5:15 PM - 7:15 PM
[MIS14-P13] Paleoenvironmental reconstruction for the last 2000 years using the Anenuma lake sediments, Aomori Prefecture, Japan
Keywords:Anenuma, Mineral composition , Lake sediments, powder X-ray diffraction (XRD) analysis
In 2019, sediments were collected by Aomori Prefecture to investigate changes in the bottom sediment environment associated with recent eutrophication in Anenuma and Uchinuma, adjacent to the north and south sides of Lake Ogawara on the Pacific side of Aomori Prefecture. The Anenuma is located in the eastern part of Aomori Prefecture, with an area of 1.57 km2 and a maximum depth of 3.9 m. Sediments were collected from the lake bottom. Volcanic ash layers associated with the 946AD Hakutouzan-Tomakomai (B-Tm) and 915AD Towada A(To-a) pyroclastic flow were found in the sediments of Anenuma at depths of 203 cm and 210 cm, respectively, and the deposition rate was estimated as approximately 0.2 cm/y. Such high sedimentation rate has the potential to reconstruct environmental changes in the catchment areas of Towada City, Tohoku Town, Misawa City, and Oirase Town in Aomori Prefecture with a high temporal resolution. The objective of this study is to evaluate how natural landscape and anthropogenic land use changes in the catchment area have affected material supply patterns to brackish water bodies such as the Anenuma and the Lake Ogawara by examining the Anenuma sediments.
We measured major mineral composition of the sediments using powder X-ray diffraction (XRD) analysis. Temporal variabilities of the peak intensities of identified minerals were compared with a previous study (Nara et al. 2021) at the Lake Ogawara, which is located downstream of Anenuma, to discuss environmental changes in the Anenuma. It was found that the pyrite/quartz peak intensity ratios of show similar variations to those of the total sulfur (TS) measurements in the Lake Ogawara. Referring to previous studies, the following five patterns of variation in the mineral compositions were identified based on the correlation of the To-a tephra layer and the pyrite yielding interval between the cores of Anenuma and Ogawara lakes: (A) before 200BC, (B1) 200BC to about 800AD, (B2) about 800AD to 915AD, (C1) 915AD to about 1600AD, ( C2) approximately 1600AD to present. Of these, the rapid fluctuation of pyrite observed before 200BC is consistent with the results of the study from the Lake Ogawara, interpreted as the influence of seawater that had flowed into the lake until that time. On the other hand, about 800AD corresponds to the conquest of Ezo by Sakanoue Tamuramaro, 915AD to the Towada A pyroclastic flow, and about 1600AD to the establishment of the shogunate system, suggesting a change in the erosion and detrital supply pattern due to changes in land use and the landscape of the catchment area caused by natural disasters. In order to reconstruct the impact of land use change in the catchment area, we plan to measure the amount of biomass combustion origin materials in the sediment cores and conducting a literature review.
We measured major mineral composition of the sediments using powder X-ray diffraction (XRD) analysis. Temporal variabilities of the peak intensities of identified minerals were compared with a previous study (Nara et al. 2021) at the Lake Ogawara, which is located downstream of Anenuma, to discuss environmental changes in the Anenuma. It was found that the pyrite/quartz peak intensity ratios of show similar variations to those of the total sulfur (TS) measurements in the Lake Ogawara. Referring to previous studies, the following five patterns of variation in the mineral compositions were identified based on the correlation of the To-a tephra layer and the pyrite yielding interval between the cores of Anenuma and Ogawara lakes: (A) before 200BC, (B1) 200BC to about 800AD, (B2) about 800AD to 915AD, (C1) 915AD to about 1600AD, ( C2) approximately 1600AD to present. Of these, the rapid fluctuation of pyrite observed before 200BC is consistent with the results of the study from the Lake Ogawara, interpreted as the influence of seawater that had flowed into the lake until that time. On the other hand, about 800AD corresponds to the conquest of Ezo by Sakanoue Tamuramaro, 915AD to the Towada A pyroclastic flow, and about 1600AD to the establishment of the shogunate system, suggesting a change in the erosion and detrital supply pattern due to changes in land use and the landscape of the catchment area caused by natural disasters. In order to reconstruct the impact of land use change in the catchment area, we plan to measure the amount of biomass combustion origin materials in the sediment cores and conducting a literature review.