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A geochemical study on trace element behavior in sediments and soil formation on atolls, central Pacific

伊藤, 理彩 東京大学 DOI:10.15083/0002001440

2021.09.08

概要

【本研究の新規性】
①有孔虫を主体とする環礁堆積物は短期間で堆積したことを示し、その上層部に形成された黒色層(土壌)を初めて物質科学的に解析した。（Ito et al., 2018a,b）
②環礁堆積物で酸性化が起きており、それが人為的影響ではなく炭素・窒素循環の活発化によること、またそれが土壌化に寄与することを解明した。（Ito et al., 2018a）
③堆積完了後に新たに付加された人為起源物質と自然起源物質を特定し、堆積物中の重金属の固定メカニズムを解明した。（Ito et al., 2018b)

【研究背景および目的】
環礁州島は、炭酸カルシウム(CaCO3)を主成分とした有孔虫やサンゴ礫などの生物遺骸が離水サンゴ上に堆積した島である。これまで、環礁州島形成過程に関する先行研究は数多くあるが(e.g., Yasukochi et al., 2014)、堆積物中に含まれる各元素の起源および挙動、また近代化に伴う人為的影響については不明な点が多い。先行研究ではDeenikら(2006)が、マーシャル諸島の堆積物の構成元素や各地点別のpHについて調査を行っているが、連続した深度から採取された試料ではなく、年代情報も欠如していたため、環礁のような孤立環境下で、かつほとんどがCaCO3のみで構成された堆積物にどのような作用が生じて土壌化が促進されたのかは明らかになっていない。また人為的影響についても、これまでの研究(e.g.,Fujita et al., 2014; Arimoto et al., 1985)では環礁付近の海域や沿岸域、またはエアロゾル中に含まれる元素の濃度分析とその因子解析が主流となっており、元素の実際の起源同定には不確かさが残っていた。そこで本研究では、環礁堆積物の堆積年代・堆積過程を明らかにすると共に、環礁堆積物・黒色層（土壌）の地球化学的・鉱物科学的分析から、これらの物質が反映する情報を明らかにすることを全体の目的とした。1、2章では、環礁堆積物の組成における特徴や堆積物中に含まれる元素の深度プロファイルを作成するとともにマジュロ環礁などにおける複数の島での堆積年代を明らかにすることを目的とした。それらを踏まえて3、4章(研究1)では、環礁堆積物の地球化学・鉱物科学的考察から、環礁堆積物の表層で酸性化が起きていることを明らかにし、土壌化プロセスを促進した要因を定量化することを目的とした。5章（研究2）では、環礁堆積物の環境化学的考察から、環礁への微量元素の起源とそれらの供給過程、また堆積物中での移行挙動を明らかにするとともに重金属が環礁堆積物から下方に流出した場合の環境への影響についても明らかにすることを目的とした。

【試料採取および分析方法】
試料は、マーシャル諸島マジュロ環礁のLaura、Calalen、Jelto、Delapの4島の内陸部・外洋側・ラグーン側から深度別に採取した。また、マジュロ環礁の東側のアルノ環礁のKebjeltakの内陸部、KebjeltakとAjeltokrok間のcausewayからもサンプルを採取した。

研究１においては、酸性化の指標として土壌pH(土壌と水を1:2.5で混合して測定されるpH; Soil Reaction Committee, 1930)を用いた。また主要堆積物の有孔虫の内部空隙率は、放射光X線μ-CT(SPring-8,BL37XU)の測定データをもとに再構築した二次元画像から得られた空隙率を積算することによって体積全体に対する空隙率に換算した。主要元素の測定には、蛍光X線分析法(XRF)を用いた。無機イオンの定量にはイオンクロマトグラフィー、インドフェノールブルー法を用いた。有機物の定量には堆積物の水抽出溶液の滴定と堆積物固相側と溶液側のTOC分析を行った。特に、固相側の有機物の同定には固体13C-核磁気共鳴(NMR)法を適用した。また硝酸中の三酸素同位体および窒素同位体の分析においては脱窒菌法を適用した。さらに、有孔虫内の鉱物割合の算出にはX線回折(XRD)においてリートベルト法を適用した。また放射性炭素(14C)年代測定法においては、加速器質量分析(AMS)を行い、得られた年代結果に対してBayesianage-depthモデルを適用し、腐植の年代を決定した。

研究２においては、堆積物中の主要元素の測定にXRF法、堆積物・地下水中の微量元素の測定に誘導結合プラズマ質量分析法(ICP-MS)を用いた。また元素の濃集部位を明らかにするために、走査型電子顕微鏡(SEM)や放射光を利用したμ-XRFを用い、表層で濃度が高くなっていることが確認された各元素の起源の特定には、X線吸収微細構造(XAFS)のうち、X線吸収端近傍構造(XANES領域)の解析を行った。また堆積物試料に対して酢酸抽出実験を行い、重金属の中でも水中の生物多様性に悪影響を与えやすい亜鉛(Zn)(e.g., Greene et al., 1975)について、溶出しやすい化学種、不溶性の化学種について特定するとともに、Znの溶出率の定量を行った。また堆積物の年代は、堆積物中の主要構成物である底生有孔虫(Calcarina.spp)に14C年代測定法を適用することにより決定した。

