The stable isotopic ratio and minor and trace element compositions in marine biogenic calcium carbonate, such as coral skeleton and bivalve shells, are useful tools for the paleoclimate reconstruction. However, these chemical compositions are also affected by biological processes (vital effect). For the accurate paleoclimate reconstruction, understanding the vital effect mechanism is essential. This study present new data and discuss the elemental incorporation mechanisms by biological processes, mainly based on the micro-scale elemental distributions.

To develop the analytical procedure using a high lateral resolution secondary ion mass spectrometer (Nano-SIMS NS50) installed at the Ocean Research Institute, the University of Tokyo, the chemical compositions of four natural calcium carbonate samples were analyzed by three analytical methods. Concentrations of minor (Mg and Sr) and trace (Ba and U) elements were first analyzed by inductively coupled plasma mass spectrometry (ICP-MS) after chemical dissolution and calibrated against a standard dolomite. Their homogeneities were checked by in situ laser ablation (LA) ICP-MS with 10-20 spots. The carbonate samples were measured by using a high lateral resolution secondary ion mass spectrometer (NanoSIMS NS50). A~4 nA O- primary beam was used to sputter a 5-6 μ m diameter crater on the sample surface and secondary positive ions were extracted for mass analysis using an accelerating voltage of 8 kV and a Mattauch-Herzog geometry. Multi-collector system was adjusted to detect (26)Mg+,(43)Ca+,(88)Sr+,(138)Ba+,(238)U(16)+and (238)U(16)O(2+)ions at the same time. Resolving power of 2500-5000 at 10% peak height was attained by entrance slit set at 40 μm and each exit slit at 50 u m with adequate flat topped peaks. Observed (26)Mg(+), (43)Ca+, (88)Sr+, 138Ba+, and (238)U(16)O(2+) ratios agreed well with those measured by LAICP-MS, confirming their reported precision and accuracy.

To evaluate the relationship among chemical compositions, environmental factors and the vital effect, minor and trace elements of modern deep-sea corals were measured in bulk individuals and skeletal micro-structure. Deep-sea corals hold great potential as a key to important aspects of paleoceanography for at least two reasons, 1) they offer temporal high resolution records of deep-sea environment, because they have growth banding structures, 2) and they are good samples for studying vital effects, because the deep-sea environment does not change over short time scales. However, the relationship between the chemical composition of deep-sea coral skeletons and environmental factors is not well understood. The oxygen isotopes and chemical compositions of deep-sea corals were measured in this study. Among the bulk individuals,δ(18)0 value and Sr/Ca ratio show a negative but weak correlation with ambient temperature. On the other hands, the Mg/Ca ratio has a positive, weak correlation with the temperature. Large variations were found among samples collected from similar temperature. The variation is up to 3.8%o for δ(18)O, 0.9 mmol/mol for Sr/Ca ratios, and 0.78 mmol/mol for Mg/Ca ratios among samples with ambient average temperature within 1℃. This variation may be due to a large vital effect. The centers of calcification (COCs), which was formed at high calcification rate, has lower Sr/Ca, U/Ca and higher Mg/Ca ratios than surrounding fasciculi. This chemical distribution supports the model that elemental incorporation derived from Rayleigh fractionation. This suggests that calcification rate is a very important factor for the chemical composition in deep-sea corals and is one of the most significant mechanisms of the vital effect. Because of the large vital effect, further investigations are essential to use the deep-sea coral as a temperature proxy.

To investigate the elemental incorporation mechanism into coral skeletons, chemical and isotopic compositions of Acropora nobilis skeleton were analyzed at various spatial resolutions. Branching corals Acropora consist of fastgrowing axial corallite and slowly growing radial corallite at the visible scale. On the other hands, at the micro-scale, there are several types of skeletal elements precipitated under different calcification rate. The chemical profiles of both axial and radial corallite along with growth axes were measured by conventional ICP-MS and Stable Isotope Mass Spectrometry. The tip and basal parts of Acropora nobilos skeletons were also analyzed at microscale. The Mg/Ca, Sr/Ca, Ba/Ca, and U/Ca ratios were measured in ~8μm diameter spots by using NanoSIMS, and Mg, Sr, Ca, and S distributions were analyzed by Electtron Probe Micro Analyzer (EPMA), with a spatial resolution of~tm. Based on the elemental distribution obtained by EPMA, the Acropora's skeleton are composed of more than three types of the skeletal elements, "Framework", "Infilling" and High Mg Low S" skeletons. Observation of skeletal structure revealed that the skeletal porosity decreased with distance from the tip, since "Infilling" skeletons filled the voids of "Framework" skeletons. Micro-scale elemental analyses (EPMA and NanoSIMS) revealed that "Infilling" skeletons have lower Mg/Ca and higher Sr/Ca and U/Ca than "Framework" skeletons. Since the "Infilling" skeletons were probably formed under the slower calcification rate than "Framework" skeletons, the elemental fractionation pattern between two skeletal elements is consistent with the Rayleigh fractionation model. The chemical profiles of axial corallite along with the growth were significantly affected by the proportions of "Infilling" skeletons.

