堀 真子

<title>Abstract</title> We report here hourly variations of Mg/Ca, Sr/Ca and Ba/Ca ratios in a Mediterranean mussel shell (<italic>Mytilus galloprovincialis</italic>) collected at the Otsuchi bay, on the Pacific coast of northeastern Japan. This bivalve was living in the intertidal zone, where such organisms are known to form a daily or bidaily growth line comprised of abundant organic matter. Mg/Ca ratios of the inner surface of the outer shell layer, corresponding to the most recent date, show cyclic changes at 25-90 mm intervals, while no valuable variations are observed in Sr/Ca and Ba/Ca ratios. High Mg/Ca ratios were probably established when the bivalve was located at low tide with a reduced supply of Ca from seawater. Immediately following the great tsunami induced by the 2011 Tohoku earthquake, Mg/Ca enrichment occurred, up to 10 times that of normal low tide, while apparent Ba/Ca enrichment was observed for only a few days following the event, implementing a proxy of the past tsunami. Following the tsunami, periodic peaks and troughs in Mg/Ca continued, perhaps due to a biological memory effect as an endogenous clock.

Terrestrial carbonates, such as tufas and stalagmites, are unique archives in terms of providing paleo-environmental information of land area, especially in mid-latitudes where records of ice cores and corals are absent. The oxygen and carbon stable isotopic values (δ¹⁸O and δ¹³C) are the most fundamental proxy records, which reflect relative changes in hydrological condition and vegetation, respectively. The recent challenge focuses mainly on how we quantitatively estimate temperature, precipitation and geochemical circulation in the past. Here, I first review the traditional δ¹⁸O studies using tufas and stalagmites, and then introduce my challenge to evaluate multiple carbon sources in karst aquifer. Finally, I extended the mixing and partitioning model to the multi-tracers, such as Sr/Ca and REE/Ca ratios. Multi-tracer analysis is one of solutions to understand details of a unique karst system, which is involved by complex geochemical circulation and associated paleo-environmental changes in the catchment area.

Chemical and isotopic compositions of water were measured at 46 carbonate hot-springs in Japan, by applying the methods avoiding the post-sampling alternation. Measured stable isotopic values indicate that CO2 of an endogenic origin was dissolved in water of a meteoric origin, in many cases. The carbonate springs are generally in intermediate temperature for sustaining a high solubility of CO2, and near-neutral pH as a result from the reactions between weak carbonate acid and rock-forming minerals. Although carbonate acid effectively dissolved carbonate minerals, this reaction is often unimportant among the water-rock reactions because the many water samples appear alkalinity-excess, relatively low Ca concentration, and high dissolved silica. However, a significant dissolution of 13C-enriched carbonate minerals was recognized in some samples showing a high carbon isotopic value and high equilibrate CO2 partial pressure. Dissolved oxygen content implies a contact with the atmosphere or mixing with oxidative underground water, both of which decrease the initial CO2 partial pressure and the concentration of dissolved chemical components.日本国内の46ヶ所の炭酸泉について、採水後の成分変質に留意した方法で、化学組成・同位体組成の潤定を行った。安定同位体の測定値は、深部起源の二酸化炭素が天水起源の水に溶解したことを示す。炭酸泉は一般に、高い二酸化炭素の溶解度を保持するために中温であり、弱酸である炭酸と造岩鉱物の中和反応により中性のpHを示す。炭酸は効果的に炭酸塩鉱物を溶解する性質を持つが、多くの水試料が過剰なアルカリ度、比較的低いカルシウム濃度、高い溶存シリカ濃度を示すことから考えると、炭酸塩鉱物の溶解反応は重要でないことが多い。ただし、高い溶存炭酸の炭素同位体比と高い平衡二酸化炭素分圧を示す試料では、13Cに富む炭酸塩鉱物の溶解が重大に起こっていた。溶存酸素濃度は大気との接触もしくは酸化的な地下水との混合を示し、このような場合、初生的な二酸化炭素分圧と溶存化学成分の濃度は低下する。

