[S-EM13] 地磁気・古地磁気・岩石磁気

[SEM13-P03] The Laschamp and a new post-Laschamp excursions repeating directional swings from Lake Suigetsu varved sediments

*兵頭 政幸^1,2、中川 毅³、松下 隼人²、北場 育子³、山田 圭太郎³、田辺 祥汰²、Bradak Balazs^1,4、McLean Danielle⁵、Staff Richard⁶、Smith Victoria ⁵、Albert Paul⁵、山崎 彬輝⁷、北川 淳子^8,7 (1.神戸大学内海域環境教育研究センター、2.神戸大学理学研究科惑星学専攻、3.立命館大学古気候学研究センター、4.ブルゴス大学物理学科、5.オックスフォード大学考古学科、6.グラスゴー大学SUERC、7.福井県立里海里山研究所、8.年縞博物館)

キーワード：古地磁気学、ラシャンエクスカーション、双極子的磁場卓越、水月湖、年縞堆積物、IntCal13/IntCal20年代

Geomagnetic excursions represent dynamic natures of the geodynamo during declined dipole-field periods. Many sedimentary records of excursions have been reported since 1970s, but none of them resolves sub-centennial scale field-changes. We report a tri-decadal resolution Laschamp Excursion record from Lake Suigetsu varved sediments that reveals many detailed features. A high resolution ¹⁴C dates and varve chronology-based Bayesian age model reveals the Laschamp Excursion ranges from 42,340 to 41,410 IntCal13 (42,400 to 41,700 IntCal20) yr BP, consisting of five short (20 to 110 yr) excursional directional-swings. We found another excursion ranging from 38,580 to 38,030 IntCal13 (39,080 to 38,580 IntCal20) yr BP composed with similar swings. Both excursions coincided with the Δ¹⁴C maxima in Lake Suigetsu as well as the ¹⁰Be flux maxima in Greenland. We simulated how the high-frequency directional swings in the high sedimentation rate (s.r.) (72 cm/kyr) sediments are altered when recorded in sediments with lower s.r., by downsizing the data assuming response functions defined by a sample thickness and a half lock-in depth of post depositional remanent magnetization. As s.r. decreases, the short directional swings are gradually filtered-out and/or combined, which causes (1) stepwise decreases in the time duration of excursion, and (2) a drastic change of virtual geomagnetic pole (VGP)-pass from multiple linear swings between northern high-latitudes and northern mid-/southern latitudes to a large clockwise open loop with a precursory small episode. These features of the downsized Lake Suigetsu data are consistent with many previous observations in sediment with lower s.r. The excursional VGPs of Lake Suigetsu repeat swinging constrained by two longitudinal bands through the Pacific Ocean and the Indian Ocean-Africa. In addition, they make four clusters centered in the North Pacific, the South Pacific, the South Indian Ocean, and the eastern Africa in chronological order, which accounts for the apparent clockwise open loop of VGP-pass in low s.r. records. The clusters include the VGPs inferred from lava flows associated with the Laschamp Excursion from various regions of the world, which indicate dominance of non-axial dipolar fields whose sources located under the four clusters during the excursion.