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[HQR05-P10] Characteristics and origin of the banded clay in the sediment core in Lake Suwa, central Japan
Keywords:lacustrine sediments, Pleistocene–Holocene, banded sediments, XRF core scanner
Introduction: Lake Suwa is located in an inland basin with high-altitude, the Japanese Alps region. The drainage area and lake environments sensitively response to climate change after the late Pleistocene [1]. The lake sediment core (upper Pleistocene to Holocene) obtained in the coast of Lake Suwa intermittently intercalates banded clay sediments [1]. In Lake Suwa, the water circulation is caused by wind and the change in the surface water density due to freezing on the lake surface water in winter [2]. The banded clay sediments can suggest the stagnation of the vertical circulation. In this study, we performed the description of the banded clay sediments based on the microscopic observation, XRF core scanner (ITRAX), XRF, and CHN elemental analyses, and discussed their formative factor.
General description of the sediment core: The sediment core (SK2021 core), of which length is 30.0 m, was drilled on the southeastern coast of Lake Suwa. The sedimentary units are divided into the following three units: Unit I (channel and floodplain deposits), Unit II (lacustrine deposits), and Unit III (delta deposits), based on grain size, sedimentary structure, composition, and paleosol feature. The radiocarbon ages of wood fragments show ca. 13.3–4.2 cal kyr BP in Unit II.
Characteristics and origin of the banded clay sediments: Unit II (lacustrine deposits) is mainly composed of dark and massive muds with abundant diatom fossils, and intercalates banded clay sediments in 17.9–17.3 m depth (ca. 12.7–11.8 cal kyr BP) and 16.4–16.3 m depth (ca. 11.3–11.2 cal kyr BP). The irregular banded pattern consists of dark or olive gray laminae, gray laminae, and yellowish-brown laminae, of which thickness shows several millimeters. On the basis of the microscopic observation and XRF core scanner analysis, these laminae are correspond to the siderite laminae rich in Fe, S, P, Cu, Pb, and Ba, diatom laminae rich in Si and Sr, and framboidal pyrite-clastics-pellet laminae rich in S, Ca, Al, and La, respectively. The siderite laminae showing precipitation of sulfide predominate in the banded pattern, suggesting the preservation under the reduction condition caused by stagnation of the bottom water with low dissolved oxygen. In the late Last Glacial to Holocene periods, the vertical circulation is considered to have been intermittently suspended in the lake. Abundant TiO2/SiO2, Al2O3/SiO2, P2O5/SiO2, and TN/TOC in the banded clay sediments indicate the increase in sediment supply from the catchment and biological productivity in the lake. The occurrence of anoxic water mass can be caused by eutrophication and increased biological productivity due to sediment (soil nutrient) supply, and/or lack in freezing on the lake surface. The climate change during the late Last Glacial and Holocene can contribute to the vertical circulation in Lake Suwa.
References: [1] Hatano, N. et al., 2023, Palaeogeogr. Palaeoclimatol. Palaeoecol. 614, 111439. [2] Toyota, M., 2010, Jpn. Jour. Limnol. 71, 45–52.
General description of the sediment core: The sediment core (SK2021 core), of which length is 30.0 m, was drilled on the southeastern coast of Lake Suwa. The sedimentary units are divided into the following three units: Unit I (channel and floodplain deposits), Unit II (lacustrine deposits), and Unit III (delta deposits), based on grain size, sedimentary structure, composition, and paleosol feature. The radiocarbon ages of wood fragments show ca. 13.3–4.2 cal kyr BP in Unit II.
Characteristics and origin of the banded clay sediments: Unit II (lacustrine deposits) is mainly composed of dark and massive muds with abundant diatom fossils, and intercalates banded clay sediments in 17.9–17.3 m depth (ca. 12.7–11.8 cal kyr BP) and 16.4–16.3 m depth (ca. 11.3–11.2 cal kyr BP). The irregular banded pattern consists of dark or olive gray laminae, gray laminae, and yellowish-brown laminae, of which thickness shows several millimeters. On the basis of the microscopic observation and XRF core scanner analysis, these laminae are correspond to the siderite laminae rich in Fe, S, P, Cu, Pb, and Ba, diatom laminae rich in Si and Sr, and framboidal pyrite-clastics-pellet laminae rich in S, Ca, Al, and La, respectively. The siderite laminae showing precipitation of sulfide predominate in the banded pattern, suggesting the preservation under the reduction condition caused by stagnation of the bottom water with low dissolved oxygen. In the late Last Glacial to Holocene periods, the vertical circulation is considered to have been intermittently suspended in the lake. Abundant TiO2/SiO2, Al2O3/SiO2, P2O5/SiO2, and TN/TOC in the banded clay sediments indicate the increase in sediment supply from the catchment and biological productivity in the lake. The occurrence of anoxic water mass can be caused by eutrophication and increased biological productivity due to sediment (soil nutrient) supply, and/or lack in freezing on the lake surface. The climate change during the late Last Glacial and Holocene can contribute to the vertical circulation in Lake Suwa.
References: [1] Hatano, N. et al., 2023, Palaeogeogr. Palaeoclimatol. Palaeoecol. 614, 111439. [2] Toyota, M., 2010, Jpn. Jour. Limnol. 71, 45–52.