Japan Geoscience Union Meeting 2015

Presentation information


Symbol M (Multidisciplinary and Interdisciplinary) » M-IS Intersection

[M-IS34] Paleoclimatology and paleoceanography

Thu. May 28, 2015 11:00 AM - 12:45 PM 301A (3F)

Convener:*Kazuyoshi Yamada(Museum of Natural and Environmental history, Shizuoka), Minoru Ikehara(Center for Advanced Marine Core Research, Kochi University), Tomohisa Irino(Faculty of Environmental Earth Science, Hokkaido University), Akira Oka(Atmosphere and Ocean Research Institute, The University of Tokyo), Yusuke Okazaki(Department of Earth and Planetary Sciences, Graduate School of Science, Kyushu University), Ikuko Kitaba(Research Centre for Palaleoclimatology, Ritsumeikan University), Akihisa Kitamura(Institute of Geosciences, Faculty of Science, Shizuoka University), Masaki Sano(Research Institute for Humanity and Nature), Takeshi Nakagawa(Ritsumeikan University), Akira Hayashida(Department of Environmental Systems Science, Doshisha University), Chair:Kazuyoshi Yamada(Museum of Natural and Environmental history, Shizuoka)

12:00 PM - 12:15 PM

[MIS34-09] M-B boundary age constrained by high-precision U-Pb zircon dating of a widespread tephra in a sedimentary sequence

*Yusuke SUGANUMA1, Makoto OKADA2, Kenji HORIE1, Hiroshi KAIDEN1, Mami TAKEHARA3, Ryoko SENDA4, Jun-ichi KIMURA4, Kenji KAWAMURA1, Yuuki HANEDA2, Osamu KAZAOKA5, Martin Head6 (1.National Institute of Polar Research, 2.Ibaraki University, 3.Kyushu University, 4.JAMSTEC, 5.Research Institute of Environmental Geology, Chiba, 6.Brock University)

Geomagnetic polarity reversals, including the Matuyama-Brunhes boundary (MBB), are critical markers in age calibrating sedimentary sequences with volcanic rocks. Most age determinations for the MBB use marine astronomically-tuned benthic and planktonic foraminiferal oxygen isotope records to date the mid-point in the transition of the virtual geomagnetic pole (VGP). During the MBB and other reversals, the Earth's geomagnetic field intensity dropped significantly, resulting in the increased production of cosmogenic radionuclides, including 10Be, in the upper atmosphere. Hence, the MBB has also been recognized as a positive spike in the 10Be flux recorded in marine sediments and an Antarctic ice core.
The MBB has a frequently cited age of 780 ka, which derives from astronomically-tuned benthic and planktonic oxygen isotope records from the eastern equatorial Pacific. This marine astronomically-dated MBB age is supported by 40Ar/39Ar ages of Maui lavas at 781-783 ka, revised by the recent reference age of Fish Canyon Tuff sanidine (FCTs) standards. However, an understanding of post-depositional remanent magnetization (PDRM) processes shows that lock-in of the geomagnetic signal occurs below the sediment?water interface in marine sediments, which then yields ages for geomagnetic events that are too old. Because this age offset is influenced by sedimentation rate, those records with higher sedimentation rates should minimize the PDRM lock-in problem. In fact, younger astrochronological MBB ages of 772-773 ka have been reported from high sedimentation rate records. These MBB ages are consistent with records of cosmogenic nuclides in marine sediments and an Antarctic ice core, although they are not supported by radiometric timescales.
Here, we present a high-precision U-Pb zircon age of 772.7 ± 7.2 ka from a marine-deposited tephra just below the MBB in a forearc basin in Japan. Because the U-series dating is relatively free from issues about standardization and decay constants, this U-Pb zircon age coupled with a newly obtained oxygen isotope chronology yields a highly accurate MBB age of 770.2 ± 7.3 ka. Our MBB age is consistent with those based on the latest orbital-tuned marine sediments. We provide the first direct comparison between orbital tuning, U-Pb dating, and magnetostratigraphy for the MBB, fulfilling a key requirement for calibrating the geological timescales.