5:15 PM - 7:15 PM
[SEM16-P16] Comparison of magnetic properties of topmost sediments at different times of dissolved oxygen concentration at the bottom of Lake Ikeda, Kagoshima Prefecture
Keywords:Magnetic property, Lake sediments, early diagenesis
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 (DO) concentration during the hypoxic condition period in the lake. In this time, we will report magnetic properties in two core samples (1B and 3A) of about 35cm long collected in 2024 May (the DO concentration of the bottom water was less than 1 mg/L), and compare the magnetic properties of the 2024 cores with those of the 2018 core collected about 5 months after the recent convection of the whole lake water in 2018 (the DO value of the bottom water was about 5 mg/L). The sampling localities of the three core samples were almost same. The core samples consisted mainly of silt with the interspersed layers of ultra to fine-grained sand.
The cores were 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). For identifying magnetic minerals, thermal demagnetization experiments of isothermal remanence imparted at 5K up to 300K with a MPMS and thermomagnetic analyses between room temperature and 720°C with a thermomagnetic balance were also performed.
Results of the low and high-temperature thermomagnetic analyses indicate that the main magnetic minerals are maghematized magnetite and titanomagnetite (x = 0.1) throughout the cores. According to the variations of hysteresis ratios denoting magnetic grain size, the following three zones are recognized in each core of 2024: Zoon-A shows Mr/Ms form 0.18 to 0.22 and Hcr/Hc from 2.3 to 2.7, and zoon-B shows smaller Mr/Ms and larger Hcr/Hc values than zoon- A, implying the existence of magnetic minerals in larger grain size. Zoon-C has similar Mr/Ms and Hcr/Hc values to Zone-B and higher Mr, Mr and Hcr values than Zone-B. In core-1B, layers of zone-A are recognized at 0-11, 19-23 and 25-35 cm below sediment surface (bss). Those of zone-B are at 11-19 and 23-25 cmbss, and those of zone-C are at 8-10 at 16-17 cmbss. In core-3A, zoon-A at 0-9, 18-21 and 23-34 cmbss, zoon-B at 9-18 and 21-23 cmbss, and zoon-C at 7-8and 17-18 cmbss.
Measurements of natural gamma-ray intensity were performed on the 2018 and 2024-1B core samples. By the comparison between the two core samples based on variation in the gamma-ray intensity of 137-Cs origin, it is found that sediments at 0-9cmbss of the 2024-1B core have been formed between 2018 and 2024, and that the sedimentation rate is about 0.17 g/cm^3/year. The hysteresis ratios and S-ratio (S-0.1) of the sediments at 0-1 cmbss show that the 2024 sample contains finer-grain sized magnetic minerals than the 2018 one, possibly implying the deposition of finer magnetic minerals or the dissolution of magnetic minerals in anoxic condition of the bottom water. Any significant evidence showing the effect of early diagenesis to magnetic minerals has not been recognized in differences of the hysteresis parameters and their ratios between the 2018 and 2024 core samples, and it is implied that the differences might have been influenced by lithological difference between the two cores.
The cores were 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). For identifying magnetic minerals, thermal demagnetization experiments of isothermal remanence imparted at 5K up to 300K with a MPMS and thermomagnetic analyses between room temperature and 720°C with a thermomagnetic balance were also performed.
Results of the low and high-temperature thermomagnetic analyses indicate that the main magnetic minerals are maghematized magnetite and titanomagnetite (x = 0.1) throughout the cores. According to the variations of hysteresis ratios denoting magnetic grain size, the following three zones are recognized in each core of 2024: Zoon-A shows Mr/Ms form 0.18 to 0.22 and Hcr/Hc from 2.3 to 2.7, and zoon-B shows smaller Mr/Ms and larger Hcr/Hc values than zoon- A, implying the existence of magnetic minerals in larger grain size. Zoon-C has similar Mr/Ms and Hcr/Hc values to Zone-B and higher Mr, Mr and Hcr values than Zone-B. In core-1B, layers of zone-A are recognized at 0-11, 19-23 and 25-35 cm below sediment surface (bss). Those of zone-B are at 11-19 and 23-25 cmbss, and those of zone-C are at 8-10 at 16-17 cmbss. In core-3A, zoon-A at 0-9, 18-21 and 23-34 cmbss, zoon-B at 9-18 and 21-23 cmbss, and zoon-C at 7-8and 17-18 cmbss.
Measurements of natural gamma-ray intensity were performed on the 2018 and 2024-1B core samples. By the comparison between the two core samples based on variation in the gamma-ray intensity of 137-Cs origin, it is found that sediments at 0-9cmbss of the 2024-1B core have been formed between 2018 and 2024, and that the sedimentation rate is about 0.17 g/cm^3/year. The hysteresis ratios and S-ratio (S-0.1) of the sediments at 0-1 cmbss show that the 2024 sample contains finer-grain sized magnetic minerals than the 2018 one, possibly implying the deposition of finer magnetic minerals or the dissolution of magnetic minerals in anoxic condition of the bottom water. Any significant evidence showing the effect of early diagenesis to magnetic minerals has not been recognized in differences of the hysteresis parameters and their ratios between the 2018 and 2024 core samples, and it is implied that the differences might have been influenced by lithological difference between the two cores.