Kyushu University Academic Staff Educational and Research Activities Database
List of Papers
Kuwahara Yoshihiro Last modified date:2024.06.03

Professor / Course for Earth Changes / Department of Environmental Changes / Faculty of Social and Cultural Studies

1. Takashima,R., Selby,D., Yamanaka,T., Kuwahara,Y., Nakamura,M., Sawada,K., Ikeda,M.A., Ando,T., Hayashi,K., Nishida,M., Usami,T., and Kameyama,D., Large igneous province activity drives oceanic anoxic event 2 environmental change across eastern Asia, Communications Earth & Environment,, 5, 1, 1-8, 2024.02, During mid-Cretaceous Oceanic Anoxic Event 2, significant increase of atmospheric carbon dioxide concentrations from the eruption of the large igneous provinces is hypothesized to have induced a humid climate and an elevation in nutrient runoff from continents to the oceans, resulting in oxygen depletion in the ocean. However, hitherto there is limited insight into the driving factors of Oceanic Anoxic Event 2 from the Pacific and Asian continental margins, even though the former and the latter were the largest ocean and landmass at that time. Here, a multiproxy analysis for the Oceanic Anoxic Event 2 interval of the Yezo Group –deposited on northwestern Pacific along the active Asian continental margin– is interpretated to identify seven volcanic pulses, five of which may have elevated humidity, weathering intensity, and vegetational change in the eastern margin of Asia. Moreover, oxygen depletion occurred simultaneously in the northwest Pacific. Given that these environmental changes in the eastern margin of Asia were penecontemporaneous with the global carbon burial intervals during Oceanic Anoxic Event 2, the elevated nutrient supply from the Asian continental margin to the Pacific Ocean may have, in part, contributed to the worldwide depletion of oxygen of the ocean during Oceanic Anoxic Event 2..
2. Kato,C., Ohno,M., Hatakeyama,T., Yamada,Y., Honda,F., Shimada,K., Nagase,T., Totsuka-Shiiki,S., Kuwahara,Y. and Ishibashi,J., Low-temperature magnetic behavior of isocubanite from seafloor hydrothermal deposits in the Okinawa Trough, Physics and Chemistry of Minerals,, 51, 5, 1-13, 2024.02, The characteristic behavior of magnetic remanence correlated with mineralogical textures and composition was observed using low-temperature magnetometry, microscopy, and chemical analysis of three isocubanite samples collected from hydrothermal deposits in the Okinawa Trough and a sample transformed from natural cubanite via heating. Both zero-field remanence acquired at 5 K and field cooling remanence acquired at 300–5 K of all samples sharply decreased with increasing temperature at approximately 100 K. In addition, low-temperature cycling of isothermal remanence at 300 K exhibited a transition at approximately 100 K; remanence increased with decreasing temperature and vice versa. The intensity of remanence at low temperature and sharpness of the transition varied across samples with different compositions and microscopic textures, that is, the presence or absence of chalcopyrite lamellae and their widths. The sample obtained from a hydrothermal chimney, in which the magnetic transition was most clearly observed, was also subjected to X-ray diffraction, Mössbauer spectroscopy, electrical resistivity, and magnetic hysteresis measurements. The obtained results were generally consistent with those reported previously for unnamed mineral CuFe3S4 with an ordered cation arrangement. The low-temperature magnetic behavior of isocubanite possibly depends on the degree of cation ordering and can be regarded as an indicator of chemical composition and cooling history. Therefore, low-temperature magnetometry is useful for the detection of isocubanite and a potentially powerful technique for the prompt estimation of its composition and texture, contributing to our understanding of the formation process of hydrothermal deposits..
3. Nguyen Van Thinh, M. Matsumoto, Zaw Myo, Y. Kuwahara, Xie Yipin, A. Ozaki, K. Kurosawa, Biogeochemical properties and potential risk of shallow arsenic-rich sediment layers to groundwater quality in Western Bangladesh, ENVIRONMENTAL GEOCHEMISTRY AND HEALTH, 10.1007/s10653-021-01087-7, Vol.44, Iissue 10, 3249-3263, 2022.10.
4. Nguyen Van Thinh, YusukeShiratori, Phuc Hoan Tu, Yoshihiro Kuwahara, Novel co-doped iron oxide and graphitic carbon nanosheets on biochar for arsenite removal from contaminated water: Synthesis, applicability and mechanism, Bioresource Technology Reports,, 17, 1-10, 2022.02, Arsenite (AsIII) is the most toxic form of arsenic in drinking water, but AsIII removal is still a challenge. In this study, α-Fe2O3-doped graphitic carbon nanosheets were coated on the surfaces of coconut biochar backbone to remove AsIII from contaminated water. The properties of the composites and the efficiency of AsIII treatment were measured by modern techniques and methods, such as XPS, SEM-EDS, ICP–MS, BET analyses. The novel composites enhance AsIII removal capacity by up to 99.85%, and exhibited high effectiveness for AsIII removal over a large pH range from 3 to 9. In addition, XPS analysis revealed that the FeCl3-derived composites have the ability to oxidize AsIII into AsV species on the composite surfaces due to its catalytic properties. The novel composites can be considered adsorbents that can treat a large AsIII concentration range to obtain safe aqueous As levels according to the WHO guidelines for drinking water..
