Periodic variation extracted from lacustrine laminated diatomite of the middle Pleistocene Hiruzen-bara Formation, Okayama Prefecture.

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  • 中期更新統蒜山原層（岡山県）の湖成縞状珪藻土層に見られる周期変動
  • チュウキ コウシントウ ヒルゼンバラ ソウ オカヤマケン ノ コセイ シマジョウ ケイソウドソウ ニ ミラレル シュウキ ヘンドウ

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Abstract

The Middle Pleistocene (∼0.5 Ma) Hiruzen-bara Formation exposed in Hiruzen-bara Highland, Okayama Prefecture, consists of lacustrine varved diatomite. It is composed of alternation of about 2 mm thick couples of light-colored and dark-colored laminae. Because light-colored laminae is made mostly of Stephanodiscus sp. breeding in winter season, and dark-colored lamina of Cyclotella comta, a single couplet of these laminae is suggested to represent an annual deposit. Thickness of 8000 varves are sequentially measured for photo film under microscope. Time series variations of lamina thickness indicating productivity of diatoms are examined by spectral analyses, and environmentally induced fluctuations were extracted. Modified time series of thickness variations by numerical filters are examined by spectral analysis (FFT : fast Fourier transformation), and dominant periods were obtained as 9-10.5 and 32 years, which are, however, not always significant through the sequence.To improve precision and quality of the thickness data, the digital images of the varves are also analyzed, where more distinct 4-5, 9.1-11.6, 22 (FFT), 34-36 and 100 year periods (MEM : maximum entropy method) are detected. However, the amplitude of 11 years periodicity even in the most distinct interval is less than 20% of the total variations, and is fluctuating from hundred to several hundred years interval. The potential factor causing 11 year periodicity in the varved diatomite is sunspot cycle, which varies from 9 to 13 years. As the pure diatomite of the Hiruzen-bara formation is a highly sensitive record of the lake environments in the past, the dominant periods obtained from microscopic measurement as well as image analysis demonstrates the time series fluctuations of diatom blooming, which is most likely affected by climatic change due to periodic solar activity, even masked by other non-periodic environmental changes.

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