11:45 〜 12:00
[SEM16-11] Paleo- and rock magnetic investigations on sediments collected at Site C9035 in the Enshu Nada
In the regions around Japan, a standard curve of declination and inclination changes since around 400 AD has been constructed and utilized through systematic paleomagnetic analysis of archaeological materials (Hatakeyama and Shibuya, 2017). On the other hand, there is a report on the record of declination and inclination changes back to about 10,000 years ago based on paleomagnetic analysis of Lake Biwa sediments (Ali et al., 1999), which is sometimes tentatively used (e.g. Kanamatsu et al., 2022), but a high-reliability standard curve has not yet been constructed on a timescale of tens of thousands of years. There is a long-standing need to create such a standard curve.
We conducted paleo- and rock magnetic measurements on sediments collected at Site C9035 in the Enshu Nada, which is located at the eastern Nankai Trough. The sediments were recovered during the D/V Chikyu SCORE Expedition 912 Leg 1, and their total length extended more than 80 m below the seafloor. This is divided into two major lithological units, of which the upper 40 m is composed of bioturbated silt and layered coarse silt–very fine sand with massive silt. There are approximately 200 turbidite layers in this interval. U-channel samples typically 1.4 m in length were systematically taken from this interval, and magnetic susceptibility and remanent magnetizations were measured every 1-cm along the samples. For the remanent magnetizations measurements, alternating field (AF) demagnetization was conducted up to 80 mT (approximately 10 treatment steps) on natural remanent magnetization using a cryogenic magnetometer with an in-line static AF demagnetizer (2G Enterprises model 760R). Discrete samples were also collected, and low-temperature magnetometry was carried out using a magnetic property measurement system (Quantum Design model MPMS-XL5) on dried subsamples.
Magnetic susceptibility showed a characteristic decrease from the top to the bottom at depths of approximately 17 m and 37 m, and the susceptibility of the three intervals bounded by these depths fluctuated around a certain average value within each interval. In the low-temperature magnetic analysis, a characteristic decrease in magnetization was observed around 110 K, which is thought to reflect the Verwey transition of magnetite over all depths, but this characteristic becomes less visible in the interval deeper than 37 m. Paleomagnetic declination and inclination exhibited gentle fluctuations around the geocentric axial dipole field direction in general, though some intervals showed abrupt fluctuations. After application of the criterion of DANG (Deviation ANGle) > 10 degree, such intervals could be mostly discarded.
Based on the visual core description, the 14C dating of planktonic foraminifera, and the estimated age of the well-known wide-spread tephra layer, the depth of the sediments can be converted to a deposition age. After eliminating the turbidite layers and applying this age model, paleomagnetic secular variation (PSV) records for both the inclination and the declination were constructed, extending back to about 45,000 years ago. They show characteristic changes over time, and these can be correlated with the published PSV records from Lake Biwa for the last ~10 kyr (Ali et al., 1999) and the last ~40 kyr (Hayashida et al., 2007). The newly constructed PSV records have a higher time resolution than the Lake Biwa records, and will form the basis for creating reliable standard curves on a timescale of tens of thousands of years.
We conducted paleo- and rock magnetic measurements on sediments collected at Site C9035 in the Enshu Nada, which is located at the eastern Nankai Trough. The sediments were recovered during the D/V Chikyu SCORE Expedition 912 Leg 1, and their total length extended more than 80 m below the seafloor. This is divided into two major lithological units, of which the upper 40 m is composed of bioturbated silt and layered coarse silt–very fine sand with massive silt. There are approximately 200 turbidite layers in this interval. U-channel samples typically 1.4 m in length were systematically taken from this interval, and magnetic susceptibility and remanent magnetizations were measured every 1-cm along the samples. For the remanent magnetizations measurements, alternating field (AF) demagnetization was conducted up to 80 mT (approximately 10 treatment steps) on natural remanent magnetization using a cryogenic magnetometer with an in-line static AF demagnetizer (2G Enterprises model 760R). Discrete samples were also collected, and low-temperature magnetometry was carried out using a magnetic property measurement system (Quantum Design model MPMS-XL5) on dried subsamples.
Magnetic susceptibility showed a characteristic decrease from the top to the bottom at depths of approximately 17 m and 37 m, and the susceptibility of the three intervals bounded by these depths fluctuated around a certain average value within each interval. In the low-temperature magnetic analysis, a characteristic decrease in magnetization was observed around 110 K, which is thought to reflect the Verwey transition of magnetite over all depths, but this characteristic becomes less visible in the interval deeper than 37 m. Paleomagnetic declination and inclination exhibited gentle fluctuations around the geocentric axial dipole field direction in general, though some intervals showed abrupt fluctuations. After application of the criterion of DANG (Deviation ANGle) > 10 degree, such intervals could be mostly discarded.
Based on the visual core description, the 14C dating of planktonic foraminifera, and the estimated age of the well-known wide-spread tephra layer, the depth of the sediments can be converted to a deposition age. After eliminating the turbidite layers and applying this age model, paleomagnetic secular variation (PSV) records for both the inclination and the declination were constructed, extending back to about 45,000 years ago. They show characteristic changes over time, and these can be correlated with the published PSV records from Lake Biwa for the last ~10 kyr (Ali et al., 1999) and the last ~40 kyr (Hayashida et al., 2007). The newly constructed PSV records have a higher time resolution than the Lake Biwa records, and will form the basis for creating reliable standard curves on a timescale of tens of thousands of years.