5. Hirokazu Kato, Shota Amekawa, Masako Hori, Chaun-Chou Shen, Yoshihiro Kuwahara, Ryoko Senda, Akihiro Kano, Influences of temperature and the meteoric water δ18O value on a stalagmite record in the last deglacial to middle Holocene period from southwestern Japan, Quaternary Science Reviews,, 256, 1 February, 1-13, 2021.02, Our understanding of stalagmite δ18O paleoclimate records would be significantly improved by determining the relative influence of the two factors, temperature and water δ18O. 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 Δ47 values from 50 stalagmite layers, calibrated using Δ47-temperature relationships observed in modern stream tufa from Japan (Kato et al., 2019) yield estimates of paleo-temperature and water δ18O variability. Several layers affected by significant prior calcite precipitation under a dry climate display strong kinetic isotope influences: positive δ18O and negative Δ47 deviations. Except for these layers, Hiro-1 Δ47 records temperature changes that are broadly consistent with climatic stages after the last glacial maximum (LGM) to the mid-Holocene, although the significant Bølling-Allerød warming does not appear in Δ47 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 °C. Using Δ47-temperature, stalagmite δ18O values, and change in seawater δ18OSW, the comprehensive isotopic depletion from vapor source (seawater) to meteoric water was reconstructed. We obtained larger 18O-depletion (−9.5 to −10.0‰) in the pre-Holocene section than in the middle Holocene section (around −8.5‰). 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, δ18O record of Hiro-1 stalagmite appear largely unaffected by rainfall amount..
6. Yu Kitahara, Daiki Nishiyama, Masao Ohno, Yuhji Yamamoto, Yoshihiro Kuwahara, Tadahiro Hatakeyama, Construction of new archaeointensity reference curve for East Asia from 200 CE to 1100 CE, Physics of the Earth and Planetary Interiors,, 310, January, 1-15, 2021.01.
7. Tatsuya Hayashi, Toshiro Yamanaka, Yuki Hikasa, Masahiko Sato, Yoshihiro Kuwahara, Masao Ohno, Latest Pliocene Northern Hemisphere glaciation amplified by intensified Atlantic meridional overturning circulation, Communications Earth & Environment,, 1, 25, 1-10, 2020.09, The global climate has been dominated by glacial–interglacial variations since the intensification of Northern Hemisphere glaciation 2.7 million years ago. Although the Atlantic meridional overturning circulation has exerted strong influence on recent climatic changes, there is controversy over its influence on Northern Hemisphere glaciation because its deep limb, North Atlantic Deep Water, was thought to have weakened. Here we show that Northern Hemisphere glaciation was amplified by the intensified Atlantic meridional overturning circulation, based on multi-proxy records from the subpolar North Atlantic. We found that the Iceland–Scotland Overflow Water, contributing North Atlantic Deep Water, significantly
increased after 2.7 million years ago and was actively maintained even in early stages of
individual glacials, in contrast with late stages when it drastically decreased because of
iceberg melting. Probably, the active Nordic Seas overturning during the early stages of
glacials facilitated the efficient growth of ice sheets and amplified glacial oscillations..
