Departmental Bulletin Paper フィリピン,ルソン島中央部のパイタン湖のコア試料の層序と古環境

山崎, 圭二  ,  鳥井, 真之  ,  中西, 利典  ,  バリソ, エリクソン  ,  リベラ, ダニコ  ,  リム, ロブジュネリエアア  ,  ポガイ, キャシー  ,  ダアグ, アルトロ  ,  中村, 俊夫  ,  ホン, ワン  ,  鹿島, 薫  ,  檀原, 徹  ,  藤木, 利之  ,  奥野, 充  ,  Yamasaki, Keiji  ,  Torii, Masayuki  ,  Nakanishi, Toshimichi  ,  Bariso, Ericson B.  ,  Rivera, Danikko J.  ,  Lim, Robjunelieaaa B.  ,  Pogay, Cathy  ,  Daag, Arturo  ,  Nakamura, Toshio  ,  Hong, Wan  ,  Kashima, Kaoru  ,  Danhara, Tohru  ,  Fujiki, Toshiyuki  ,  Okuno, Mitsuru

27pp.145 - 150 , 2016-03 , 名古屋大学宇宙地球環境研究所年代測定研究部
Description
Paitan Lake located on the northern part of Luzon Central Plain, is a tuff ring in the Amorong monogenic volcanoes. The lake has 0.8km^2-surface area, 38.5km^2-drainage basin, and one outflow-river with a few meter of width in the northern side. In order to establish the tephrostratigraphy in Luzon Island, the Philippines, two sediment cores, PPL-1 (13m in depth) and PPL-2 (28m in depth), were obtained from this lake. These cores consisted of two units: an upper alternating bed of mud layers and peaty layers in a depth range from 0 to 6m, and a lower dark mud bed from 6 to 28m. Seven tephra layers were observed in the cored sediments. Phenocryst assemblage implies that these are originated from Pinatubo volcano located ca. 90km SW from this lake. To detect cryptotephra, initial magnetic susceptibility was measured at 2-cm interval. Soft X-ray photographs were taken to interpret deformation structures during the drilling process. Radiocarbon dates of seven plant fragments and twelve organic sediments were measured by the AMS facility of KlGAM, the Institute of Accelerator Analysis Ltd. and ISEE of Nagoya University, respectively. Inferred age for four tephra horizons is consistent with previous dates for Pinatubo eruptions. To discuss paleoenvironment changes of this lake, diatom analysis with 20-cm intervals is conducted. Attached diatoms indicating marsh or peat land have emerged in cored sediments of 0 to 3m in depth with high CN ratio. From 3.2 to 28m in depth, the freshwater planktonic diatoms become dominant species and indicete freshwater lake environment. Aulacoseira granulate is dominant in 3.2 to 6m in depth, on the other hand, deeper part than 6.6m, Aulacoseira muzzanensis is dominant. Both species may indicate changes of water depth of the lake. Furthermore, only around 25m in depth, Cyclotella radiosa is dominant. At this horizon (ca. 20 cal kBP), organic carbon content and CN ratio also increase. This signal may indicate some event at Last Glacial Maximum (LGM). フィリピン,ルソン島のパイタン湖はAmorong単成火山群のタフリングで,閉鎖的な環境が持続されてきた.筆者らは,湖畔で深度28mのコア試料を採取し,珪藻群集分析,軟X線写真撮影,14C年代測定,火山灰分析,初磁化率測定,炭素・窒素含有量測定を行った.植物片7点と有機質堆積物12点の14C年代から,得られたコア試料は,最近約24,000年間を記録し,その堆積速度は 0.8~2.4m/kyrである.6 枚のテフラ層が検出され,岩石記載学的特徴からピナツボ火山が起源である.珪藻群集では,深度0~3mで付着性珪藻が出現し,沼沢もしくは泥炭地と推定された.3m以深は淡水浮遊性珪藻へと優占種が変化し,淡水湖沼環境であったことを示す.さらに深度6m付近でAulαcoseira granulateからAulacoseira muzzanensisへ優占種が変化した.最終氷期極相期(LGM)に当る深度25mでのみCyclotella radiosaが優占種となり,何らかのイベントを示すと考えられる.
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