| 1. |
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, doi.org/10.1038/s43247-020-00023-4, 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.. |
| 2. |
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.. |
| 3. |
Y. Kuwahara, M. Makio, In situ AFM study on barite (001) surface dissolution in NaCl solutions at 30 C, Applied Geochemistry, http://dx.doi.org/10.1016/j.apgeochem.2014.10.008, 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).. |
| 4. |
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.. |
| 5. |
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.. |
| 6. |
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. |
| 7. |
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. |
| 8. |
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. |
| 9. |
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. |
| 10. |
Yoshihiro Kuwahara, In-situ AFM study of smectite dissolution under alkaline conditions at room temperature, American Mineralogist, Vol. 91, 1142-1149., 2006.07. |
| 11. |
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. |
| 12. |
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. |
| 13. |
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. |
| 14. |
Yoshihiro KUWAHARA and Yoshikazu AOKI, Dissolution kinetics of phlogopite under acid conditions, Clay Science, Vol.11, 31-45, 1999.01. |
| 15. |
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. |
