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Supplementary Document Background JAEA has conducted research on long-term geomorphic development due to uplift/denudation and climate/sea-level changes, and research on the effects of the geomorphic changes on the geological environment (such as groundwater flow), as part of the research for geological disposal of HLW. The purpose of this research is to develop a method for precise paleoclimate reconstruction in inland areas of Japan. Outline of the research Climatic change during the past several hundred thousand years is characterized by repetition of glacial-interglacial cycles. This global climatic change was determined by analysis of δ18O (oxygen Isotope ratios; see glossary 1) from deep-sea sediments. Around Japan, climatic change during the past several hundred thousand years was reconstructed based on analysis of deep-sea sediments from the Japan Sea. However, in inland areas of Japan, long-term (more than several hundred thousand years) continuous reconstruction of paleoclimates has rarely been conducted, although many studies for reconstruction of paleoclimate during the last several tens of thousands of years were conducted using lacustrine sediments. For this study, JAEA produced continuous stratigraphic sections by drilling cored boreholes through the sediments of the Okute Basin, Mizunami City, Gifu Prefecture (Figure 1), and reconstructed the paleoclimate (temperature and precipitation) during the past 300,000 years based on pollen species and their compositional ratios in the pollen records. For continuous reconstruction of paleoclimates over periods of several hundred thousand years, a quiescent depositional environment where fine sediments (sand or mud) could accumulate and remain undisturbed is required. Such environments are rare in Japan, where intense crustal movement and severe fluvial erosion is the norm. However, because the Okute Basin is elevated, and has a peculiar landform, where there is almost no river inflow (Figure 1), the Basin formed in a quiescent environment and is therefore ideal for the study. JAEA conducted sample borings at several points in the Okute Basin. Because fine sediments have accumulated almost continuously during the past 300,000 years in the basin, the Okute Basin is the best place for the reconstruction of paleoclimate and the confirmation of the agreement with global climate change. Samples of the basin sediments were collected from the core every 30 cm. Then paleoprecipitation and paleotemperatures were estimated by the Modern Analogue Technique (see glossary 2), using pollen species and compositional ratios in the pollen records. Figure 2 shows a photograph and the geological column of the core and paleoprecipitation estimated from the pollen records. Figure 3 shows pollen composition in the Okute Basin core and paleotemperatures reconstructed by pollen analysis. The age of each stratum can be estimated based on the age of tephras in the stratum. Reconstructed paleotemperature of the Okute Basin is almost consistent with global climate change (change of δ18O) reconstructed from analysis of deep-sea sediments (Figure 3). This fact indicates that climate change in the Tono area (Central Japan) is linked with global climate change. Tree pollen was used in this reconstruction because the tree pollen composition is representative of vegetation from a larger area than pollen from grasses, and therefore the reconstructed paleoclimate is considered to be representative of paleoclimate for a relatively large area such as Tono. Considering the details of paleotemperature change precisely, the estimated paleotemperature change around 50,000 years ago was more intense than the global change at that time. This difference might indicate local change in paleotemperature in the Okute Basin area. The reconstructed paleoprecipitation change has a tendency to decrease in glacial (cool) periods and to increase in interglacial (warm) periods. However, the relationship between paleoprecipitation change and δ18O change in these records is unclear. Change of precipitation in Japan may be affected not only by global climate change but also by local factors, such as cessation of the Tsushima Warm Current in the Japan Sea. A topic for future research is to determine the cause of paleoprecipitation changes.  Figure 1. Location of the study area (Okute Basin, Mizunami City, Gifu Prefecture). Air photograph issued by Geographical Survey Institute is used.  Figure 2. Photograph and geological column of core and reconstructed paleoprecipitation Modified from Sasaki et al. (2006), Quaternary Research (Daiyonki-Kenkyu), Vol. 45.  Figure 3. Pollen composition in the Okute Basin core and paleotemperature reconstructed by pollen analysis. (a) Percentage pollen diagram of the Okute core, (b) Paleotemperature reconstructed by modern analogue technique (left) and global climate change (right). Modified from Sasaki et al. (2006), Quaternary Research (Daiyonki-Kenkyu), Vol. 45. Future research Reconstruction of paleo-landforms and the geomorphic development is planned to evaluate the effect of climatic changes on landscape morphology. - BACK - |