8. Hirokazu Kato, Shota Amekawa, Akihiro Kano, Taiki Mori, Yoshihiro Kuwahara, Jay Quade, Seasonal temperature changes obtained from carbonate clumped isotopes of annually laminated tufas from Japan
Discrepancy between natural and synthetic calcites, Geochimica et Cosmochimica Acta, 10.1016/j.gca.2018.10.016, 244, 548-564, 2019.01, Carbonate clumped isotopes provide a novel paleo-thermometer that does not require knowledge of the isotopic compositions of environmental water. Despite considerable effort, there are substantial inter-laboratory disagreements in empirical calibrations between the abundance anomaly of the clumped isotopes (Δ47) and absolute temperature, and discrepancies in paleo-temperature estimates from natural samples. In this study, clumped isotopes using two types of 17O corrections were analyzed for two sample sets of synthetic calcites and tufas collected monthly from two sites in southwest Japan with known δ18O values and temperatures of parent water. These measurements form the basis for two new temperature calibrations of carbonate clumped isotopes, the first from our Kyushu University laboratory in Japan. The calcites synthesized at four different temperatures (2.9–61.0 °C) yield a temperature-Δ47 relationship of Δ47=0.0354±0.0013×106/T2+0.290±0.015 (R2 = 0.986), which is indistinguishable from the calibrations of other recent studies based on acid digestion experiments at high temperature (70–100 °C). The Δ47 thermometry for the natural tufa samples display the seasonal pattern of water temperature better than the δ18O thermometry. However, the Δ47 values of the tufa samples deposited at temperatures ranging from 5.6 to 16.0 °C were lower than the values expected from the calibration using synthetic calcites, thus providing another temperature calibration of Δ47=0.0336±0.0036×106/T2+0.301±0.048 (R2 = 0.735). Therefore, our results clearly identify a detectable, but fairly uniform, discrepancy in Δ47-temperature calibration between synthetic calcite and natural samples (tufas). The amplitude of Δ47 offset between the tufa and synthetic samples appears to be independent of the seasonal change in water temperature and the chemical properties of water, including pH, pCO2, and precipitation rate of calcite. The lower Δ47 values have been previously reported from other tufas and travertines, and therefore appear to be associated with processes inherent to tufas and travertines, such as CO2 degassing. We also find that our results are insignificantly influenced by types of 17O corrections and almost independent with bulk isotopic composition..
9. Yu Kitahara, Yuhji Yamamoto, Masao Ohno, Yoshihiro Kuwahara, Shuichi Kameda, Tadahiro Hatakeyama, Archeointensity estimates of a tenth-century kiln
first application of the Tsunakawa–Shaw paleointensity method to archeological relics, Earth, Planets and Space, 10.1186/s40623-018-0841-5, 70, 1, 2018.12, Paleomagnetic information reconstructed from archeological materials can be utilized to estimate the archeological age of excavated relics, in addition to revealing the geomagnetic secular variation and core dynamics. The direction and intensity of the Earth’s magnetic field (archeodirection and archeointensity) can be ascertained using different methods, many of which have been proposed over the past decade. Among the new experimental techniques for archeointensity estimates is the Tsunakawa–Shaw method. This study demonstrates the validity of the Tsunakawa–Shaw method to reconstruct archeointensity from samples of baked clay from archeological relics. The validity of the approach was tested by comparison with the IZZI-Thellier method. The intensity values obtained coincided at the standard deviation (1σ) level. A total of 8 specimens for the Tsunakawa–Shaw method and 16 specimens for the IZZI-Thellier method, from 8 baked clay blocks, collected from the surface of the kiln were used in these experiments. Among them, 8 specimens (for the Tsunakawa–Shaw method) and 3 specimens (for the IZZI-Thellier method) passed a set of strict selection criteria used in the final evaluation of validity. Additionally, we performed rock magnetic experiments, mineral analysis, and paleodirection measurement to evaluate the suitability of the baked clay samples for paleointensity experiments and hence confirmed that the sample properties were ideal for performing paleointensity experiments. It is notable that the newly estimated archaomagnetic intensity values are lower than those in previous studies that used other paleointensity methods for the tenth century in Japan..
10. Nguyen Van Thinh, Yasuhito Osanai, Tatsuro Adachi, Phong K. Thai, Nobuhiko Nakano, Akinori Ozaki, Yoshihiro Kuwahara, Ryosuke Kato, Masato Makio, Kiyoshi Kurosawa, Chemical speciation and bioavailability concentration of arsenic and heavy metals in sediment and soil cores in estuarine ecosystem, Vietnam, Microchemical Journal, 10.1016/j.microc.2018.03.005, 139, 268-277, 2018.06, The first investigation of arsenic (As) contamination in soil cores in the biggest estuarine ecosystem in northern Vietnam was conducted using FE-EPMA, ICP-MS and XRD techniques. Concentrations of arsenic and five heavy metals were measured and mineralogical characteristic of soils and sediments in different land uses were analyzed. Most of the soils contained high proportions of As and heavy metals in the potential mobility fractions and silicate minerals. High concentrations of As and heavy metals were observed in river sediments and top-layers of mangrove forest soil cores. Qualitative analyses using WDS technique on FE-EPMA found As on the iron (hydro)oxides and framboidal pyrite surfaces in the samples. The principal component analysis indicated the enrichment of As and heavy metals in the upper soil layers, suggesting that intensive human activities in upstream of the Red river in recent decades could be the cause of elevated levels of these toxic elements in the estuarine ecosystem..