【結果および考察】
研究１：深度1m以浅において、土壌pHの低下に伴い有孔虫の空隙率が増加し、一方で高マグネシアンカルサイト（Mg-calcite）の割合が減少していた。SEMのマッピングの結果とXRDのリートベルト法により、土壌pHの低下によって、Mg-calciteが選択的に溶出し、有孔虫の周辺部やチャンバー・仮足痕の内側にcalciteが再沈殿していることが分かった。また、土壌pHの低下とともに硝酸イオン(NO3-)濃度と有機酸濃度が増加する傾向があり、これらの酸が土壌pHの低下に寄与していることが示唆された。これらの酸性物質の起源を特定するため、NO3-については脱窒菌を用いてN2Oを抽出し、硝酸由来の窒素と酸素について同位体比測定を行ったところ、採取した試料の全てのΔ17Oがほぼ0‰に近い値を示したことから、試料中のNO3-は大気由来ではなく、土壌有機物由来のアンモニアが微生物により硝化されて生成したものである可能性が高いことが示された。一方、堆積物中に含まれる有機物については、13C-NMRの結果より、腐植物質(フルボ酸)であることが判明した。特にNO3-濃度がpH7.5-8.2の範囲で上昇したのは、土壌pHが有機酸によって硝酸菌の至適pH(e.g.,Antoniou et al., 1990)まで下がったことにより、硝酸菌の活動が活発化した可能性が高いことが考えられる。一般的な土壌では、硝化が活発に行われると土壌pHがさらに低下するため、至適pHを下回った環境下で硝酸菌の活動は抑制されるが(e.g.,Painter and Loveless, 1983)、環礁のようなCaCO3主体の堆積物においては、CaCO3による中和反応が起き、硝酸菌の至適pHが維持されている可能性が高い。これらの結果により、(i)高い土壌pHの範囲では有機酸が、(ii)硝化細菌の至適pHの範囲ではNO3-が特に土壌pHの低下に寄与したと考えられる。NO3-は植物の養分になるほか、土壌pHの低下に寄与し、生育可能な植物種の増加に寄与する。また、土壌pHの低下は有孔虫の溶解による堆積物粒子の細粒化を促進し、堆積物の保水力を上げることからも植生の発達に寄与し、腐植の増加につながったと考えられる。例えば、隆起サンゴ礁では初期土壌が形成されるまで約4000年以上の年月がかかるが(Maejima et al., 2011)、本研究の腐植の年代測定の結果から、環礁州島の場合は島形成後、約200年以内という短期間での土壌の生成が起こったことが明らかとなった。この結果からも、環礁においては活発な窒素循環が起きていること、植物などの有機物の生成が盛んに行われ、土壌化が促進されたことが示唆される。

研究２：有孔虫の14C年代測定の結果より、Lauraの最上部の堆積物の年代は最も新しいもので約1670calBPを示したため、1900年代以降の島の都市化が起こる前に、現在の表層堆積物は既に堆積が完了していたことがわかった。Alは表層で濃度が高かったが、本試料から求めた2000年間のマジュロ環礁でのエアロゾル由来のAlの沈着量は、マジュロ環礁近傍の遠洋性堆積物中のAlの総量や観測によるエアロゾル中のAl濃度と整合的であった。深度50cm以浅で濃集している元素について得たEnrichment Factor (EF)(Alで規格化;Kempetal.,1976)から、Zn、Cu、Pは人為等の外来起源、Ti、Fe、希土類元素（REE）は自然起源である可能性が高いことがわかった。中部太平洋のダスト輸送に関する先行研究(Merrilletal.,1989)の結果も踏まえると、Alなどの陸源物質は、約2000年前の島の急速な堆積の後、表層が長期間露出した間に大陸から輸送され、堆積物表層に付加したと考えられる。またREE濃度とそのパターンからも、堆積物の上層は濃度が増加し、下層に比べて負のCe異常が小さいことからCe異常のない大陸物質が付加したことが示唆される。一方、人為起源と考えられるZnは、XANES解析の結果、亜鉛と銅の合金の微粒子（真鍮と考えられる）として存在しているほか、鳥の糞中などに含まれているリンが有孔虫のカルシウムと反応して二次的に生成されたアパタイトや有孔虫のカルサイトに共沈し、固定されることが分かった。その結果、Znは地下水（淡水レンズ）には移行しにくく、環礁においては天然の有害元素除去機構が働いていることが、本研究によって明らかとなった。

【本研究の成果まとめ】
①複数の環礁島で、2000～700年前に一度に表層まで堆積が起こったことを明らかにした。
②島の堆積完了後、表層が長期間露出したことにより、約2000年間に渡って大陸地殻由来の物質が大気経由で付加したことを明らかにした。
③初期の植物により有機酸が放出され、硝化細菌の至適pHとなり、硝化が活発に行われるが、同時にCaCO3による中和反応も起き、硝化細菌にとっての至適pHが維持された環境となったと考えられる。その結果、約200年以内という短期間で腐植が生成されたことを明らかにした(隆起サンゴ礁より早く土壌化が進行していたことが分かった)。
④近現代に排出された重金属は、有孔虫表面に二次的に生成したアパタイトに共沈するため、堆積物の上層部で固定され、地下水には移行しにくいこと、すなわち、環礁には天然の重金属元素除去機構が備わっていることを明らかにした。

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