Small scale wetland based sedimentary environmental change during the Holocene on the Konsen Plateau, eastern Hokkaido, Japan based on TOC and flowcytometry-pollen radiocarbon dating

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

1Atmosphere and Ocean Research Institute, The University of Tokyo, , Japan, 2Graduate School of Global Food Resources, Hokkaido University, , Japan, 3Graduate School of Agriculture, Hokkaido University, , Japan, 4Research Faculty of Agriculture, Hokkaido University, , Japan, 5International and Regional Studies, Hokkaido University of Education, , Japan, 6College of Humanities and Sciences, Nihon University, , Japan, 7GEOMAR Helmholtz Centre for Ocean Research Kiel, , Germany, 8Faculty of Horticulture, Chiba University, , Japan, 9The Field Science Center for Northern Biosphere, Hokkaido University, , Japan

Widespread peatland and wetland distribution is common in eastern Hokkaido, Japan. However, few studies of paleoenvironmental transition history in small alluvial lowland wetland systems located on the Konsen plateau have been conducted. A major obstacle is the reconstruction of a robust age model. Bulk radiocarbon dating, generally used for determining age of peat, must be employed with caution in selection of samples since peat and wetland sediments contain both plant remains at the time of sedimentation and various organic materials such as roots that are mixed from the layer above after sedimentation (Shinozaki, 2013). Radiocarbon dating on macrofossils and marker tephra provide accurate ages, but they cannot always be found from wetland sediments. On the other hand, pollen fossils show accurate radiocarbon dates (e.g. Brown et al., 1992). In this study, we measured radiocarbon from pollen fossils contained in a sediment core taken from a wetland of Chanai region on the Konsen Plateau, eastern Hokkaido, Japan. This study marks the first use of pollen to measure radiocarbon concentrations of peat by using flowcytometry, which allows for substantially more efficient pollen screening compared to conventional methods. To reconstruct paleoenvironments, we conducted high resolution elemental analysis on samples to interpret the origin of the wetland sediments and their decomposition rate.
Tephra stratigraphic analysis of the core indicates that the transition to a wetland setting occurred between ca. 7.5-2.5 ka. Bulk radiocarbon dates suggest that the core sample preserved mostly continuous sediments from about 5.0 ka to the present. Results from elemental analysis showed that Total Organic Carbon (TOC) fluctuated between 20 and 40% at 90 to 60 cm below surface (cmbs), and between 40 and 50% at 40 to 5 cmbs, with a peak of 53% at 36 cmbs. The resulted carbon and nitrogen (C/N) ratios were about 14-20 between 90 and 40 cmbs, and were about 25 in the upper 40 cm of the core, with a sharp rise around 40 cmbs. Since C/N ratios decrease with the decomposition of peat (Kondo et al., 1997), this sharp rise suggests the changes in sedimentary preservation status occurred around this depth. We will further examine the transition of sedimentary environment in the wetland together with the results of radiocarbon dating using pollen fossils purified by newly developed flowcytometry system.


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Date

Nov 08 - 19 2021