11. Jaeguk Jo, Toshiro Yamanaka, Tomoki Kashimura, Yusuke Okunishi, Yoshihiro Kuwahara, Isao Kadota, Youko Miyoshi, Jun-ichiro Ishibash and Hitoshi Chiba, Mineral nitrogen isotope signaturein clay minerals formed under high ammonium environment conditions in sediment associated with ammonium-rich sediment-hosted hydrothermal system, Geochemical Journal, doi:10.2343/geochemj.2.0518, Vol.52, No.4, 317-333, 2018.05, The shallow-seafloor Wakamiko hydrothermal field in Kagoshima Bay, Japan, has been characterized by high ammonium contents (~17 mM) in the discharge fluid. The sediment layer covering the field also contained a high concentration of ammonium in the pore fluids. The fluid chemistry suggests that the high concentration was attributed to whether mixing with hydrothermal fluid or the mineralization of organic matter. The occurrence of sediments such as smectite, illite and mica has been reported, and is associated with hydrothermal activity. Those 2:1 clay minerals can fold ammonium as mineral nitrogen (MN) in their interlayers. Extracted clay fractions from the sediment core samples obtained from the field were mainly composed of smectite. It contained a small amount of MN and its d15N values reflected that of pore fluid ammonium in the case of a high ammonium (>4 mM) environment. On the other hand, d15N values of MN in the low ammonium (£4 mM) environment were similar to those of the associated adsorbed organic nitrogen. The high ammonium environment associated with hydrothermal activity is expected to enhance formation of ammonium-bearing clay minerals, and then the d15N value of MN fixed in the clay minerals is possibly preserved a signal of pore fluid ammonium..
12. Yoshihiro Nakamuta, Kana Urata, Yoko Shibata, Yoshihiro Kuwahara, Effect of NaCrSi2O6 component on Lindsley’s pyroxene thermometer: An evaluation based on strongly metamorphosed LL chondrites, Meteoritic and Planetary Science, 10.1111/maps.12808, 52, 3, 511-521, 2017.04.
13. Y. Kuwahara, W. Liu, M. Makio, K. Otsuka, In Situ AFM Study of Crystal Growth on a Barite (001) Surface in BaSO4 Solutions at 30°C, Minerals, 10.3390/min6040117, Special Issue Nucleation of Minerals: Precursors, Intermediates and Their Use in Materials Chemistry), 117, 1-18, 2016.11, The growth behavior and kinetics of the barite (001) surface in supersaturated BaSO4 solutions (supersaturation index (SI) = 1.1–4.1) at 30°C were investigated using in situ atomic force microscopy (AFM). At the lowest supersaturation, the growth behavior was mainly the advancement of the initial step edges and filling in of the etch pits formed in the water before the BaSO4 solution was injected. For solutions with higher supersaturation, the growth behavior was characterized by the advance of the and [010] half-layer steps with two different advance rates and the formation of growth spirals with a rhombic to bow-shaped form and sector-shaped two-dimensional (2D) nuclei. The advance rates of the initial steps and the two steps of 2D nuclei were proportional to the SI. In contrast, the advance rates of the parallel steps with extremely short step spacing on growth spirals were proportional to SI2, indicating that the lateral growth rates of growth spirals were directly proportional to the step separations. This dependence of the advance rate of every step on the growth spirals on the step separations predicts that the growth rates along the [001] direction of the growth spirals were proportional to SI2 for lower supersaturations and to SI for higher supersaturations. The nucleation and growth rates of the 2D nuclei increased sharply for higher supersaturations using exponential functions. Using these kinetic equations, we predicted a critical supersaturation (SI =4.3) at which the main growth mechanism of the (001) face would change from a spiral growth to a 2D nucleation growth mechanism: therefore, the morphology of bulk crystals would change..
14. I. Kita, T. Yamashita, S. Chiyonobu, S. Hasegawa, T. Sato, Y. Kuwahara, Mercury content in Atlantic sediments as a new indicator of the enlargement and reduction of Northern Hemisphere ice sheets, Journal of Quaternary Science, 10.1002/jqs.2854, 31, 3, 167-177, 2016.03, The total mercury content of Quaternary sediments over the last 1.1Ma from IODP Site U1308 drilled in the North Atlantic Ocean has changed, varying with periodicities of 100 000 and 41 000 years. The Hg content of the sediment increased during glacial periods, associated with increases in ice-rafted debris (IRD), and correlates positively with the total organic carbon (TOC) content and inversely with the absolute abundance of surface-dwelling nannoplankton species. TOC/total nitrogen ratios and d13Corg values of TOC indicate that the TOC deposited during glacial periods was marine and associated with increased Hg deposition. These observations suggest that marine Hg is incorporated into the organic matter produced by deep-dwelling phytoplankton in the lower photic zone. Mercury- and IRD bearing icebergs flowed out to this area during glacial periods melt, increasing the Hg content of photic water, and depositing IRD on the ocean floor, where increased marine Hg during glacial periods was consumed by deep-dwelling phytoplankton. Therefore, the change in the sediment Hg.
content from the North Atlantic Ocean is controlled by climatic change and can be used as a chemical indicator of
Northern Hemisphere ice sheet variability.
