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[HQR05-P08] Reconstruction of lake water level fluctuations since the Jomon period based on sediment core analysis from an offshore archaeological site of Lake Suwa, central Japan
Keywords:Lacustrine sediments, Submerged ruins, Paleosol, Lake level fluctuations, Lake Suwa
Introduction
Paleosols and offshore archaeological sites intercalated within lacustrine sediments serve as indicators of paleoenvironment changes, as they are formed by fluctuations in lake water levels and subsidence due to fault activities. In Lake Suwa in the Chubu Mountain Region of central Japan, some paleosols have been identified in the lacustrine sediments, and it has been reported that lake water levels fluctuated from the Late Pleistocene to Holocene [1]. On the other hand, an offshore archaeological site (the Sone Site) is found in Lake Suwa. The site has yielded artifacts dating from the Paleolithic to Jomon periods and is considered evidence of past water level declines [2]. However, no geological evidence has yet been reported to substantiate this. In this study, detailed sediment descriptions and radiocarbon dating were conducted on the sediment core collected from the Sone Site to elucidate the factors contributing to its submergence.
Methods
The Sone Site is located at a water depth of 1.5–2.0 m, approximately 300–500 m from the mouth of the Senbongi River on the eastern shore of the lake, toward the lake center [3]. In this study, a sediment core was collected from within the site at a location approximately 320 m from the eastern shore, at a water depth of about 2 m. Drilling was conducted using a Mackerras piston corer, yielding a sediment core 260 cm in length. The core was split, and the color, lithology, sedimentary structures, and plant fossils were described. Additionally, plant fragments were collected from ten stratigraphic units and subjected to AMS radiocarbon dating. Based on these results, an age-depth model was constructed. Sediment samples were collected at approximately 15 cm intervals from the core, and the major elemental composition was analyzed using an XRF at the Marine Core Research Institute, Kochi University.
Results
The age of the core is estimated to be ca. 4.2 cal kyr BP at its deepest part. The overall sedimentation rate is approximately 52.6 cm/kyr on average, but it is remarkably lower (approximately 2.4 cm/kyr) in 250–240 cm in depth. The core is mainly composed of massive carbonaceous mud layers, with a graded sand layer present at 166–156 cm in depth and a cross-stratified sandy layer that shows an upward coarsening trend and is rich in root fossils at 118–101 cm in depth. Fine rootlet fossils are densely concentrated in the mud layer at 260–250 cm in depth. The SiO2 concentration exceeds 60 wt.% in most stratigraphic units and fluctuates at values higher than those of the suspended sediments from the Kamikawa and Miyagawa rivers flowing into the lake [4]. This suggests that the SiO2 is determined by the amount of diatom accumulation within the lake. SiO2 gradually decreases from 240 cm to 118 cm. However, within this interval, there are pulsed decreases in the layers where fine rootlet fossils are densely concentrated (i.e., between 260–250 cm and 118–101 cm in depth). In the mud layer above 101 cm, the SiO2 remains stable between 60.74 and 64.67 wt.%, with no significant changes.
Discussion
The carbonaceous mud layers are interpreted as lacustrine deposits, whereas the graded sand layers are considered to have formed by density flows associated with events such as floods. The root fossil rich layer at 118–101 cm, which comprises a cross-stratified sandy layer exhibiting an upward coarsening trend, is interpreted as having formed in association with exposure on land due to the development of deltaic sandbars. In contrast, in the mud layer between 260 and 250 cm, the dense concentration of fine rootlet fossils and the abrupt decrease in SiO2 suggest that diatom deposition ceased, implying that the site was exposed above water. Since no evidence of intercalated sand layers or changes in grain size is observed in this unit, the exposure is considered to result from a rapid drop in water levels rather than from depositional processes or topographic changes associated with deltaic sandbar development. Based on these results, it is inferred that the lake bottom around the site was exposed on land at least twice since the early Jomon period (ca. 4.2–4.0 cal kyr BP and ca. 1.8 cal kyr BP), with a particularly pronounced decline in lake water levels indicated at ca. 4.2–4.0 cal kyr BP. Future work will focus on comparing this core with artifact-bearing strata.
[1] Hatano et al., 2024, Geomorphology 455, 109194. [2] Fujimori, 1965, Chigaku Zasshi 74, 76–94. [3] Mikami, 2016, The Hunters of Lake Suwa, Shinsensha, 96p. [4] Hatano, 2022, Nagano Pref. Environ. Conserv. Res. Inst. Res. Rept. 18, 61–71.
