4:15 PM - 4:30 PM
[HCG21-10] Varve in Lake Ichinome-gata revealed by monthly sediment trap sampling
Keywords:Varve, Depositional Process, Lake Ichinome-gata, Maar, Sediment trap
Varve sediments, without bioturbation, have the potential for high-resolution reconstruction of paleoclimate and paleoenvironment with annual/seasonal scale. Evaluation of depositional processes is essential to reveal paleoenvironmental variation using sediments. However, there are several types of varve sediments, such as organic, calcareous (Kelt & Hsu, 1978), clastic (Strum, 1979), and ferruginous (Anthony, 1977). Lake Suigetsu is a brackish lake and is famous for the deposition of varve sediments. Varves in Lake Suigetu are divided into light gray layers deposited after main diatom bloom in spring, opaque mineral-rich layers formed in summer under anoxic water, and dark gray layers deposited after diatom bloom in autumn and winter (Fukuzawa, 1995). Lake Ichinome-gata in Akita Prefecture is a freshwater lake formed in the maar and famous for the deposition of varve sediments. Yamada et al. (2014) reported stratigraphy and paleoenvironment using sediment core samples of Lake Ichinome-gata. They assumed the seasonality of depositional processes from core samples and observed light gray layers with diatom remains due to spring bloom and dark gray layers with terrigenous materials containing phytoclasts possibly deposited from autumn to winter. In this presentation, we discuss depositional process of varve sediment in Lake Ichinome-gata using the data of monthly sediment trap sampling between April 2023 and May 2025.
We collected water samples at depths of 0 m and 20 m and particles settled into a sediment trap every ~30 days around the center of the lake. Sediment traps were moored at 20 m depth and 1 m above the bottom of the lake. Plankton observations using water samples and smear slide, Rock-Eval, and palynofacies analyses using sediment trap samples were performed.
Depth profiles of water temperature showed maximum stratification and whole circulation occurred in August–September and December–February, respectively. Cell counts of Asterionella sp., as the main primary producer in Lake Ichinome-gata, indicate spring bloom started in February at depths of 0 m and 20 m. In 2024, the highest cell concentration of Asterionella sp. was 7.1×102 cells/ml on March 19 and these higher concentrations continued until June 21 at least. However, the diatom flux, estimated from diatom remains and sediment flux, is highest in May, with ~2 months lag between cell count data. Cell concentrations were gradually decreased due to stratification, and there was no cell of Asterionella sp. after November. From Rock-Eval analysis, sediment trap samples of both middle and bottom layers collected from April to September, characterized by higher productivity, have higher Hydrogen Index (HI; 350-400 mg CH/g TOC) values and lower Oxygen Index (OI; 120-160 mg CO2/g TOC) values. In contrast, HI values decreased and OI values increased in the samples collected after November. Finally, in February, HI values reached a minimum (middle: 205; bottom: 203), and OI values reached a maximum (middle: 246; bottom: 226). In addition, pyrograms of sediment trap samples collected from November to February indicated that labile organic matter decomposed, compared with other months.
Hence, from these results, we summarized the depositional process of varve sediment in Lake Ichinome-gata as follows. First, a diatom bloom occurred in February with a 2-month lag due to whole circulations starting from late December supplying nutrients to the surface. Diatom production continued until September and formed a diatom-rich light gray layer in the bottom from March to October. In contrast, humic substances with higher OI values due to the decomposition of labile organic matter formed dark gray layers from November to February. These humic substances also possibly accumulate by flowing up of suspended materials by whole circulation.
We collected water samples at depths of 0 m and 20 m and particles settled into a sediment trap every ~30 days around the center of the lake. Sediment traps were moored at 20 m depth and 1 m above the bottom of the lake. Plankton observations using water samples and smear slide, Rock-Eval, and palynofacies analyses using sediment trap samples were performed.
Depth profiles of water temperature showed maximum stratification and whole circulation occurred in August–September and December–February, respectively. Cell counts of Asterionella sp., as the main primary producer in Lake Ichinome-gata, indicate spring bloom started in February at depths of 0 m and 20 m. In 2024, the highest cell concentration of Asterionella sp. was 7.1×102 cells/ml on March 19 and these higher concentrations continued until June 21 at least. However, the diatom flux, estimated from diatom remains and sediment flux, is highest in May, with ~2 months lag between cell count data. Cell concentrations were gradually decreased due to stratification, and there was no cell of Asterionella sp. after November. From Rock-Eval analysis, sediment trap samples of both middle and bottom layers collected from April to September, characterized by higher productivity, have higher Hydrogen Index (HI; 350-400 mg CH/g TOC) values and lower Oxygen Index (OI; 120-160 mg CO2/g TOC) values. In contrast, HI values decreased and OI values increased in the samples collected after November. Finally, in February, HI values reached a minimum (middle: 205; bottom: 203), and OI values reached a maximum (middle: 246; bottom: 226). In addition, pyrograms of sediment trap samples collected from November to February indicated that labile organic matter decomposed, compared with other months.
Hence, from these results, we summarized the depositional process of varve sediment in Lake Ichinome-gata as follows. First, a diatom bloom occurred in February with a 2-month lag due to whole circulations starting from late December supplying nutrients to the surface. Diatom production continued until September and formed a diatom-rich light gray layer in the bottom from March to October. In contrast, humic substances with higher OI values due to the decomposition of labile organic matter formed dark gray layers from November to February. These humic substances also possibly accumulate by flowing up of suspended materials by whole circulation.