17:15 〜 18:30
[SCG45-P03] Deep-tow magnetics for 29–33 Myr seafloor of the Southeast Indian Ridge
キーワード：深海磁気異常、C11n12n, C11r, C12n, & C12r、南東インド洋海嶺
Knowledge of the Earth's magnetic field intensity variations provides an essential constraint on the age benchmark and geodynamo model. A record of absolute palaeointensity has been compiled from archaeomagnetic and volcanic materials, and long-term relative palaeointensity has been obtained from sedimentary sequences. Although geomagnetic intensity signal recorded as the oceanic crust's thermoremanence should provide a higher-time-resolution record of crustal accretion at mid-ocean ridges (e.g., Gee et al., 2000), available data is essentially limited due to observation difficulty. Here, we present new observation data of near-seafloor magnetic anomalies of 29–33 Myr seafloor at the Southeast Indian Ridge.
We conducted a deep-tow operation during the R/V Hakuho-maru cruise of KH-20-1 (Cape Town to Fremantle, January – February 2020). Underway geophysical data (e.g., multibeam bathymetry and total magnetic field) of the target area has been obtained during the KH-07-4 and KH-16-1 cruises. The deep-sea total magnetic fields were measured using the deep-tow cesium magnetometer developed by the Atmosphere and Ocean Research Institute, The University of Tokyo. The towed proton magnetometer data was simultaneously observed at the sea-surface. The deep-tow sensor was kept 30 m away from the mainframe using the electric cable. The stainless mainframe consists of the magnetometer body, a pressure sensor, a motion sensor, and a transponder for acoustic positioning. The mainframe is 100 m away from the 600kg-weight tied to the ship No.1 wire. Another transponder was also set to the ship No.1 wire 20 m above the weight. The data of total magnetic field, depth, and attitude are merged and recorded at 2Hz. This observation covers the signal from the northern off-axis area of the Southeast Indian Ridge on 13–14 February. The sensor reached ~4,500m deep. The total length of obtained data at >3,000m deep is up to 40 miles.
Obtained near-seafloor magnetic anomalies ranges 60,700 to 62,300 nT. This variation is three times larger than the total intensity variation observed at sea-surface. Long-wavelength signatures are well correlated with each other. Comparing with geomagnetic polarity chrons (Gee and Kent, 2015), previously obtained surface data (Quesnel et al., 2009), and globally picked isochrons (Seton et al., 2014), we individually picked magnetic isochrons of C11n12n, C11r, C12n, and C12r from obtained near-seafloor magnetic anomalies. The corresponding age is 29.401 to 33.058 Myr. Several tiny wiggles with an amplitude of up to 300 nT are remarkably observed in C12r. This result is generally consistent with summarized cryptochrons (Cande and Kent, 1992; Cande and Kent, 1995). A few tiny wiggles with an amplitude of up to 200 nT were also observed in C12n, which have never been reported. We consider that this result can be a new finding of short-term fluctuations in the geomagnetic field, but careful examination considering crustal processes is needed. In this presentation, we aim to discuss these observed tiny wiggles' origin by comparing our results with sea-surface anomaly data of faster spreading seafloor segments (western slopes of the East Pacific Rise 22–33°S and Pacific Antarctic Ridge 33–40°S). Besides, Oligocene sediment record from IODP Sites U1331, U1332, and U1333 (140°W Pacific equatorial high-productivity zone; Yamazaki et al., 2013; Yamamoto et al., 2013), and DSDP Site 522 (Walvis Ridge, South Atlantic Ocean; Hartl et al., 1993; Tauxe and Hartl, 1997) are utilized to discuss palaeointensity variation. We will also compare our results with 30Myr lava flow data from the Lima Limo section of the Ethiopian trap (Ahn et al., 2021; Yoshimura et al., 2020).
We conducted a deep-tow operation during the R/V Hakuho-maru cruise of KH-20-1 (Cape Town to Fremantle, January – February 2020). Underway geophysical data (e.g., multibeam bathymetry and total magnetic field) of the target area has been obtained during the KH-07-4 and KH-16-1 cruises. The deep-sea total magnetic fields were measured using the deep-tow cesium magnetometer developed by the Atmosphere and Ocean Research Institute, The University of Tokyo. The towed proton magnetometer data was simultaneously observed at the sea-surface. The deep-tow sensor was kept 30 m away from the mainframe using the electric cable. The stainless mainframe consists of the magnetometer body, a pressure sensor, a motion sensor, and a transponder for acoustic positioning. The mainframe is 100 m away from the 600kg-weight tied to the ship No.1 wire. Another transponder was also set to the ship No.1 wire 20 m above the weight. The data of total magnetic field, depth, and attitude are merged and recorded at 2Hz. This observation covers the signal from the northern off-axis area of the Southeast Indian Ridge on 13–14 February. The sensor reached ~4,500m deep. The total length of obtained data at >3,000m deep is up to 40 miles.
Obtained near-seafloor magnetic anomalies ranges 60,700 to 62,300 nT. This variation is three times larger than the total intensity variation observed at sea-surface. Long-wavelength signatures are well correlated with each other. Comparing with geomagnetic polarity chrons (Gee and Kent, 2015), previously obtained surface data (Quesnel et al., 2009), and globally picked isochrons (Seton et al., 2014), we individually picked magnetic isochrons of C11n12n, C11r, C12n, and C12r from obtained near-seafloor magnetic anomalies. The corresponding age is 29.401 to 33.058 Myr. Several tiny wiggles with an amplitude of up to 300 nT are remarkably observed in C12r. This result is generally consistent with summarized cryptochrons (Cande and Kent, 1992; Cande and Kent, 1995). A few tiny wiggles with an amplitude of up to 200 nT were also observed in C12n, which have never been reported. We consider that this result can be a new finding of short-term fluctuations in the geomagnetic field, but careful examination considering crustal processes is needed. In this presentation, we aim to discuss these observed tiny wiggles' origin by comparing our results with sea-surface anomaly data of faster spreading seafloor segments (western slopes of the East Pacific Rise 22–33°S and Pacific Antarctic Ridge 33–40°S). Besides, Oligocene sediment record from IODP Sites U1331, U1332, and U1333 (140°W Pacific equatorial high-productivity zone; Yamazaki et al., 2013; Yamamoto et al., 2013), and DSDP Site 522 (Walvis Ridge, South Atlantic Ocean; Hartl et al., 1993; Tauxe and Hartl, 1997) are utilized to discuss palaeointensity variation. We will also compare our results with 30Myr lava flow data from the Lima Limo section of the Ethiopian trap (Ahn et al., 2021; Yoshimura et al., 2020).