堀 真子

We report here hourly variations of Mg/Ca, Sr/Ca, and Ba/Ca ratios in a Mediterranean mussel shell (Mytilus galloprovincialis) 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 mu m intervals, while no interpretable variations are observed in Sr/Ca and Ba/Ca ratios. High Mg/Ca ratios were probably established by (1) cessation of the external supply of Ca and organic layer forming when the shell is closed at low tide, and (2) the strong binding of Mg to the organic layer, but not of Sr and Ba. 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, therefore serving 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.

In the East Asian monsoon area, stalagmites generally record lower and higher oxygen isotope (delta O-18) levels during warm humid interglacial and cold dry glacial periods, respectively. Here, we report unusually low stalagmite delta O-18 from the last glacial period (ca. 32.2-22.3 ka) in Fukugaguchi Cave, Niigata Prefecture, Japan, where a major moisture source is the East Asian winter monsoon (EAWM) that carries vapor from the warm surface of the Japan Sea. The delta O-18 profile of this stalagmite may imply millennial-scale changes, and high delta O-18 intervals that are related to Dansgaard-Oeschger (D-O) interstadials. More importantly, the stalagmite exhibits low overall delta O-18 values; the mean delta O-18 (- 8.87 parts per thousand) is distinctly lower than the mid-Holocene mean of another stalagmite from the same cave (4.2-8.2 ka, - 7.64 parts per thousand). An interpretation assuming a more intense EAWM and greater vapor transportation during the last glacial period, compared with the mid-Holocene, contradicts the limited inflow of the Tsushima Warm Current into the Japan Sea because of lowered sea level. Additionally, our model calculation using delta O-18 data from meteoric water indicated that the amount effect of winter meteoric water was insignificant (1.2 parts per thousand/1000 mm). Low stalagmite delta O-18 for the last glacial period in Fukugaguchi Cave most likely resulted from O-18-depleted surface water, which developed in the isolated Japan Sea. The estimated amplitude of the delta O-18 decrease in surface water was similar to 3 parts per thousand at most, consistent with the abnormally low values for foraminifera (by similar to 2.5 parts per thousand) in sediment during the last glacial period, shown by samples collected from the Japan Sea. This is the first terrestrial evidence of O-18 depletion in Japan Sea surface water during the last glacial period.

Our understanding of stalagmite delta O-18 paleoclimate records would be significantly improved by determining the relative influence of the two factors, temperature and water delta O-18 Here we apply carbonate clumped isotope thermometry to a well-dated (18.1-4.5 ka) stalagmite, Hiro-1 from Maboroshi Cave in Hiroshima Prefecture, southwestern Japan. The Delta(47) values from 50 stalagmite layers, calibrated using Delta(47) -temperature relationships observed in modern stream tufa from Japan (Kato et al., 2019) yield estimates of paleo-temperature and water delta O-18 variability. Several layers affected by significant prior calcite precipitation under a dry climate display strong kinetic isotope influences: positive delta O-18 and negative Delta(47) deviations. Except for these layers, Hiro-Delta(47) records temperature changes that are broadly consistent with climatic stages after the last glacial maximum (LGM) to the mid-Holocene, although the significant Bolling-Allerod warming does not appear in Delta(47) values. The estimated temperature difference between the period after the LGM (18.0-16.0 ka) and mid-Holocene (7.7-4.9 ka) was 8.3 degrees C. Using Delta(47) temperature, stalagmite delta O-18 values, and change in seawater delta O-18(SW), the comprehensive isotopic depletion from vapor source (seawater) to meteoric water was reconstructed. We obtained larger 18(O)-depletion (-9.5 to -10.0 parts per thousand) in the pre-Holocene section than in the middle Holocene section (around -8.5 parts per thousand). The larger 18O(-)depletion of the pre-Holocene period was ascribed to a larger fractionation during vapor generation under a lower temperature, change in seasonality of precipitation, or a longer vapor trajectory due to the subaerially exposed Seto Inland Sea. Unlike Chinese stalagmites, delta O-18 record of Hiro-1 stalagmite appear largely unaffected by rainfall amount. (C) 2020 The Author(s). Published by Elsevier Ltd.

