著者

渡邊 隆広 奈良岡 浩 西村 弥亜 中村 俊夫 仙田 量子 河合 崇欣

出版者

名古屋大学年代測定資料研究センター

雑誌

名古屋大学加速器質量分析計業績報告書

巻号頁・発行日

no.15, pp.199-205, 2004-03

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バイカル湖湖底堆積物試料中の高分子直鎖状炭化水素、及びバクテリア由来有機分子であるホパノイド化合物の分子レベル安定炭素同位体比分析を行い、過去における湖内バクテリア活動の復元と環境変化の考察を行った。最終氷期から完新世への移行期において、メタン酸化バクテリアの寄与が増大していたことが明らかになった。この結果は、過去における気候変動に対応して湖水循環が停止、もしくは極めて弱くなっていたことを示唆する。\\Several organic geochemical studies on sedimentary photosynthetic pigments, lignin phenols and lipids from vascular plants have been conducted using Lake Baikal sediment core [Orem et al., 1997; Tani et al., 2002]. However, these investigations have not elucidated past limnological conditions such as redox changes in Lake Baikal. The sulfur isotope compositions of pyrite indicated a variation of SRB activity in response to glacial/interglacial climate changes [Watanabe et al., 2004]. In this study prokaryote biomarkers including hopanoid and acyclic isoprenoid compound will be examined for the sedimentary profiles of bacterial activities including methanotrophic bacteria. Organic compounds of higher plant origin, such as high molecular n-alkanes, n-ketones and n-alkanols are relatively abundant in the Ver98-1 St.5 sediment core (up to 5.6, 1.8, and 11.7 μg/g dry sed., respectively). The abundance of partly autochthonous organic matter, such as phytol and low molecular alkanols is relatively low. It has revealed significant difference in the molecular composition between the warm periods and the cool periods, which is consistent with the organic carbon and total nitrogen concentration and TOC/TN ratios. The increase in terrigenous organic molecules in the warm periods can be explained by the increased inflow of river water. In the deepest part of modern Lake Baikal, ca. 9 mg/l dissolved oxygen occurs, which suggests a large-scale vertical convection in the lake in spite of its great depth (1634m). Watanabe et al. [2003] suggested less-oxic conditions of Lake Baikal bottom water in climate transition periods, such as Younger Dryas (YD), based on high TS contents (up to 13mg/g dry sed) and high TS/TOC ratios (up to 0.5 atomic ratio), being much larger than average TS/TOC ratios of freshwater and normal oxic marine sediment. The high TS/TOC are usually observed not only as a result of less-oxic condition but also diagenetic pyritization. We have measured δ^<13>C of bacterial biomakers and δ^<34>S of pyrite through the YD event (ca.12kyr B.P.). Hop-17(21)-ene is the most abundant among unsaturated hopanoids throughout the Holocene and YD sediment (32-68 ng/g dry sediment). Furthermore, high amounts of saturated hopanoids are present with 17δ(H), 21δ(H)-homohopane and 17δ(H), 21δ(H)-hopane dominating (up to 1045 and 381 ng/g dry sediment, respectively) in the Holocene. 17δ(H), 21δ(H)-homohopane and 17δ(H), 21δ(H)-hopane vary isotopically in a narrow range from -30.8 to -34.7‰, which suggests that the δδ hopanes could be derived from chemotrophic bacteria. Hop-17(21)-ene in the YD becomes more depleted in ^<13>C (〜-43‰) than that in the Holocene by 〜10‰, implying an increase of methanotrophic bacterial activity. In addition, squalane is depleted in ^<13>C (-46.2‰) compared to the total organic carbon by 20‰. Such ^<13>C-depleted squalane suggests that archea have directly or indirectly introduced ^<13>C-depleted methane-derived carbon into the biomass. The methane-involving anaerobic bacterial activity suggests the decrease of dissolved oxygen content in deeper part of Lake Baikal at the YD rapid cooling event. These isotopic signatures suggest that less oxic conditions associated with a water circulation change occurred rapidly within a time interval of less than 1000 yr in response to a global climatic change at the late Quaternary. Since lake water circulation exerts a great influence especially on the geochemical cycle and biological activity in Lake Baikal, reconstruction of paleo-redox conditions and water circulation changes in the lake are indispensable to identify detailed changes in biological activity and lake ecosystems with respect to climate changes.タンデトロン加速器質量分析計業績報告 Summaries of Researches Using AMS 2003 (平成15)年度