To evaluate the effects of hydrothermal and/or biological activity on trace element/Ca ratios in the bivalve shell, the chemical compositions in the cross sections of deep-sea mussel (Bathymodiolus platifrons) shell were analyzed by using micro analytical technique. The Mg/Ca, Sr/Ca, Mn/Ca, and Ba/Ca ratios were measured in ~8μm diameter spots by using NanoSIMS, and Mg, Sr, Ca, and S distributions were analyzed by Electtron Probe Micro Analyzer, with a spatial resolution of ?2μm. After these analyses, shell microstructures were observed by SEM to evaluate the relationship between elemental distributions and shell microstructure. The inner aragonitic layer is composed of three kinds of shell microstructures. The organic-rich layer and etchresistant minerals were interlaminated sporadically among ordinary deposited nacreous layers. All elements showed large variations associated with shell microstructure changes, implying that they are mainly controlled by biological processes. Comparing with the nacreous layers, Mg, Sr, Ba, and S were concentrated in intracrystalline organic materials located at organic-rich layers. In contrast, Mn/Ca variations were primarily coupled with shell microstructure; with the low Mn/Ca in etchresistant minerals. Such element profiles are not consistent with the expected variation from temperature and/or environmental change around hydrothermal vents, indicating that these element ratios are not direct hydrothermal proxies. To use trace element/Ca ratios as paleoceanographic proxies, observation of shell structures and evaluation of the vital effect are essential.

At the microscale, chemical compositions show large heterogeneity associate with skeletal microstructures, indicating that the biological effects are dominant. Both deep-sea coral and branching coral, the Rayleigh fractionation may be the dominant process affecting the micro-scale elemental distribution. In the case of deep-sea hydrothermal mussel, localized organic materials are the main controlling factors for the micro-scale elemental distributions.

本論文は7章からなる.第1章はイントロダクションであり,生物源炭酸塩中の微量元素組成に関する背景について述べられている.生物源炭酸塩中の微量元素組成は定量的に環境を記録していると考えられており,古環境復元の研究に広く応用されてきた.しかし,微量元素組成は生物活動によっても変化する事がわかっており,古環境復元の不確かさの要因となっている.微小領域の元素分布は石灰化による元素分別に関して重要な知見を与えてくれ,微小領域の元素変動パターンから元素取り込みの仕組みを評価することが本論文の主旨である.

第2章では二次元高空間分解能二次イオン質量分析法(NanoSIMS)を用いた炭酸塩分析法の開発について述べられている.本論文では新しく開発された二次元高空間分解能二次イオン質量分析計を用いて天然の炭酸カルシウムを分析する手法を確立した.その結果,従来の二次イオン質量分析法の空間分解能の半分以下である,約5μmの領域でMg/Ca, Sr/Ca, Ba/Ca, U/Ca比を同時分析する手法を確立した.

第3章では深海サンゴ骨格の微量元素変動について述べられている.深海サンゴは海洋深層循環を高解像度で記録している可能性がある試料であり,元素取り込みに与える生物活動の影響を評価するのに適した試料でもある.本論文では,深海サンゴのバルク組成と水温との比較を行なった結果,水温と弱い相関を示すことを明らかにした.また,深海サンゴ骨格を微小領域で分析した結果,骨格構造に関係した大きな元素不均質があることが明らかになった.

第4章では枝サンゴ骨格の微量元素変動について述べられている.枝サンゴはサンゴ礁の主要な構成要素であるため,古環境復元に重要な試料である.本論文では枝サンゴを様々な空間分解能で分析を行い,化学組成と環境因子の関係について議論している.枝サンゴの主成長軸に沿ってバルク分析した結果,Sr/Ca, Mg/Ca, U/Caは水温依存性を示さなかった.微小領域分析の結果から,隙間を埋めるように形成される二次骨格が成長に従い割合が増加し,骨格構造を形成する一次骨格と比較して,二次骨格はより高いSr/Ca, U/Ca比と低いMg/Ca比を示す事を明らかにした.

第5章ではシンカイヒバリガイ殻の微量元素変動について述べられている.本論文では,NanoSIMSを用いた定量分析,電子プローブマイクロアナライザーによる定性面分析,マトベイズ染色液を用いた有機物染色,電子顕微鏡による骨格構造観察を組み合わせることで,元素取り込み過程を総合的に評価している.その結果,シンカイヒバリガイに関しては,有機物が局所的に濃集している部位にMg, Sr, Baが濃集しており,有機物含有量が微量元素組成に大きな影響を与える可能性があることを明らかにした.

第6章では本論文で得られた微量元素変動パターンを基に,古環境復元への応用の可能性について議論している.

第7章では本論文で得られた結果の要約が述べられている.

本研究は,NanoSIMSという最先端の機器を用いて,初めて微小領域での深海サンゴなどの生物源炭酸塩についてUを始めMg, Sr, Ba, Sの元素について定量的に測定を可能にしたもので新しい分析方法を確立したと言える.さらに,骨格には微細構造に高い不均一性が認められ,生物源炭酸塩の構造と化学組成との間に相関のあることが初めて見いだされた.これらの研究成果は,将来の古環境解析に新しい手法と分野を切り開いたと言える.

なお,共同研究に関しては.本論文第2章は,佐野有司・高畑直人・平田岳史・Niel C. Sturchioと,第3章は日下部実・中井俊一・石井輝明・渡邊剛・比屋根肇・佐野有司と,第4章は渡邊剛・高畑直人・天川裕史・川島龍憲・岨康輝・佐野有司と,第5章は小俣珠乃・山本啓之・高畑直人・佐野有司との共同研究の成果である.しかし,論文提出者が主に分析及び検証を行なったもので,論文提出者の論文への貢献は本質的な部分で特に高く,寄与は十分であると審査委員全員が判断した.

以上の理由より,審査委員会は本論文を提出した白井厚太朗氏に博士(理学)の学位を授与できると認めた.