent studies have shown that global warming will enhance peat decomposition and consequently reduce the carbon storage in wetlands (e.g. Wilson et al., 2021). To estimate the effect of climate change on peat decomposition and its capacity as a carbon reservoir, high-resolution chronologies of wetland sediments are required.
Tephrochronology and radiocarbon (¹⁴C) dating have often been used to estimate the age of sedimentation of peat. However, indicator tephras are not always found in the surface layers of sediments. ¹⁴C dating method is difficult to apply on the topmost sediment layers, which are likely to contain ¹⁴C from nuclear testing around 1950-1960. Topsoils have shown increased disturbances by human influence during the Anthropocene (Crutzen, 2006), which renders them unusable. Although the beginning of the Anthropocene is still controversial and has not been formally defined as a geological age unit, human activities have clearly affected the Earth's surface environment as exemplified by the rapid increase in carbon dioxide concentration and the release of anthropogenic radioactive materials since the mid-20th century (e.g., Aoyama and Hirose, 2004). In order to understand the influence of human activities on the sedimentary environment of wetlands, it is necessary to obtain high-resolution chronological information on the scale of several years to several decades. In this study, we used cesium-137 (¹³⁷Cs) and lead-210 (²¹⁰Pb) to date the topmost wetland sediment samples. These methods can estimate sediments ages and can be applied in areas with relatively high sedimentation rates that are difficult to analyze by ¹⁴C dating. In this study, we measured the radioactivity concentrations of ¹³⁷Cs and ²¹⁰Pb in the topmost wetland sediment samples, and attempted to clarify the sedimentation and development process of wetland sediments by high resolution in the past several decades.
The sedimentation rate of surface peat can be estimated from the depth of the peak ¹³⁷Cs radioactivity concentration which corresponds to the time of global fallout. More detailed sedimentation rates can be calculated from the decay trend of ²¹⁰Pb. The sedimentation rate at the surface layer was calculated to be ~0.18 cm/yr. The low ²¹⁰Pb concentration at 0-5 cmbs (centimeters below surface) is estimated to be due to the contribution of mainly of Sphagnum remains which contain few soil particles that adsorb ²¹⁰Pb within the stratum. The sedimentation age of this area was calculated from the ²¹⁰Pb concentration excluding this surface layer and the sedimentation rate estimated from ¹³⁷Cs peak. The result of this estimation implied that the sediment in the surface 12 cm is about 65 years old. In addition, it takes about 30 years for the plant remains to be converted to soil in the study area, based on the changes in facies from Sphagnum remains to decomposed peat.