[A-CC21] アイスコアと古環境変動

[ACC21-08] アイスコアと堆積物の¹⁰Beが示す17〜20万年前の長周期太陽変動

★招待講演

*堀内 一穂¹、鎌田 佳苗¹、前島 舜¹、佐々木 祥¹、佐々木 宣欣¹、山崎 俊嗣²、藤田 秀二³、本山 秀明³、松崎 浩之⁴ (1.弘前大学大学院理工学研究科、2.東京大学大気海洋研究所、3.国立極地研究所、4.東京大学総合研究博物館)

Centennial to millennial periodicities of past solar activity in the Holocene epoch have been well investigated by using cosmogenic-nuclides records from tree-ring (¹⁴C) and ice-core (¹⁰Be) archives. However, those of the older ages are quite unclear because high-resolution records of cosmogenic nuclides are scarce. We obtained multiple high-resolution ¹⁰Be records (atmospheric ¹⁰Be flux and authigenic ¹⁰Be/⁹Be ratio) of 170–200 kyr ago, the period that includes the Iceland Basin geomagnetic excursion interval (Horiuchi et al., in press). Strong consistency among the records enables us to construct a robust ¹⁰Be stack of this interval. A wavelet analysis of the stacked record reveals 4-kyr and 8-kyr periodicities, both of which can be interpreted as intrinsic geomagnetic cycles. We also found a cycle of 1.7 kyr in the ¹⁰Be stack record, but it is significant only near the ¹⁰Be maximum (the minimum of the geomagnetic paleointensity). A relative enhancement of solar modulation in ¹⁰Be production is predicted with reduced paleointensity (Masarik and Beer, 1999; Beer et al., 2012). Therefore, it is possible to attribute this cosmogenic 1.7-kyr cycle to the long-term solar cycle that is identified in this period. The wavelet spectrum of the highest-resolution ¹⁰Be record (obtained from the Dome Fuji ice core) shows more a detailed structure than the stacked record. The 4- and 1.7-kyr cycles show similar tendencies to, and are clearer than, those of the stack. The spectrum also shows somewhat intermingled multi-centennial (0.3, 0.5 and 0.7 kyr) cycles around the maxima of ¹⁰Be, which likely represent solar cycles in this period (note: 200-yr Suess/de Vries cycle is under the detection limit of our record). On the other hand, the 2.2–2.4-kyr Hallstatt cycle and 1-kyr Eddy cycle, which are frequently documented in Holocene cosmogenic records, are not dominant in our records.