15. M. Ohno, T. Hayashi, M. Sato, Y. Kuwahara, A. Mizuta, I. Kita, T. Sato, A. Kano, Millennial-scale interaction between ice sheets and ocean circulation during Marin Isotope Stage 100, Frontiers in Earth Science, 10.3389/feart.2016.00055, 4, 1-9, 2016.05, Waxing/waning of the ice sheets and the associated change in thermohaline circulation have played an important role in global climate change since major continental ice sheets appeared in the northern hemisphere about 2.75 million years ago..
16. Y. Kuwahara, M. Makio, In situ AFM study on barite (001) surface dissolution in NaCl solutions at 30 C, Applied Geochemistry,, 51, 12, 246-254, 2014.12, This paper reports in situ observations on barite (001) surface dissolution behavior in 0.1–0.001 M NaCl solutions at 30°C using atomic force microscopy (AFM). The step retreating on barite (001) surfaces changed with increasing NaCl solution concentrations. In solutions with a higher NaCl concentration (>0.01 M), many steps showed curved or irregular fronts during the later experimental stage, while almost all steps in solutions with a lower NaCl concentration exhibited straight or angular fronts, even during the late stage. The splitting phenomenon of the initial hhk0i one-layer steps (7.2 Å) into two half-layer steps (3.6 Å) occurred in all NaCl solutions, while that of the initial [010] one-layer steps observed only in the 0.1 M NaCl solution. The step retreat rates increased with an increasing NaCl solution concentration. We observed triangular etch pit and deep etch pit formation in all NaCl solutions, which tended to form late in solutions with lower NaCl concentrations. The deep etch pit morphology changed with increasing NaCl solution concentrations. A hexagonal form elongated in the [010] direction was bounded by the {100}, {310}, and (001) faces in a 0.001 M NaCl solution, and a rhombic form was bounded by the {510} and (001) faces in 0.01 M and 0.1 M NaCl solutions. An intermediate form was observed in a 0.005 M NaCl solution, which was defined by {100}, a curved face tangent to the [010] direction, {310}, and (001) faces: the intermediate form appeared between the hexagonal and rhombic forms in solutions with lower and higher NaCl concentrations, respectively. The triangular etch pit and deep etch pit growth rates also increased with the NaCl solution concentration. Combining the step and face retreat rates in NaCl solutions estimated in this AFM study as well as the data on the effect of water temperature on the retreat rates reported in our earlier study, we produced two new findings. One finding is that the retreat rates increase by approximately two-fold when the NaCl solution concentration increases by one order of magnitude, and the other finding is that the retreat rate increase due to a one order of magnitude increase in the NaCl concentration corresponds to an increase of approximately 8°C in water temperature. This correlation may help to understand and evaluate increasing dissolution kinetics induced by the different mechanisms where barite dissolution is promoted by the catalytic effect of Na+ and Cl ions (through an increase in the NaCl solution concentration) or by an increase in the hydration of Ba2+ and SO4 2- (through an increase in water temperature)..
17. Toshiro Yamanaka, Kotaro Maeto, Hironori Akashi, Junichiro Ishibashi, Youko Miyoshi, Kei Okamura, Takuroh Noguchi, Yoshihiro Kuwahara, Tomohiro Toki, Urumu Tsunogai, Tamaki Ura, Takeshi Nakatani, Toshihiro Maki, Kaoru Kubokawa, Hitoshi Chiba, Shallow submarine hydrothermal activity with significant contribution of magmatic water producing talc chimneys in the Wakamiko Crater of Kagoshima Bay, southern Kyushu, Japan, JOURNAL OF VOLCANOLOGY AND GEOTHERMAL RESEARCH, 10.1016/j.jvolgeores.2013.04.007, 258, 78-84, 2013.05, Active hydrothermal venting fromshallow seafloor (200-m depth) with talc chimneys has been discovered at the Wakamiko Crater floor in the Aira Caldera, southern Kyushu, Japan. The major chemical composition of the fluids suggests that the fluids are supplied froma single reservoir. The fluid is characterized by a low chloride concentration, low δD value, and a high δ18O value, suggesting that the endmember hydrothermal fluid is a mixture of seawater and andesitic water and possibly contribution of meteoric water and/or phase separation. Such noticeable magmatic input may be supported by high helium isotopic ratio (6.77 RA) of fumarolic gas discharging from the crater. Silica and alkaline geothermometers indicate that the fluid–rock interaction in the reservoir occurs in the temperature range of 230 to 250 °C. The high alkalinity and high ammoniumand dissolved organicmatter concentrations in the fluid indicate interaction of the fluidwith organic matter in sedimentary layers. At least three hydrothermal vents have been observed in the crater. Two of these have similar cone-shaped chimneys. The chimneys have a unique mineralogy and consist dominantly of talc (kerolite and hydrated talc) with lesser amounts of carbonate (dolomite and magnesite), anhydrite, amorphous silica, and stibnite. The precipitation temperature estimated from δ18O values of talcwas almost consistent with the observed fluid temperature. Geochemical modeling calculations also support the formation of talc and carbonate upon mixing of the endmember hydrothermal fluid with seawater and suggest that the talc chimneys are currently growing from venting fluid..