Two species of spinose planktonic foraminifera (Globigerinoides ruber and Globigerina bulloides), cultured under controlled seawater temperatures (19, 21, 23, and 25 °C) with newly precipitated walls labeled with Ca isotopes, were subjected to quantitative analyses of Mg/Ca, Sr/Ca and Ba/Ca ratios at a spatial resolution of 2 μm. Mg mapping of calcite walls showed banding structures parallel to the wall surfaces for both species. Significant Mg accumulation was evident, particularly on walls precipitated at higher temperature, together with irregularly shaped chambers. Our experiment reproduced little cross-sectional variation in Sr/Ca, suggesting constant ratio levels in the calcification fluid and seawater. The Ba/Ca, however, showed different behavior in the two species: a relatively constant profile in Gs. ruber but a positive correlation with Mg/Ca in Ga. bulloides. The different geochemical properties in the two species cultured under the same conditions suggest different elemental regulation processes in the calcification sites. Positive coupling of Mg–Ba (but constant Sr/Ca) is unlikely to be explained only by a Mg-specific removal process, whereas Sr and Ca transport may follow a similar pathway.

The vestiges of life in Eoarchean rocks have the potential to elucidate the origin of life. However, gathering evidence from many terrains is not always possible(1-3), and biogenic graphite has thus far been found only in the 3.7-3.8 Ga (gigayears ago) Isua supracrustal belt(4-7). Here we present the total organic carbon contents and carbon isotope values of graphite (d13Corg) and carbonate (delta(13)Ccarb) in the oldest metasedimentary rocks from northern Labrador(8,9). Some pelitic rocks have low delta(13)Corg values of -28.2, comparable to the lowest value in younger rocks. The consistency between crystallization temperatures of the graphite and metamorphic temperature of the host rocks establishes that the graphite does not originate from later contamination. A clear correlation between the delta(13)Corg values and metamorphic grade indicates that variations in the delta(13)Corg values are due to metamorphism, and that the pre-metamorphic value was lower than the minimum value. We concluded that the large fractionation between the delta(13)Ccarb and delta(13)Corg values, up to 25%, indicates the oldest evidence of organisms greater than 3.95 Ga. The discovery of the biogenic graphite enables geochemical study of the biogenic materials themselves, and will provide insight into early life not only on Earth but also on other planets.

Mercenaria stimpsoni (Stimpson's hard clam) is a shallow-water bivalve species distributed along mid- to high latitude coasts of the Northwest Pacific influenced by the Oyashio cold current. As this animal can live for decades, oxygen isotopes of its shell can potentially provide long-term seawater temperature and salinity data. However, little is known about the life history traits of M. stimpsoni, which hampers their use in paleoclimatology. Thus, we investigated the growth patterns of three live-caught M. stimpsoni individuals from Funakoshi Bay, especially focusing on juvenile ontogenetic stages. Sclerochronological and high-resolution oxygen isotope analyses suggest that young (below age 10) specimens of this species grow predominantly between spring and fall, but ceased growing during the cold season when sea surface temperature drops below approximately 10 degrees C. The results also show that ontogenetically older (>10 yr) specimens grow shell material almost exclusively during summer. Oxygen isotopes in ontogenetically younger portions of the shells are a good indicator of past summer temperatures. (C) 2016 Elsevier B.V. All rights reserved.

