5:15 PM - 7:15 PM
[SEM16-P07] Detailed geomagnetic variation records at the lower Mammoth and upper Kaena boundaries from the marine Pliocene, southernmost part of the Boso Peninsula
Keywords:Paleomagnetism, Geomagnetic reversal, Pliocene
Geomagnetic reversal records preserved in geologic material serve as globally synchronous markers, providing critical correlation points for comparing paleoclimate and paleoceanographic records obtained from deep-sea cores and other stratigraphic sequences. However, the temporal resolution of geomagnetic reversal records that occurred before the Pliocene is low, and their correspondence with marine oxygen isotope stages (MIS) remains unclear, leading to discrepancies between deep-sea cores (e.g., Andersson et al., 2002; Channell et al., 2016). In addition, the sedimentation rate of deep-sea cores is often only a few centimeters per thousand years, so the paleomagnetic signal is smoothed over several hundred years within a single sample. As a result, rapid variations during geomagnetic reversals cannot be adequately captured, and it is difficult to resolve detailed temporal variations.
In this study, paleomagnetic analyses at stratigraphic intervals of approximately 1 m to 10 cm and benthic foraminiferal oxygen isotope measurements at stratigraphic intervals of 3~1 m were conducted on marine Pliocene succession in the southernmost part of the Boso Peninsula with sedimentation rates exceeding several tens of centimeters per thousand years. A detailed age model was constructed by correlating the results with the LR04 stack curve (Lisiecki and Raymo, 2005). As a result, it was revealed that the lower boundary of the Mammoth reversed subchron corresponds to MIS MG1, and the upper boundary of the Kaena reversed subchron corresponds to MIS G21.
At the lower Mammoth boundary, the relative paleointensity (RPI) exhibits a rapid decline followed by a two-step recovery consisting of a gradual phase and a rapid phase, indicating an asymmetry in the geomagnetic intensity variations associated with the reversal. The RPI minimum period was about 3.7 kyr from 3319.62 to 3315.90 ka. Including the gradual recovery phase, the period during which the geomagnetic field was weaker than usual is estimated to be 5.6 kyr. During the RPI minimum, the virtual geomagnetic pole (VGP) shows highly variable behavior, migrating in a complex manner over regions such as the Australian, African, and eastern equatorial Pacific. After the polarity reversal, the RPI begins to recover gradually, accompanied by a characteristic transition towards the South Pacific. The VGP then shifts back to the high southern latitudes, where the RPI undergoes a rapid recovery and converge on the high southern latitudes.
At the upper Kaena boundary, the RPI shows a two-step decay consisting of a rapid attenuation followed by a gradual attenuation. The recovery of RPI begins gradually, similar to the pattern observed at the lower Mammoth boundary—the minimum period was about 3.5 kyr from 3033.72 to 3030.27 ka. Including gradual weakening and recovery periods, the total time during which the geomagnetic field was weaker than usual is estimated to be 12 kyr. During the RPI minimum, the VGP shows highly variable behavior, migrating complexly over regions such as the Eurasian continent, the northwestern part of the Australian continent, and the northern Indian Ocean. The VGP then passes through mid- to the high-latitude areas of the Northern Hemisphere, followed by a rapid shift toward northwestern Australia, and converge on high northern latitudes.
These results indicate that RPI decay and recovery patterns during geomagnetic reversals are not uniform but vary from one reversal to another. Additionally, during the RPI minimum periods, the VGP migrates through regions where non-dipolar magnetic fields are predominant, as Hoffman et al. (2020) suggested. This indicates that non-axial dipolar magnetic fields become prominent during periods of RPI minima.
References
Lisiecki and Raymo (2005) doi: 10.1029/2004PA001071; Andersson et al. (2002) doi: 10.1016/S0031-0182(01)00494-1; Channell et al. (2016) doi: 10.1016/j.quascirev.2015.10.011; Hoffman et al. (2020) doi: 10.1093/gji/ggz480.
In this study, paleomagnetic analyses at stratigraphic intervals of approximately 1 m to 10 cm and benthic foraminiferal oxygen isotope measurements at stratigraphic intervals of 3~1 m were conducted on marine Pliocene succession in the southernmost part of the Boso Peninsula with sedimentation rates exceeding several tens of centimeters per thousand years. A detailed age model was constructed by correlating the results with the LR04 stack curve (Lisiecki and Raymo, 2005). As a result, it was revealed that the lower boundary of the Mammoth reversed subchron corresponds to MIS MG1, and the upper boundary of the Kaena reversed subchron corresponds to MIS G21.
At the lower Mammoth boundary, the relative paleointensity (RPI) exhibits a rapid decline followed by a two-step recovery consisting of a gradual phase and a rapid phase, indicating an asymmetry in the geomagnetic intensity variations associated with the reversal. The RPI minimum period was about 3.7 kyr from 3319.62 to 3315.90 ka. Including the gradual recovery phase, the period during which the geomagnetic field was weaker than usual is estimated to be 5.6 kyr. During the RPI minimum, the virtual geomagnetic pole (VGP) shows highly variable behavior, migrating in a complex manner over regions such as the Australian, African, and eastern equatorial Pacific. After the polarity reversal, the RPI begins to recover gradually, accompanied by a characteristic transition towards the South Pacific. The VGP then shifts back to the high southern latitudes, where the RPI undergoes a rapid recovery and converge on the high southern latitudes.
At the upper Kaena boundary, the RPI shows a two-step decay consisting of a rapid attenuation followed by a gradual attenuation. The recovery of RPI begins gradually, similar to the pattern observed at the lower Mammoth boundary—the minimum period was about 3.5 kyr from 3033.72 to 3030.27 ka. Including gradual weakening and recovery periods, the total time during which the geomagnetic field was weaker than usual is estimated to be 12 kyr. During the RPI minimum, the VGP shows highly variable behavior, migrating complexly over regions such as the Eurasian continent, the northwestern part of the Australian continent, and the northern Indian Ocean. The VGP then passes through mid- to the high-latitude areas of the Northern Hemisphere, followed by a rapid shift toward northwestern Australia, and converge on high northern latitudes.
These results indicate that RPI decay and recovery patterns during geomagnetic reversals are not uniform but vary from one reversal to another. Additionally, during the RPI minimum periods, the VGP migrates through regions where non-dipolar magnetic fields are predominant, as Hoffman et al. (2020) suggested. This indicates that non-axial dipolar magnetic fields become prominent during periods of RPI minima.
References
Lisiecki and Raymo (2005) doi: 10.1029/2004PA001071; Andersson et al. (2002) doi: 10.1016/S0031-0182(01)00494-1; Channell et al. (2016) doi: 10.1016/j.quascirev.2015.10.011; Hoffman et al. (2020) doi: 10.1093/gji/ggz480.