18. Hidenao Hasegawa, Itsuro Kita, Shingo Tsukamoto, Shun Chiyonobu, Yoshihiro Kuwahara, Synchronous Changes of Nitrogen and Carbon Isotopic Ratios and Nannoplankton Assemblage in Marine Sediments off Peru at 250 ka: A Role of Phytoplankton in Primary Ocean Productivity, Open Journal of Geology, DOI: 10.4236/ojg.2013.32015, 3, 2, 113-120, 2013.04, We report the mechanism controlling changes of δ15Norg and δ13Corg values of marine organic matter, based on the change of calcareous nannoplankton assemblage during the last 500,000 years in core samples from ODP Hole 846B off Peru in the equatorial Pacific Ocean. The δ15Norg values fluctuate in a range from 2.1‰ to 6.7‰, giving an abrupt increase since about 250 ka with the averages of 3.8‰ and 5.0‰ during the older and younger periods, respectively. The δ13Corg values change in a range from ?23.5‰ to ?20.1‰ in an inverse correlation with δ15Norg values, describing an increase of average values at 250 ka as well as those of δ15Norg values. The total organic carbon content also shows averages of 0.6% and 0.3% during the younger and older periods, respectively. The numerical increase of deep dwelling species (Florisphaera profunda) of calcareous nannoplankton during the younger period in comparison with the older period indicates that these chemical and isotopic jumps synchronized with nannoplankton assemblage changes in marine sediments are caused by activity of deep dwellers in photic zone more stratified by an abrupt decline of trade wind strength on this sea area since about 250 ka. A study coupling δ15Norg and δ13Corg values and nannoplankton assemblage can be a useful method for evaluating the extent of stratification of photic zone and the roles of surface and deep dwellers of phytoplankton in producing primary organic matter.
19. Y. Kuwahara, K. Ishida, S. Uehara, I. Kita, Y. Nakamuta, T. Hayashi, and R. Fujii, Cool-stage AFM, a new AFM method for in situ observations of mineral growth and dissolution at reduced temperature: Investigation of the responsiveness and accuracy of the cooling system and a preliminary experiment on barite growth, Clay Science, 16, 4, 111-119, 2012.12, Using in situ cool-stage atomic force microscopy (AFM), we can observe sample surfaces in air or fluid directly at the site or step level at low or reduced temperatures (~ -35°C in air or ~ 4°C in fluid). The main components of the new AFM system include a heater/cooler element (cool-stage), specialized heater/cooler piezoelectric scanner with a heat exchanger, Thermal Applications Controller, AFM fluid cell, and fluid cooling system to cool the piezo scanner. We investigate the responsiveness and accuracy of the temperature control in the cooling AFM system by varying the setpoint temperature of the cool-stage and flow rate of the water flowing through the AFM fluid cell and thereby obtain suitable experimental conditions for in situ cool-stage AFM observations of mineral growth and dissolution in aqueous solutions at low or reduced temperatures. We also report the results of a preliminary experiment on in situ observations of the growth behavior on the barite (001) surface in a supersaturated BaSO4 solution at reduced temperature (25°C to 15°C) using the new AFM method..
20. Paleosynecology of diatoms in the Paleo-Kathmandu Lake during the middle to late pleistocene: Variations in productivity and species diversity of planktonic and benthic assemblages.
21. Yoshihiro Kuwahara, In situ hot-stage AFM study of the dissolution of the barite (001) surface in water at 30–55 °C, American Mineralogist, doi:10.2138/am.2012.4130, 97, 10, 1564-1573, 2012.10, This paper reports in situ observations of the dissolution behavior of the barite (001) surface in pure water at 30–55 °C using hot-stage atomic force microscopy (AFM). The dissolution at 30 and 40 °C occurred in three stages; however, at 55 °C, the dissolution behavior observed at the former temperatures started immediately after injecting water into the AFM fluid cell. The first stage of the dissolution was characterized by the retreat of the initial steps and continued for about 60 min at 30 °C and about 10 min at 40 °C. The second stage of the dissolution was characterized by the splitting of the initial one-layer step into two half-layer steps [fast (“f”) and slow (“s”) retreat steps] with different retreat rates and by the formation of etch pits. The large difference in the retreat rate of the “f” and “s” steps led to the formation of a new one-layer step, which showed slightly faster retreat rates than the “s” half-layer step at all temperatures. The splitting of the [010] one-layer step into two half-layer steps was observed only at 55 °C. During the third stage, the development of angular deep etch pits from an initial form with a curved outline differed at each temperature. The deep etch pits grew rapidly at higher temperature, but showed at least two different retreat rates for the (001) plane at each temperature, indicating the development of the pits along different dislocations (screw and edge dislocations).
