10:45 AM - 12:15 PM
[MIS15-P29] A study of the Formation Mechanism of Annual lamination (varve) in Hokkaido and Mongolian Lakes
Keywords:Lacustrine varve, Rhodochrosite, Paleoclimate
Annual laminations (varves) have been identified at more than 200 lakes around the world (Ojala et al., 2012; Zolitschka et al., 2015) and have been found in strata from a wide range of geological periods from the Precambrian to the Cenozoic (e.g., Hasegawa et al., 2022). The varves have a variety of compositions, such as clastic, biogenic, and evaporitic (e.g., Martin-Puertas et al., 2017), and can be a powerful tool for reconstructing past distribution of climatic zones. In this study, we examine the formation mechanism of varves in Lake Onuma in southwestern Hokkaido, where rhodochrosite is found (Katsuta et al., 2020), and Sangiin Dalai Lake in northern Mongolia. The core samples used is this study are 16ONM03 (90 cm) and 22ONM01 core (upper 4m section) drilled at Lake Ohnuma, and 19SD04 core drilled at Sangiin Dalai Lake in Mongolia.
The 22ON01 core consists of a dark-gray clay layer in the upper 2 m, a yellowish-brown lamina developed clay layer from 2 to 2.8 m, and a brownish-brown silt layer from 2.8 to 3.0 m. The lamination part is thought to correspond to 1640-1929 CE, based on the correlation of the tephra layers and the eruption event at Hokkaido-Komagatake volcano (Katsuta et al., 2020). In this case, the average sedimentation rate is estimated as c. 2.5 mm/yr, which is consistent with a lamina thickness of about 2-3 mm, indicating that the lamina is most likely an annual stripe. We also performed principal component analysis (PCA) of the Itrax elemental composition data of 16ONM03 core. The results revealed that the component 1 reflects the composition of the tephra layer while the component 2 reflects variations in Mn content.
The 19SD04 core from Sangiin Dalai Lake, northern Mongolia, consists of alternating layers of well-laminated calcareous clay layer and weakly-laminated dark-gray clay laye in the upper 1.5 m (Holocene), and greenish gray silt and greyish sand layer in the 1.5 to 6 m (Last glacial). Based on the 14C age, it is likely that the lamina of Sandiin Dalai Lake is also varve origin. PCA of elemental composition data for the 19SD04 core revealed that the component 1 reflect water content, while component 2 reflect variations in alternations of well-laminated calcareous clay and weakly-laminated clay layer. The results imply that varve in Sangiin Dalai Lake is formed by the precipitation of calcite due to the enhanced productivity and increased alkalinity of the lake surface water.
The 22ON01 core consists of a dark-gray clay layer in the upper 2 m, a yellowish-brown lamina developed clay layer from 2 to 2.8 m, and a brownish-brown silt layer from 2.8 to 3.0 m. The lamination part is thought to correspond to 1640-1929 CE, based on the correlation of the tephra layers and the eruption event at Hokkaido-Komagatake volcano (Katsuta et al., 2020). In this case, the average sedimentation rate is estimated as c. 2.5 mm/yr, which is consistent with a lamina thickness of about 2-3 mm, indicating that the lamina is most likely an annual stripe. We also performed principal component analysis (PCA) of the Itrax elemental composition data of 16ONM03 core. The results revealed that the component 1 reflects the composition of the tephra layer while the component 2 reflects variations in Mn content.
The 19SD04 core from Sangiin Dalai Lake, northern Mongolia, consists of alternating layers of well-laminated calcareous clay layer and weakly-laminated dark-gray clay laye in the upper 1.5 m (Holocene), and greenish gray silt and greyish sand layer in the 1.5 to 6 m (Last glacial). Based on the 14C age, it is likely that the lamina of Sandiin Dalai Lake is also varve origin. PCA of elemental composition data for the 19SD04 core revealed that the component 1 reflect water content, while component 2 reflect variations in alternations of well-laminated calcareous clay and weakly-laminated clay layer. The results imply that varve in Sangiin Dalai Lake is formed by the precipitation of calcite due to the enhanced productivity and increased alkalinity of the lake surface water.