The Holocene stalagmite FG01 collected at the Fukugaguchi Cave in Itoigawa, central Japan provides a unique high-resolution record of the East Asian winter monsoon. Because of the climate conditions on the Japan Sea side of the Japanese islands, the volume of precipitation during the winter is strongly reflected in the stalagmite O-18 signal. Examination of the carbon isotopes and the Mg/Ca ratio of FG01 provided additional information on the Holocene climate in Itoigawa, which is characterized by two different modes separated at 6.4 ka. Dripwater composition and the correlation between the C-13 and Mg/Ca data of FG01 indicate the importance of prior calcite precipitation (PCP), a process that selectively eliminated C-12 and calcium ions from infiltrating water from CO2 degassing and calcite precipitation. In an earlier period (10.0-6.4 ka), an increase in soil pCO(2) associated with warming and wetting climate trends was a critical factor that enhanced PCP, and resulted in an increasing trend in the Mg/Ca and C-13 data and a negative correlation between the C-13 and O-18 profiles. A distinct peak in the C-13 age profile at 6.8 ka could be a response to an increase of approximately 10% in C4 plants in the recharge area. At 6.4 ka, the climate mode changed to another, and correlation between O-18 and C-13 became positive. In addition, a millennial-scale variation in O-18 and pulsed changes in C-13 and Mg/Ca became distinct. Assuming that O-18 and PCP were controlled by moisture in the later period, the volume of precipitation was high during 6.0-5.2, 4.4-4.0, and 3.0-2.0 ka. In contrast, the driest interval in Itoigawa was during 0.2-0.4 ka, and broadly corresponds to the Little Ice Age.

Insolation is an important component of meteorological data because solar energy is the primary and direct driver of weather and climate. Previous analyses of cultivated giant clam shells revealed diurnal variation in the Sr/Ca ratio, which might reflect the influence of the daily light cycle. We applied proxy method to sample from prehistoric era, a fossil giant clam shell collected at Ishigaki Island in southern Japan. The specimen was alive during the middle Holocene and thus exposed to the warmest climate after the last glacial period. This bivalve species is known to form a growth line each day, as confirmed by the analysis of the Sr enrichment bands using EPMA and facilitated age-model. We analyzed the Sr/Ca, Mg/Ca and Ba/Ca ratios along the growth axis, measuring a 2-mmspot size at 2-mminterval using NanoSIMS. The Sr/Ca ratios in the winter layers are characterized by a striking diurnal cycle consisting of narrow growth lines with high Sr/Ca ratios and broad growth bands with low Sr/Ca ratios. These variations, which are consistent with those of the cultivated clam shell, indicate the potential for the reconstruction of the variation in solar insolation during the middle Holocene at a multi-hourly resolution.

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.

This study examines the distribution of rare earth elements (REEs) in an 18-4.5 ka stalagmite in southwestern Japan, with the aim of assessing the utility of such data in paleoenvironmental reconstructions. Shale-normalized REEs (REEN) of this stalagmite generally contain two features: (1) a positive Eu anomaly and (2) heavy REE (HREE) enrichments. The magnitude of the positive Eu anomaly is most likely controlled by the relative contribution of REEs from weathered wall-rock andesites, which is estimated to be 19-96% of the total REEs in the stalagmite. The HREE enrichments in the stalagmite are likely to have formed by ionic radii-dependent fractionation between mineral and aqueous phases, where parental waters for the stalagmite preferentially incorporated the HREEs with relatively low partition coefficients (D-REE) during bedrock dissolution. In contrast, calcite precipitation causes quantitative incorporation of dissolved REEs by the stalagmite, all of which are highly compatible with calcite (D-REE > 100). This indicates that stalagmite REE patterns should be identical to dripwater REE patterns. The REE characteristics of the stalagmite also changed temporally with the period of deglaciation (16.6-7.1 ka) that was associated with slow stalagmite growth rates, and this coincided with high La-N/Nd-N ratios (>1). La (and Ce) have effective ionic radii that are smaller than that of Ca2+ (100 pm), and these were enriched in aqueous phases during slow dissolution of the bedrock. The data presented here indicate that the stalagmite REE patterns may be useful as potential indices for local weathering intensity.