We verified the appropriate conditions and the precisions of stable isotopic analyses for carbonate and water samples using a mass spectrometer with Gas Bench. Standard deviation and reproducibility of the analyses are dependent on the amount of carbon dioxide obtained from samples, and are minimized when the amount of carbonate sample of 0.09-0.14 mg. Isotopic equilibrium is obtained in l hour at 50℃ after the reaction of carbonate and purified phosphoric acid. On these conditions, the reproducibility was±0.19‰ forδ^13C, and±0.22‰ for δ^18O(2SD). In the case of water samples, the appropriate sample amount was 0.75-1.0 mL for seawater, and 0.5 mL for calcareous freshwater that includes 3-4 mM of dissolved inorganic carbon. Reproducibility of ±0.04‰ (2SD) is gained for δ^13C after the reaction for 20 hours at 25℃. Oxygen isotope reaches the equilibrium in 24 hours after the reaction, which reveals the reproducibility of ±0.18‰(2SD). Standard deviation and reproducibility in any cases are within a required level for geoscientific researches on the marine and terrestrial specimens.

To assess the reliability of the Liquid Scintillation Counting method, we monitored the weight and counting ratio of a benzene sample (STD059) prepared in 2000. Both benzene weight and the counting ratio generally decreased during the 2 years of analysis, at rates of 7 mg/yr and 0.073 cpm/gC/yr, respectively. The total decrease in the counting ratio was 97% of the initial value, as calculated using a linear approximation. The counting efficiency of STD059 was 71–73%, less than that for IAEA standards prepared in 2006–2007 (74.3–78.5%). Spectral quenching parameters (SQP) and Channel ratios (R) were similar between the STD059 and IAEA standards (C-1 to C-5), suggesting insignificant quenching of STD059. The temporal reduction in counting efficiency was possibly caused by density quenching, although the scintillator concentration of the solvent was lower than 15.9 g/L during the study period. According to the counting efficiency of STD059, we determined that the ¹⁴C activities of the IAEA standards were within 0.10-1.81 pMC of the values recommended by Rozanski et al. (1992), while the C-1 and C-4 samples showed~2% of modern carbon contamination, which resulted in a +1.0 pMC excess.

岡山県西部の北部大賀台長屋の沢は,全長約1200mに及び,大規模にトゥファが発達する.ここでは,石灰岩とその下位の砂岩頁岩相の低角スラスト境界から湧出した水が,長く急傾斜の流路沿いを流れる間にトゥファを堆積させている.湧水の水質の時期的変化は,これまで他地域で記載されてきた変化と類似する.ただし,夏期に溶存Ca^<2+>濃度が低く,水温の季節的変化幅が大きいという特徴があり,これらは長屋の地下水塊が小さいことを示唆する.長屋の沢のトゥファには夏〜秋の緻密層と冬〜春の孔隙質層による,明瞭かつ規則的な年縞が発達する.孔隙質層中にはシアノバクテリアや珪藻が豊富に認められるが,緻密層には,径の大きな方解石結晶が発達し,微生物の痕跡は少ない.堆積物の組織的変化は,水質から計算された方解石沈殿速度のデータと矛盾し,むしろ降水や農業用水の流入を反映した水量の激変に関係すると考えられる.

北部中国山西省娘子関のトゥファの産状と安定同位体組成には古気候情報が記録されている.従来の研究によると,酸素同位体ステージ3にあたる5.5-3.5万年前にトゥファの堆積が最も盛んであった.この時期に堆積したトゥファの中には方解石結晶の粒度の違いを反映した縞状組織があり,酸素安定同位体比の高解像度分析結果により,これが年縞であることが示された.また,この古トゥファの酸素・炭素安定同位体比はいずれも現世トゥファの値よりも低い.水の酸素同位体比の値が同一であったと仮定すれば,この古トゥファが堆積した時期は現在よりも2-3℃気温が高かったと見積もられる.一方,低い炭素同位体比は土壌層での二酸化炭素生産が活発であるか,地下水系がより閉鎖的であった事を示唆する.トゥファの堆積作用が現在より活発であった酸素同位体ステージ3の期間には,大量の湧水が倒木を水没させる程度に深い流れを形成していた.活発なトゥファの堆積はより湿潤な気候を示す.この見積りは北部中国でのレス・古土壌シークエンスの研究結果と符合している.