5:15 PM - 6:45 PM
[U03-P02] Change in depositional process of sediment due to heavy rain fall event: Implication from sediment trap in Lake Ichinome-gata, Akita, Japan.
Keywords:Varve, Heavy Rain Fall, Lake Ichinome-gata, maar, Sediment trap
Lake Ichinome-gata, a marl lake with varve sediments on the bottom, is located in the Oga Peninsula of Akita Prefecture in northeastern Japan. The sediment cores with varve are suitable for paleoenvironmental research because seasonal-level change can be reconstructed from annual lamina. Recently, the frequency of heavy rainfall events has been increasing due to rapid climate change. For example, the highest daily precipitation in recorded history, called as the Akita Heavy Rainfall, was observed in the Oga Peninsula on July 15, 2023. These events such as floods, earthquakes disturb the depositional process of sediments, and finally, these records are preserved as event deposits intercalated with varved sediments. However, the detailed time-series process for the formation of event deposits is not clear. In this study, we investigated how environmental changes are reflected in the depositional process of varve and event deposits in Ichinome-gata, especially focusing on the Akita Heavy Rainfall in 2023.
Sediment traps are moored in the middle and bottom layers of Lake Ichinome-gata and collected settling particle every month. We obtained water samples for plankton community observation, and sediment trap samples for microscope observation and geochemical analyses. Smear slide and palynofacies analyses using a microscope were performed for the characterization of sediment. The kerogen samples for palynofacies and FT-IR analyses were treated with HCl and HF.
Vertical measurements of water temperature at the center of the lake showed that stratification of the lake water increased during the summer. Dinoflagellates and diatoms thrived in the surface layer (0 m depth) and the middle layer (20 m depth), respectively, and their numbers gradually decreased from April to July, which was related to nutrient depletion due to stratification. However, after Akita Heavy Rainfall, dinoflagellates and diatoms flourished again. This suggests that the inflow of nutrients was supplied to each layer by the heavy rainfall.
The fluxes are six and ten times higher in the middle and bottom layers, respectively, compared to June. Microscope observations indicated that the main particle size of sediment trap samples without July is clay, but those after heavy rainfall (July) are silt. The result from palynofacies shows an increase in plant fragments as the main component of organic matter in July. However, from Rock-Eval analysis, the higher Oxygen Index (OI) values of bulk sediments from sediment trap are higher than those of lignin and cellulose standards composing plant tissues. This result shows the sample includes a higher content of fulvic acid. The FT-IR analysis showed high peaks of functional groups such as -OH (~3340 to 3400 cm-1), which were consistent with the results from Rock-Eval analysis.
This evidence suggested that sediments of streams and lakesides are entrained with water flow due to heavy rainfall and spread to the surface water or the thermocline layer.
On the other hand, water thermometers equipped on the lake bottom indicated that the temperature rose rapidly at the time of heavy rainfall event and maintained the elevated temperature after some months. The suspended sediment in the bottom water was observed as aspherical and colloidal particles with a microscope and SEM. The IR spectra of these suspended particles are characterized by the lower peaks of free -OH. These results suggest that colloidal particles were formed due to the formation of fulvic acid complexes and/or dilution by clay mineral, accumulated as high-density water mass, and flowed along the lake slope to the bottom water.
As a result, it can be pointed out that two types of water masses with sediments flowed into Lake Ichinome-gata due to the heavy rainfall. One type is coarse with a high sedimentation rate, and the other one is fine and suspended. Therefore, the sediment deposited by heavy rainfall in Lake Ichinome-gata might be characterized by the normal grading layer, although further investigation is needed.
Sediment traps are moored in the middle and bottom layers of Lake Ichinome-gata and collected settling particle every month. We obtained water samples for plankton community observation, and sediment trap samples for microscope observation and geochemical analyses. Smear slide and palynofacies analyses using a microscope were performed for the characterization of sediment. The kerogen samples for palynofacies and FT-IR analyses were treated with HCl and HF.
Vertical measurements of water temperature at the center of the lake showed that stratification of the lake water increased during the summer. Dinoflagellates and diatoms thrived in the surface layer (0 m depth) and the middle layer (20 m depth), respectively, and their numbers gradually decreased from April to July, which was related to nutrient depletion due to stratification. However, after Akita Heavy Rainfall, dinoflagellates and diatoms flourished again. This suggests that the inflow of nutrients was supplied to each layer by the heavy rainfall.
The fluxes are six and ten times higher in the middle and bottom layers, respectively, compared to June. Microscope observations indicated that the main particle size of sediment trap samples without July is clay, but those after heavy rainfall (July) are silt. The result from palynofacies shows an increase in plant fragments as the main component of organic matter in July. However, from Rock-Eval analysis, the higher Oxygen Index (OI) values of bulk sediments from sediment trap are higher than those of lignin and cellulose standards composing plant tissues. This result shows the sample includes a higher content of fulvic acid. The FT-IR analysis showed high peaks of functional groups such as -OH (~3340 to 3400 cm-1), which were consistent with the results from Rock-Eval analysis.
This evidence suggested that sediments of streams and lakesides are entrained with water flow due to heavy rainfall and spread to the surface water or the thermocline layer.
On the other hand, water thermometers equipped on the lake bottom indicated that the temperature rose rapidly at the time of heavy rainfall event and maintained the elevated temperature after some months. The suspended sediment in the bottom water was observed as aspherical and colloidal particles with a microscope and SEM. The IR spectra of these suspended particles are characterized by the lower peaks of free -OH. These results suggest that colloidal particles were formed due to the formation of fulvic acid complexes and/or dilution by clay mineral, accumulated as high-density water mass, and flowed along the lake slope to the bottom water.
As a result, it can be pointed out that two types of water masses with sediments flowed into Lake Ichinome-gata due to the heavy rainfall. One type is coarse with a high sedimentation rate, and the other one is fine and suspended. Therefore, the sediment deposited by heavy rainfall in Lake Ichinome-gata might be characterized by the normal grading layer, although further investigation is needed.