Stalagmite oxygen stable isotopic records from Chinese and Japanese caves have described the intensity of the East Asian summer monsoon (EASM) in the late Quaternary. In this study, we present a stalagmite delta O-18 record over the past 10,000 years from Fukugaguchi Cave, Itoigawa, Honshu, Japan, facing the Japan Sea. The regional climate is characteristically wet in winter and nearly 60% of the annual precipitation occurs from November to March when the East Asian winter monsoon (EAWM) brings moisture from the Japan Sea warmed by the Tsushima Warm Current. Rainwater delta O-18 values near the cave generally decrease with the precipitation amount, indicating an amount effect. The stalagmite 8180 profile has had a concurrent trend with winter precipitation observed near the cave since 1924 in addition to high-resolution records of the eolian fraction observed in China. These agreements suggest that the Fukugaguchi stalagmite delta O-18 record reflects the EAWM intensity. In comparison of our profile with the EASM record obtained from Chinese stalagmites, inverse correlation was recognized only in the earlier interval of 10.0-5.2 ka, suggesting that the southward migration of the intertropical convergence zone intensified the EAWM. From a peak at 5.2 ka, the EAWM intensity quickly decreased and remained at a lower level between 4.5 and 3.0 ka. Since 3.0 ka, the Fukugaguchi record has high-amplitude changes of millennial time scales, including two peak intervals in 2.9-2.51 ka and 1.3-0.7 ka. EAWM-related winter precipitation might have been amplified with intensification of the Tsushima Warm Current that enhanced the land-sea thermal contrast during winter season. A co-variation presumed between the Fukugaguchi delta O-18 record and the Chinese EAWM records implies that the thermal contrast between East Asia and NW Pacific may have influenced the winter monsoon in throughout the East Asian climate system. (c) 2013 Elsevier Ltd. All rights reserved.

We measured Sr/Ca, Ba/Ca, Sr-87/Sr-88 ratios, delta O-18 and delta C-13 values in a stalagmite, which developed 18.0-4.5 thousand years ago (ka) in southwestern Japan. Dripwater and two major bedrocks (limestone and andesite) in the locality were also studied. The Sr-87/Sr-88 ratios of the stalagmite are relatively homogeneous (0.706852-0.706921), suggesting a steady source mixing ratio of similar to 40% from high-Sr-87/Sr-88 limestone and similar to 60% from low-Sr-87/Sr-88 andesite. The stalagmite Sr/Ca and Ba/Ca ratios were higher than the ratio expected from the dissolved fraction of limestone and andesite. The covariance among Sr/Ca, Ba/Ca, and delta C-13 profiles suggests a significant role of prior calcite precipitation (PCP), i.e., carbonate precipitation from infiltrating water before the water drips on a stalagmite. The relationships among stalagmite Sr/Ca, Ba/Ca ratios and delta C-13 values are consistent with the Rayleigh-type fractionation model, supporting that PCP results in successive enrichment of Sr, Ba and C-13 in the aqueous phase and the resulting stalagmite. The degree of PCP calculated for the stalagmite is highly variable from 0 to 85%, and generally decreased from the last glacial period to the middle Holocene. The large degree of PCP observed during 18-15 ka implies a relatively dry climate during this period, which is consistent with weak monsoon intensity inferred by the delta O-18 values. The Sr-87/Sr-88 ratios of the stalagmite show a slight decrease through the entire period. The increase in the andesite-derived fraction with relatively high Sr-87/Sr-88 may result from accelerated silicate weathering in the epikarst with increasing temperature, humidity, and soil pCO(2). (C) 2013 Elsevier B.V. All rights reserved.