The activation energies (62–74 kJ/mol) for the step and face retreats in this study were significantly higher than those reported in earlier studies. Recalculations performed using only data obtained under similar conditions in previous studies led to activation energies of 66–79 kJ/mol. These results and the earlier report showing that the form of the deep etch pits changed from angular to bow-shaped at about 60 °C may indicate that the activation energy of barite dissolution in water is higher at lower temperatures as compared with higher temperatures, thus changing the rate-limiting step. Whether the vertical and lateral retreat rates of the barite (001) plane differ in dependence of temperature remains unclear; however, the activation energies of the retreat of the (001) face in deep etch pits tended to be slightly higher than that of the lateral retreat rates of steps or other faces in deep etch pits..
22. Meng Zhao, Masao Ohno, Yoshihiro Kuwahara, Tatsuya Hayashi and Takafumi Yamashita, Magnetic minerals in sediments from IODP Site U1314 determined by low-temperature and high-temperature magnetism, Bulletin of the Graduate School of Social and Cultural Studies, Kyushu University, 第17号, 77-84, 2011.04.
23. Yoshihiro Kuwahara, In situ Atomic Force Microscopy study of dissolution of the barite (001) surface in water at 30°C, Geochimica et Cosmochimica Acta, 10.1016/j.gca.2010.10.003, 75, 41-51, 2011.01.
24. Kuwahara, Y., Masudome, Y., Paudel, M.R., Fujii, R., Hayashi, T., Mampuku, M., Sakai, H., Controlling weathering and erosion intensity on the southern slope of the Central Himalaya by the Indian summer monsoon during the last glacial, Global and Planetary Change, 71, 1-2, 73-84, Vol. 71, 73-84., 2010.03.
25. Tashuya Hayashi, Yoshihiro Tanimura, Yoshihiro Kuwahara, Masao Ohno, Mami Mampuku, Rie Fujii, Harutaka Sakai, Toshiro Yamanaka, Takeshi Maki, Masao Uchida, Wataru Yahagi, and Hideo Sakai, Ecological variations in diatom assemblages in the Paleo-Kathmandu Lake linked with global and Indian monsoon climate changes for the last 600,000 yr, Quaternary Research, 72, 3, 377-387, Vol. 72, 377-387, 2009.10.
26. Miyoshi, Y., Ishibashi, J., Matsukura, S., Kuwahara, Y., Omura, A., Maeto, K., Chiba, H., and Yamanaka, T., Fluid-sediment interactions in a marine shallow-water hydrothermal system at the Wakamiko submarine crater, Japan, Geochimica et Cosmochimica Acta, 73, 13, A888-A888, Suppl. S., 2009.06.
27. X-ray fluorescence analysis of major, trace and light rare earth elements using low dilution glass bead.
28. Yoshihiro Kuwahara and Seiichiro Uehara, AFM Study on Surface Microtopography, Morphology and Crystal Growth of Hydrothermal Illite in Izumiyama Pottery Stone from Arita, Saga Prefecture, Japan, The Open Mineralogy Journal, Vol. 2, 34-47., 2008.12.
29. Yoshihiro Kuwahara, In situ observations of muscovite dissolution under alkaline conditions at 25-50°C by AFM with an air/fluid heater system, American Mineralogist, Vol.93, 1028-1033, 2008.08.
30. Yoshihiro Kuwahara, In-situ AFM study of smectite dissolution under alkaline conditions at room temperature, American Mineralogist, Vol. 91, 1142-1149., 2006.07.
31. Yoshihiro Kuwahara, In-situ,real time AFM study of smectite dissolution under high pH condtions at 25°-50°C., Clay Science, Vol. 12, Supplement 2, 57-62., 2006.04.
32. Harutaka Sakai, Minoru Sawada, Yutaka Takigami, Yuji Orihashi, Tohru Danhara, Hideki Iwano, Yoshihiro Kuwahara, Qi Dong, Huawei Cai, and Jianguo Li, Geology of the summit limestone of Mount Qomolangma (Everest) and cooling history of the Yellow Band under the Qomolangma detachment, The Island Arc, 10.1111/j.1440-1738.2005.00499.x, 14, 4, 297-310, Vol. 14, 297-310., 2005.12.
