17:15 〜 18:30
[HCG28-P04] 諏訪盆地の上部更新統~完新統に記録された堆積環境と湖水位の変動
キーワード：更新世末～完新世、堆積相、古土壌、湖水位、湖沼堆積物、諏訪湖
Introduction: The Suwa Basin is an inland basin located at a high altitude in central Japan. The sedimentation processes and sediment production in such a basin are regulated by a balance between the paleoclimate and tectonics (e.g., Shanley and McCabe, 1994; Miall, 1996). Furthermore, this basin, which has long daylight hours, can be expected to have archived paleoclimatic conditions. In this study, the sedimentary facies analysis, paleosol description, major elemental analysis, and CN elemental analysis were conducted for the late Pleistocene and Holocene boring core obtained at a site on the southern coast of Lake Suwa, to clarify the sedimentary processes occurring there and fluctuations in the lake water level.
Sedimentary facies and paleosols: The succession in the core can be divided into three sedimentological units based on the sedimentary facies, paleosols, TOC, TN, and C/N ratios. In ascending order, they are as follows: meandering fluvial, lacustrine, and delta units. The meandering fluvial unit is composed of inversely graded sand beds (crevasse splay deposits), alternating beds between sand and carbonaceous mud (fluvial and back-swamp deposits), and normally graded gravel–sand beds (channel deposits). The C/N ratios of this unit are relatively higher (10.25 on average) than those of the other units. Some paleosols are developed in this unit. These paleosols can be recognized by fine rootlet fossils, fecal pellets, illuviated clay, and brownish-reddish soil colors. Some of these paleosols underwent groundwater and surface-water gleizations. The lacustrine unit is composed of carbonaceous mud beds (pond deposits) and silt-clay beds (lake deposits) and has a low C/N ratios (8.19 on average). In this unit, some paleosols can be recognized by fine rootlet fossils and illuviated clay. The paleosol and lacustrine mud bed which has a high content of SiO2 crowded with diatoms alternated between the depths of 14.5 and 18.0 m. The delta unit is dominated by gravel–sand and exhibits upward-coarsening successions.
Discussion: The accumulation curve constructed based on the calendar dates calculated from the 14C ages provides the period of environmental changes in the Suwa Basin. Meandering fluvial depositions were laid from the last glacial maximum to ca. 15,000 cal years BP. The lacustrine (pond and marsh) environment expanded onto the study site from ca. 15,000 cal years BP. The primary productivity in the lake is considered to have been higher during the period ca. 12,000–6,000 cal years BP, based on the lower and static C/N ratios and higher TN. Delta progradation occurred at ca. 4,000–3,000 cal years BP.
The repetition of paleosols and diatom-rich mud beds in the lacustrine unit suggests short-term fluctuations in the lake-level during the period between 12,000–7,000 cal years BP. The lake-level underwent declines in the period ca. 12,000–11,000 cal years BP and ca. 9,000–8,200 cal years BP. The high lake-level developing during the period ca. 7,000–4,000 cal years BP coincides with the middle Holocene climatic optimum (e.g., Wanner et al., 2008). These fluctuations of the sedimentary environments and the lake-level could have been caused by climate change and/or basement subsidence.
This study was carried out under an ordinary research project (Research on the medium to long term management and maintenance of lakes, ponds, and their catchment areas in Nagano Prefecture) at the Nagano Environmental Conservation Research Institute.
References: Miall, A.D., 1996. Springer-Verlag, Berlin Heidelberg, p. 598. Shanley, K.W., McCabe, P.J., 1994. Am. Assoc. Petrol. Geol. Bull. 78, 544–568. Wanner, H. et al., 2008. Quat. Sci. Rev., 27, 1791–1828.
Sedimentary facies and paleosols: The succession in the core can be divided into three sedimentological units based on the sedimentary facies, paleosols, TOC, TN, and C/N ratios. In ascending order, they are as follows: meandering fluvial, lacustrine, and delta units. The meandering fluvial unit is composed of inversely graded sand beds (crevasse splay deposits), alternating beds between sand and carbonaceous mud (fluvial and back-swamp deposits), and normally graded gravel–sand beds (channel deposits). The C/N ratios of this unit are relatively higher (10.25 on average) than those of the other units. Some paleosols are developed in this unit. These paleosols can be recognized by fine rootlet fossils, fecal pellets, illuviated clay, and brownish-reddish soil colors. Some of these paleosols underwent groundwater and surface-water gleizations. The lacustrine unit is composed of carbonaceous mud beds (pond deposits) and silt-clay beds (lake deposits) and has a low C/N ratios (8.19 on average). In this unit, some paleosols can be recognized by fine rootlet fossils and illuviated clay. The paleosol and lacustrine mud bed which has a high content of SiO2 crowded with diatoms alternated between the depths of 14.5 and 18.0 m. The delta unit is dominated by gravel–sand and exhibits upward-coarsening successions.
Discussion: The accumulation curve constructed based on the calendar dates calculated from the 14C ages provides the period of environmental changes in the Suwa Basin. Meandering fluvial depositions were laid from the last glacial maximum to ca. 15,000 cal years BP. The lacustrine (pond and marsh) environment expanded onto the study site from ca. 15,000 cal years BP. The primary productivity in the lake is considered to have been higher during the period ca. 12,000–6,000 cal years BP, based on the lower and static C/N ratios and higher TN. Delta progradation occurred at ca. 4,000–3,000 cal years BP.
The repetition of paleosols and diatom-rich mud beds in the lacustrine unit suggests short-term fluctuations in the lake-level during the period between 12,000–7,000 cal years BP. The lake-level underwent declines in the period ca. 12,000–11,000 cal years BP and ca. 9,000–8,200 cal years BP. The high lake-level developing during the period ca. 7,000–4,000 cal years BP coincides with the middle Holocene climatic optimum (e.g., Wanner et al., 2008). These fluctuations of the sedimentary environments and the lake-level could have been caused by climate change and/or basement subsidence.
This study was carried out under an ordinary research project (Research on the medium to long term management and maintenance of lakes, ponds, and their catchment areas in Nagano Prefecture) at the Nagano Environmental Conservation Research Institute.
References: Miall, A.D., 1996. Springer-Verlag, Berlin Heidelberg, p. 598. Shanley, K.W., McCabe, P.J., 1994. Am. Assoc. Petrol. Geol. Bull. 78, 544–568. Wanner, H. et al., 2008. Quat. Sci. Rev., 27, 1791–1828.