To facilitate the measurement of U-Th isotopic compositions suitable for high-precision and high-resolution Th-230 dating of coral and speleothem carbonates, secondary electron multiplier (SEM) protocol techniques for multicollector inductively coupled plasma mass spectrometry (MC-ICP-MS) have been developed. The instrumental sensitivities are 1-2%, with a precision of +/- 1-2 parts per thousand (2 sigma) for abundance determination of 50-200 fg U-234 (1-4 ng U-238) or Th-230. This method features chemistry refinements, improvements to procedural and instrumental blanks, spectral inference reductions, and careful consideration of non-linear SEM behavior. Measurement consistency of this MC-ICP-MS combined with previous mass spectrometric results on U-Th standards and a variety of carbonates demonstrates the validity of the SEM protocol method. For fossil corals, a routine U-Th isotopic determination at permil-level precision requires only 10-50 mg of carbonate. As little as 200 mg of young coral with an age of less than 20 yr can be dated with a precision of +/- 0.3-0.8 yr. About 20-200 mg speleothem samples with sub-ppm-to-ppm U are required to earn a 5 parts per thousand precision on ages from 5 to 100 kyr. Requirement of small sample size, 10-100s mg carbonate, can permit high temporal resolution to date speleothems with slow growth rates, i.e., 1-10 mm/kyr. This high-precision Th-230 chronology is critical to accurately establish age models, date events and splice geochemical proxy time series records from multiple samples in the fields of paleoclimatology and paleoceanography. The U-Th isotopic determination techniques described here can also be applied to different environmental samples, such as waters, rocks, and sediments. (C) 2012 Elsevier Ltd. All rights reserved.

East Asian monsoon (EAM) evolution during the last deglaciation has been shown repeatedly to be aligned with paleoclimatic changes in the North Atlantic based on climate reconstructions comparing Asian speleothem records with Greenland ice cores In contrast to this finding paleoclimatic reconstructions based on Lake Suigetsu sediment cores suggest that past EAM variability in Japan was not always coherent with climatic variability recorded in Greenland ice cores We resolve this discrepancy using an absolute-dated stalagmite delta O-18 record that covers the period between 15 5 and 107 thousand years before present (ka before AD 1950) This stalagmite record is from nearby Maboroshi cave Japan and shows climate changes that are synchronous with those of Chinese caves and Greenland ice cores Our results support an effective teleconnection between low and high latitudinal climate systems during the transitions into the Bolling-Allerod (BA) warming at 146 ka and the Younger Dryas cooling at 12 8 ka in the North Atlantic However our results also indicate monsoonal intensification during the BA coincident with decreasing temperatures in Greenland from 146 to 12 8 ka We explain this decoupling as a result of the interhemispheric bipolar seesaw climate system Discrepancies between Lake Suigetsu radiocarbon age data sets and other radiocarbon calibration archives can also be removed when our Maboroshi record is used to adjust the Lake Suigetsu age model as well as resolving temporal ambiguities in the Lake Suigetsu paleoclimate record (C) 2010 Elsevier Ltd All rights reserved

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。

Tufas, which are freshwater carbonates, are potential archives of terrestrial paleoclimate. Time series of stable isotopic compositions commonly show regular seasonal patterns controlled by temperature-dependent processes, and some perturbation intrinsic to the locality. We examined three tufa-depositing sites in southwestern Japan with similar temperate climates, to understand the origin of local characteristics in the isotopic records. Seasonal change in the oxygen isotope is principally reflected by temperature-dependent fractionation between water and calcite but was perturbed after heavy rainfalls overwhelming the stability of the delta(18)O value of the groundwater at one site. Isotopic mass balance indicates an undersaturated and relatively small aquifer at this locality. Water delta(18)O values at the other two sites were stable, reflecting a regular seasonal change in the delta(18)O value of tufa. Perturbation of the delta(13)C values in tufa is largely due to CO(2) degassing from the stream, which significantly increases the delta(13)C values of dissolved inorganic carbon (DIC). At a site with remarkably high pCO(2) in springwater and a sensitive response of flow rate to rainfall, the amount of CO, degassing changed distinctly with flow rate. In contrast, the other two sites having low pCO(2) springwater reflect a regular seasonal pattern of delta(13)C in DIC and tufa specimens. (C) 2009 Elsevier Ltd. All rights reserved.

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.

