Masako Hori

<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.

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.

Tufas are largely developed in a ca. 1200-m-long stream at Nagaya in the northern Oga Limestone Plateau (Takahashi City, Okayama Prefecture, SW Japan). The water mainly issues from the spring located at the thrust boundary between the Nakamura Limestone and the lower lying shale and sandstone unit, and deposits tufa along the steep stream. These conditions allow the water being supersaturated with respect to calcite by increasing Ca^<2+> content (>60 mg/L) and decreasing PCO_2 (<2.0 matm). Variation in chemical properties of the springwater, such as PCO_2 and SIc, resembles to the seasonal change, which has been previously described in other tufa localities in SW Japan. However, the springwater of Nagaya is characteristic in low Ca^<2+> content in summer and large seasonal variation in temperature, which suggest a small water mass in the underground system. Some of the tufa specimens display very regular annual laminations, which display clear boundary between summer dense laminae and winter porous laminae. Cyanobacteria and diatoms densely occur in the porous laminae, whereas the dense laminae is poor in microbial remnants and characterized by large rhombic calcite crystals. The textural change of the annual lamination is inconsistent to inorganic precipitation rate (PWP-rate) calculated by chemical data, but is rather ascribed to change in water flow due to agricultural drainage and seasonal rainfall pattern.

Paleoclimatic analysis was made on the basis of occurrence feature and stable isotopic compositions of freshwater carbonate tufas in Niangziguan, Shanxi Province, north China. Published dating study suggests that the tufas were most extensively deposited in a period of 55-35ka, which corresponds to oxygen isotopic stage 3. The tufas deposited in this period exhibit mm-order lamination. Results of high-resolution analysis of oxygen isotope indicate that it is annual lamination consisting of summer coarse-crystalline and winter fine-crystalline layers. Stable isotopic analysis reveals that the laminated paleotufa is lower in both of δ18O and δ13C values than the modern tufa. The difference in the δ 18O values indicates higher temperatures by two to three degrees for the deposition of the paleotufa, if is assumed that the isotopic compositions of water have been constant for several tens ky. The lower values in δ 13C can be ascribed to active production of CO2 in the soil layer, or to a relatively closed condition in the underground water system. The tufa may have deposited more actively during the oxygen isotopic stage 3 than today. Spring water issued in a significant amount, which formed deep flow covering bunches of fallen wood. The active deposition of paleotufa indicates a wetter climate. Assumption of this study agrees with the paleoclimatic analyses of loess-paleosol sequences in north China.