5:15 PM - 6:45 PM
[SEM13-P10] Magnetic properies of topmost sediments in Lake Ikeda, Kagosima Prefecture.
Keywords:Magnetic property, Lake sediments
In Lake Ikeda, Kagoshima Prefecture, the lake bottom remains anoxic for a long period of time because the convection of the whole water supplying oxygen to the bottom occurs every several years, resulting in a long hypoxic condition at the lake bottom. In order to clarify the effect of early diagenesis to magnetic minerals in sediments, we have planned to compare magnetic properties of topmost sediments sampled at different conditions of dissolved oxygen concentration during the hypoxic condition period in the lake. In this time, we will report magnetic properties in a core sample of about 20 cm long collected about 5 months after the convection of the whole lake water in 2018.
The core was extruded vertically at 1 cm interval and sliced. Each slice of sediments was subjected to freeze-drying. By using dried powder samples, we performed magnetic hysteresis measurements and direct-field demagnetization experiments with an alternating-gradient force magnetometer (AGM2900-2). Thermal demagnetization experiments of isothermal remanence imparted at 6K up to 300K with a MPMS and thermomagnetic analyses between room temperature and 620°C with a thermomagnetic balance were also performed for identifying magnetic minerals.
Results of the rock magnetic analyses indicate that the main magnetic minerals are maghematized magnetite and titanomagnetite (x = 0.1) throughout the core. Based on variations of hysteresis ratios denoting magnetic grain size, the core is divided into the following five units: Unit A with finer grains between 0 and 1 cm below sediment surface (cmbss), Unit B between 1 to 7 cmbss, Unit C with coarser grains between 7 and 10 cmbss, Unit D between 11 and 16 cmbss, and Unit E with finer grains below 16 cmbss. The variation of magnetic grain size corresponds well to lithological change in the core: Units A and E are silt, Units B and D are very fine sand, and Unit C includes a layer of fine sand between 8 and 10 cmbss. Differences in the amount of magnetic mineral were also observed among the units: the amount was larger in Unit C and smaller in Units A and E. Any changes of magnetic properties implying the effect of early diagenesis have bee detected. It is suggested that variations in grain size and amount of magnetic mineral mainly reflect the primary information about changes in transportation processes of detrital materials at the time of deposition. Unit C is regarded as a characteristic layer with a large amount of coarser-grained magnetic grains. It is implied that the unit was formed associated with an event in the sedimentation process at Lake Ikeda.
The core was extruded vertically at 1 cm interval and sliced. Each slice of sediments was subjected to freeze-drying. By using dried powder samples, we performed magnetic hysteresis measurements and direct-field demagnetization experiments with an alternating-gradient force magnetometer (AGM2900-2). Thermal demagnetization experiments of isothermal remanence imparted at 6K up to 300K with a MPMS and thermomagnetic analyses between room temperature and 620°C with a thermomagnetic balance were also performed for identifying magnetic minerals.
Results of the rock magnetic analyses indicate that the main magnetic minerals are maghematized magnetite and titanomagnetite (x = 0.1) throughout the core. Based on variations of hysteresis ratios denoting magnetic grain size, the core is divided into the following five units: Unit A with finer grains between 0 and 1 cm below sediment surface (cmbss), Unit B between 1 to 7 cmbss, Unit C with coarser grains between 7 and 10 cmbss, Unit D between 11 and 16 cmbss, and Unit E with finer grains below 16 cmbss. The variation of magnetic grain size corresponds well to lithological change in the core: Units A and E are silt, Units B and D are very fine sand, and Unit C includes a layer of fine sand between 8 and 10 cmbss. Differences in the amount of magnetic mineral were also observed among the units: the amount was larger in Unit C and smaller in Units A and E. Any changes of magnetic properties implying the effect of early diagenesis have bee detected. It is suggested that variations in grain size and amount of magnetic mineral mainly reflect the primary information about changes in transportation processes of detrital materials at the time of deposition. Unit C is regarded as a characteristic layer with a large amount of coarser-grained magnetic grains. It is implied that the unit was formed associated with an event in the sedimentation process at Lake Ikeda.