Fluvial tufa deposits in southwest Japan commonly develop biannual lamination consisting of dense summer layers and porous winter layers, and the clearness of the laminae varies among the sites. The laminae have been largely attributed to a seasonally variable inorganic precipitation rate of calcite. This rate-controlled hypothesis was examined by using quantitative data for calcite packing-density (CPD) and the precipitation rate of calcite (PWP rate) calculated from water chemistry. The results for four tufa-depositing sites in SW Japan show that a positive correlation between CPD and PWP rate becomes less certain with increasing PWP rate. In the temperature realm of SW Japan, tufas develop regular distinct seasonal change in CPD when deposited in water containing Ca values less than 65 mg/l, which results in a relatively low precipitation rate. The CPD of tufa deposits rarely exceeds 65%, owing to pore space between fine-grained calcite crystals and to porosity derived from decomposed cyanobacteria and other microorganisms. By increasing the Ca content to more than 65 mg/l, the CPD often attains an upper limit and becomes insensitive to seasonal changes in the PWP rate. Therefore, seasonal variations in CPD at sites with a higher Ca content are unclear, as seen in two examples from tropical islands in southern Japan and in one locality in a temperate climate. The flow rate and microbial density on the tufa surface are subordinate factors with respect to the CPD. Seasonal changes in these two factors often enhance the porous/dense contrast of biannual lamination in SW Japan. (C) 2008 Elsevier B.V. All rights reserved.

Annually laminated carbonates, known as tufas, commonly develop in limestone areas and typically record seasonal patterns of oxygen- and carbon-isotope compositions. 6180 values arc: principally controlled by seasonal changes of water temperature, whereas delta C-13 values are the result of complex reactions among the gaseous, liquid, and solid sources of carbon in the system. We examined the processes that cause the seasonal patterns of delta C-13 in groundwater systems at three tufa-depositing sites in southwestern Japan by applying model calculations to geochemical data. Underground inorganic carbon species are exchanged with gaseous CO2, which is mainly introduced to the underground hydrological system by natural atmospheric ventilation and by diffusion of soil air. These processes control the seasonal pattern of delta C-13, which is low in summer and high in winter. Among the three sites we investigated, we identified two extreme cases of the degree of carbon exchange between liquid and gaseous phases. For the case with high radiocarbon composition (Delta C-14) and low pCO(2), there was substantial carbon exchange because of a large contribution of atmospheric CO2 and a small water mass. For the other extreme case, which was characterized by low Delta C-14 and high pCO(2), the contribution of atmospheric CO2 was small and the water mass was relatively large. Our results suggest that at two of the three sites water residence time within the soil profile was longer than 1 year. Our results also suggested a short residence time (less than 1 year) of water in the soil profile at the site with the smallest water mass, which is consistent with large seasonal amplitude of the springwater temperature variations. The Delta C-14 value of tufas is closely related to the hydrological conditions in which they are deposited. If the initial Delta C-14 value of a tufa-depositing system is stable, C-14-chronology can be used to date paleo-tufas. (C) 2007 Elsevier Ltd. All rights reserved.

The stable-isotope composition of a tufa collected on subtropical Miyako Island, southern Japan, records climatic and hydrologic changes over a 15-year period (1989-2003). A total of 450 subsamples taken at increments of 0.2 mm define the high-resolution isotopic profile of a 9-cm-long sample. The oxygen isotope values clearly exhibit cyclical changes that generally correlate with the annual lamination pattern of the tufa. The observed stability of water delta O-18 values confirms that the seasonal records of water temperature are reflected in the cyclic change of tufa delta O-18 values. The oxygen isotope profile also exhibits a long-term decreasing trend that is equivalent to a 2.5 degrees C temperature increase over the 15-year period. The range of the long-term change in delta O-18 values is at least partly compensated for by a warming trend of 0.95 degrees C seen in metrological data for Miyako Island. However, the carbon isotope profile does not vary cyclically, and the values are clearly higher than for previously reported tufas from the temperate Japanese mainland. Vegetation on Miyako Island, which is dominated by C4-type sugar cane, is largely responsible for the high delta C-13 values of the tufa. The high-resolution isotopic profile provides data on the annual deposition rate, which clearly increased from 1996 to 1998, when the groundwater table was raised by the construction of two underground dams near the tufa locality. Increased water flow from the spring activated calcite precipitation and increased the depositional rate in the upper part of the core up to > 1 cm/year. This rate is much larger than those recorded for tufas from temperate settings in Japan.

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.