Surface-renewal motions in the interfacial region below a gas-liquid interface were experimentally investigated in relation to bursting motions in the wall region. To estimate the frequency of the appearance of surface-renewal eddies, mass-transport experiments with methylene-blue solution, together with velocity measurements, were done in an open-channel flow. The instantaneous concentration of methylene-blue tracer emitted from a point source positioned in the buffer layer was measured at the free surface downstream from the source by an optical probe. Instantaneous streamwise velocity was measured using a laser-Doppler velocimeter at a position in the buffer region. Frequencies of both surface-renewal and bursting events were computed from these concentration and velocity signals using a conditional-averaging method. In order to clarify whether the surface-renewal eddies actually dominate mass transfer across the gas-liquid interface, gas-absorption experiments were added. Carbon dioxide was absorbed into the water flow across the calm free surface and its mass-transfer coefficient on the liquid side was measured under the same flow conditions as used in the above mass-transport experiments. The results show that the surface-renewal motions originate in the bursting motions which vigorously occur in the buffer region. That is, the decelerated fluid which is strongly lifted towards the outer layer by bursting almost always arrives at the free surface and renews the free surface. The frequency of the surface renewal, as well as the bursting frequency, is uniquely determined by the wall variables or the outer-flow variables and the Reynolds number. Mass transfer across the gas-liquid interface is dominated by the large-scale surface-renewal eddies, and the mass-transfer coefficient on the liquid side is proportional to the square-root of the surface-renewal frequency.

To understand the chemical nature of hydrothermal fluids in the komatiite-hosted seafloor hydrothermal system in the Hadean, we conducted two hydrothermal serpentinization experiments involving synthetic komatiite and a CO 2 -rich acidic NaCl fluid at 250 and 350 °C, 500 bars. During the experiments, the komatiites were strongly carbonated to yield iron-rich dolomite (3-9 wt.% FeO) at 250 °C and calcite (<0.8 wt.% FeO) at 350 °C, respectively. The carbonation of komatiites suppressed H 2 generation in the fluids. The steady-state H 2 concentrations in the fluid were approximately 0.024 and 2.9 mmol/kg at 250 and 350 °C, respectively. This correlation between the Fe content in carbonate mineral and the H 2 concentration in the fluid suggests that the incorporation of ferrous iron into the carbonate mineral probably limited magnetite formation and consequent generation of hydrogen during the serpentinization of komatiites. The H 2 concentration of the fluid at 350 °C corresponds to that of modern H 2 -rich seafloor hydrothermal systems, such as the Kairei hydrothermal field, where hydrogenotrophic methanogens dominate in the prosperous microbial ecosystem. Accordingly, the high-temperature serpentinization of komatiite would provide the H 2 -rich hydrothermal environments that were necessary for the emergence and early evolution of life in the Hadean ocean. In contrast, H 2 -rich fluids may not have been generated by serpentinization at temperatures below 250 °C because carbonate minerals become more stable with decreasing temperature in the komatiite-H 2 O-CO 2 system.

A technique for the simultaneous measurements of the instantaneous concentrations of two species being mixed in a turbulent flow is developed using a combined laser-induced fluorescence and laser-scattering technique, and the quantitative measurements are demonstrated in a simple grid-generated turbulent flow with two streams of nonpremixed species. It is shown that the present technique enables one to measure simultaneously the instantaneous concentrations of two species with spatial and time resolutions far less than the Kolmogorov length and time scales.

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