33. Harutaka Sakai, Rie Fujii, Yoshihiro Kuwahara, Paleo-Kathmandu Lake Drilling Project, Journal of Geological Society of Japan, Vol.111, No.11, XX, 2005.11.
34. Yoshihiro Kuwahara, Mukunda Raj Paudel, Takeshi Maki, Rie Fujii, Harutaka Sakai, Variations of paleoclimate and paleoenvironment during the last 40kyr recorded in clay minerals in the Kathmandu Basin sediments, Himalayan Journal of Sciences, Vol.2, 190-191, 2004.06.
35. Mukunda Raj Paudel, Yoshihiro Kuwahara, Harutaka Sakai, Changes in mineral composition and depositional environments recorded in the present and past basin-fill sediments of the Kathmandu Valley, central Nepal, Himalayan Journal of Sciences, Vol.2, 222-223, 2004.06.
36. Harutaka SAKAI, Rie FUJII, and Yoshihiro KUWAHARA, Changes in the depositional system of the Paleo-Kathmandu Lake caused by uplift of the Nepal Lesser Himalayas, Journal of Asian Earth Sciences, 10.1016/S1367-9120(01)00046-3, 20, 3, 267-276, Vol.20, 267-276, 2002.04.
37. Yoshihiro KUWAHARA, Rie FUJII, Yukiko MASUDOME, and Harutaka SAKAI, Measurement of crystallinity and relative amount of clay minerals in the Kathmandu Basin sediments by decomposition of XRD patterns (profile fitting), Journal of Nepal Geological Society, Vol.25, 71-80, 2001.12.
38. Rie Fujii, Yoshihiro KUWAHARA, and Harutaka SAKAI, Mineral composition changes recorded in a 284-m-long drill-well in central part of the Kathmandu Basin, Journal of Nepal Geological Society, Vol.25, 63-70, 2001.12.
39. Harutaka SAKAI, Rie Fujii, Yoshihiro KUWAHARA, B.N. UPRETI, and S.D. SHRESTHA, Core drilling of the basin-fill sediments in the Kathmandu Valley for palaeoclimatic study: preliminary results, Journal of Nepal Geological Society, Vol.25, 9-18, 2001.12.
40. Massimo NESPOLO and Yoshihiro KUWAHARA, Apparent polytypism of the micas: an XRD-AFM study of the Ruiz Peak ferric phlogopite, European Journal of Mineralogy, Vol.13, 1047-1056, 2001.12.
41. Yoshihiro KUWAHARA, Seiichiro UEHARA, and Yoshikazu AOKI, Atomic force microscopy study of hydrothermal illite in Izumiyama pottery stone from Arita, Saga prefecture, Japan, Clays and Clay Minerals, 10.1346/CCMN.2001.0490404, 49, 4, 300-309, Vol.49, 300-309, 2001.04.
42. Yoshihiro KUWAHARA, Comparison of the surface structure of the tetrahedral sheets of muscovite and phlogopite by AFM, Physics and Chemistry of Minerals, 10.1007/s002690000126, 28, 1, 1-8, Vol.28, 1-8, 2001.01.
43. Yoshihiro KUWAHARA, The surface structure of trioctahedral mica phlogopite by fluid contact mode AFM, Bulletin of the Graduate School of Social and Cultural Studies, Kyushu University, Vol.6, 87-95, 2000.12.
44. Observation of microtopography and lattice images on mineral surfaces by AFM and its problems.
45. Yoshihiro KUWAHARA and Yoshikazu AOKI, Dissolution kinetics of phlogopite under acid conditions, Clay Science, Vol.11, 31-45, 1999.01.
46. Yoshihiro KUWAHARA, Muscovite surface structure imaged by fluid contact mode AFM, Physics and Chemistry of Minerals, 10.1007/s002690050177, 26, 3, 198-205, Vol.26, 198-205, 1999.02.
47. Yoshihiro KUWAHARA, Seiichiro Uehara, and Yoshikazu AOKI, Surface microtopography of lath-shaped hydrothermal illite by tapping-mode and contact-mode AFM, Clays and Clay Minerals, 10.1346/CCMN.1998.0460511, 46, 5, 574-582, Vol.46, 574-582, 1998.05.
48. Weathering and hydrothermal alteration of mica minerals.
49. Yoshihiro KUWAHARA and Yoshikazu AOKI, Dissolution process of phlogopite in acid solutions, Clays and Clay Minerals, 10.1346/CCMN.1995.0430105, 43, 1, 39-50, Vol.43, 39-50, 1995.01.