Paleosols and offshore archaeological sites intercalated within lacustrine sediments serve as indicators of paleoenvironment changes, as they are formed by fluctuations in lake water levels and subsidence due to fault activities. In Lake Suwa in the Chubu Mountain Region of central Japan, some paleosols have been identified in the lacustrine sediments, and it has been reported that lake water levels fluctuated from the Late Pleistocene to Holocene [1]. On the other hand, an offshore archaeological site (the Sone Site) is found in Lake Suwa. The site has yielded artifacts dating from the Paleolithic to Jomon periods and is considered evidence of past water level declines [2]. However, no geological evidence has yet been reported to substantiate this. In this study, detailed sediment descriptions and radiocarbon dating were conducted on the sediment core collected from the Sone Site to elucidate the factors contributing to its submergence.
Methods
The Sone Site is located at a water depth of 1.5–2.0 m, approximately 300–500 m from the mouth of the Senbongi River on the eastern shore of the lake, toward the lake center [3]. In this study, a sediment core was collected from within the site at a location approximately 320 m from the eastern shore, at a water depth of about 2 m. Drilling was conducted using a Mackerras piston corer, yielding a sediment core 260 cm in length. The core was split, and the color, lithology, sedimentary structures, and plant fossils were described. Additionally, plant fragments were collected from ten stratigraphic units and subjected to AMS radiocarbon dating. Based on these results, an age-depth model was constructed. Sediment samples were collected at approximately 15 cm intervals from the core, and the major elemental composition was analyzed using an XRF at the Marine Core Research Institute, Kochi University.
Results
The age of the core is estimated to be ca. 4.2 cal kyr BP at its deepest part. The overall sedimentation rate is approximately 52.6 cm/kyr on average, but it is remarkably lower (approximately 2.4 cm/kyr) in 250–240 cm in depth. The core is mainly composed of massive carbonaceous mud layers, with a graded sand layer present at 166–156 cm in depth and a cross-stratified sandy layer that shows an upward coarsening trend and is rich in root fossils at 118–101 cm in depth. Fine rootlet fossils are densely concentrated in the mud layer at 260–250 cm in depth. The SiO2 concentration exceeds 60 wt.% in most stratigraphic units and fluctuates at values higher than those of the suspended sediments from the Kamikawa and Miyagawa rivers flowing into the lake [4]. This suggests that the SiO2 is determined by the amount of diatom accumulation within the lake. SiO2 gradually decreases from 240 cm to 118 cm. However, within this interval, there are pulsed decreases in the layers where fine rootlet fossils are densely concentrated (i.e., between 260–250 cm and 118–101 cm in depth). In the mud layer above 101 cm, the SiO2 remains stable between 60.74 and 64.67 wt.%, with no significant changes.
Discussion
The carbonaceous mud layers are interpreted as lacustrine deposits, whereas the graded sand layers are considered to have formed by density flows associated with events such as floods. The root fossil rich layer at 118–101 cm, which comprises a cross-stratified sandy layer exhibiting an upward coarsening trend, is interpreted as having formed in association with exposure on land due to the development of deltaic sandbars. In contrast, in the mud layer between 260 and 250 cm, the dense concentration of fine rootlet fossils and the abrupt decrease in SiO2 suggest that diatom deposition ceased, implying that the site was exposed above water. Since no evidence of intercalated sand layers or changes in grain size is observed in this unit, the exposure is considered to result from a rapid drop in water levels rather than from depositional processes or topographic changes associated with deltaic sandbar development. Based on these results, it is inferred that the lake bottom around the site was exposed on land at least twice since the early Jomon period (ca. 4.2–4.0 cal kyr BP and ca. 1.8 cal kyr BP), with a particularly pronounced decline in lake water levels indicated at ca. 4.2–4.0 cal kyr BP. Future work will focus on comparing this core with artifact-bearing strata.
[1] Hatano et al., 2024, Geomorphology 455, 109194. [2] Fujimori, 1965, Chigaku Zasshi 74, 76–94. [3] Mikami, 2016, The Hunters of Lake Suwa, Shinsensha, 96p. [4] Hatano, 2022, Nagano Pref. Environ. Conserv. Res. Inst. Res. Rept. 18, 61–71.