Mechanisms of uranium mineralization by the yeast Saccharomyces cerevisiae

We determined the association of uranium in yeast cells S. cerevisiae grown in medium containing high (1 g · L^-1) or low (0.2 g · L^-1) concentrations of phosphate after exposure for 96 h to a 4 × 10^-4 mol · L^-1 U(VI) solution at pH 3.2 or 4.7. The analysis was made using a field emission scanning electron microscope equipped with energy dispersive spectroscopy (FESEM-EDS), transmission electron microscopy (TEM), and visible diffuse reflectance spectrometry. Cells grown in the high-phosphate medium rapidly accumulated U(VI) from solution at pH 3.2 over the first 24 h, followed by a slow uptake until 96 h, whereas in cells grown in low-phosphate medium, U(VI) accumulation reached a steady state within 24 h. FESEM-EDS analyses revealed the formation of a U(VI)-bearing precipitate on the yeast cells grown in high-phosphate medium after only 48 h exposure; no precipitate was detected on cells grown in low-phosphate medium up to 96 h. These results suggest that sorption onto the cell surfaces was the dominant process initially. Analysis of the U(VI)-bearing precipitates by all three methods demonstrated the presence of H-autunite, HUO₂PO₄ · 4H₂O. Thermodynamic calculations suggest that the chemical compositions of the solutions containing yeast grown in high-phosphate medium were undersaturated with respect to H-autunite, but were supersaturated with ten times more U(VI) and P than were actually observed. Apparently, the sorbed U(VI) on the cell surfaces reacts with P released from the yeast to form H-autunite by local saturation. The U(VI) uptake by yeast cells grown in high phosphate medium at pH 4.7, along with the thermodynamic calculation, indicated that more H-autunite is precipitated in neutral pH solution than in acid solution. Thus, U(VI)-phosphate mineralization on the cells of microorganisms should be taken into account for predicting U(VI) mobility in the environment.     

Neogene geological history of the Tohoku Island Arc system

The Pacific-type orogeny in the Tohoku Island Arc is discussed using marine geological and geophysical data from both Pacific and Japan Sea along the Tohoku region. The Tohoku Arc is divided into three belts; inner volcanic and sedimentary belt, intermediate uplifted belt and outer sedimentary trench belt. Thick Neogene sediments which are distinguished in several layers by continuous seismic reflection profiling occur on both sides of the intermediate belt. The dominant structural trend of the Neogene layers is approximately parallel to the coast line and to the axis of the Japan Trench and has a extension of approximately 100 km in each unit on the Pacific side. The trench slope break is an uplifted zone of Neogene layers. The structural trend of the upper continental slope and outer shelf is relative uplift of the landward side. Tilted block movement toward the west is the dominant structural trend on the Japan Sea side. Structural trends which can be seen in both the inner and outer belts may suggest horizontal compressional stress of east to west. Orogenesis and tectogenesis in the Tohoku Arc has been active since early Miocene or latest Oligocene. It may be implied that the Japan Trench was not present during Late Cretaceous to Paleogene, as is suggested by the volcanism of the Tohoku Arc. The basic framework of the present structure was formed during late Miocene to early Pliocene in both the inner and outer belts. Structural movements were reactivated during late Pleistocene.     

Subduction erosion and accretion in the Solomon Sea region

The Solomon Sea region is an area of intense tectonic activity characterized by structural complexity, a high level of seismicity and volcanism, and rapid evolution of plate boundaries. There is little accretion in the eastern New Britain Trench. Accretion gradually increases westward with thick accretion in the western New Britain Trench and in the Trobriand Subduction System. The thick accretion in the western part of the New Britain Trench may be a result of collision from the north of Finisterre-Huon block with New Guinea mainland. The present boundary of the collision is along the Ram-Markham fault. Deformation structures and present day seismicity suggest that the northern block is under compression.

Accretion has occurred in the sediment filled trenches in the Solomon Sea. The scale of the accretionary wedge depends on the amount of trench-fill sediment available. It is unlikely that there is no sediment supply to the eastern part of the New Britain Trench where no accretion is observed and subduction erosion may be occurring. There are two possible mechanisms for subduction erosion of sediment; either a rapid rate of subduction relative to the supply of sediment inhibiting sediment accumulation in the trench; or horizontal tensional force superimposed on both the forearc and backarc regions of the arc. Seafloor spreading in both the Manus and Woodlark basins is fan-like with nearby poles in the western margins of the basins. This may be a reflection of a horizontally compressional field in the western part and a tensional field in the eastern part of the Solomon Sea. Therefore it is possible to conclude that the consumption of sediment in the eastern New Britain Trench is related to the horizontal tensional field superimposed on both the forearc and backarc regions of the subduction system.

Imbricated thrust and overthrust faults in the western New Britain Trench and Trobriand Trough are not linear over long distance, but form wavy patterns in blocks with unit distance of approximately 10 km.     

Diurnal variation of CO2 concentration, Δ14C and δ13C in an urban forest: estimate of the anthropogenic and biogenic CO2 contributions

Diurnal variation in the atmospheric CO₂ concentration and the carbon isotopic composition (Δ¹⁴C and δ¹³C) was measured in a forest in an urban area on 9 February 1999. The carbon isotope approach used in the present study differentiated between the quantitative contributions from anthropogenic and biogenic CO₂ sources in the urban atmosphere. The anthropogenic (fossil fuel) and biogenic (soil respiration) contributions was estimated, and they ranged from 1 to 16% and from 2 to 8% of the total atmospheric CO₂. The diurnal variation of the anthropogenic CO₂ was the major cause of the total atmospheric CO₂ variation, while the biogenic CO₂ remained relatively constant throughout the day. Estimating the contribution of soil respired CO₂ provided the mean residence time of soil respired CO₂ within the forest atmosphere.     

Effects of flow dimension in faulted or fractured rock on natural reductions of inflow during excavation: a case study of the Horonobe Underground Research Laboratory site,Japan

Major inflows of groundwater can occur during excavation in faulted or fractured rock masses, even if pre-excavation grouting is applied; postexcavation grouting may then be required to reduce these inflows. However, the diffusion equation for fluid pressure suggests that inflows may reduce naturally by 50–90% or more within days or weeks when the dimension of the flow in faults or fractures that feed the inflow (flow dimension) is close to 1, but inflow reduction is minimal when the flow dimension is close to 3. Therefore, if the flow dimension is close to 1, the natural reduction in inflow may obviate countermeasures. Nevertheless, this natural reduction being dependent on flow dimension is seldom considered explicitly when planning excavations or countermeasures. To verify the applicability of the relationship between natural changes of inflow and the flow dimension, this study measured changes in inflow at six locations at the Horonobe site, Japan, where major inflows occurred during excavations of tunnels or shafts in faulted or fractured siliceous rocks. The flow dimension at each location was assessed using pre-excavation packer tests in surface-based boreholes. The results confirm that changes in the inflows during the days and weeks immediately after their commencement depended consistently on the assessed flow dimension. Natural reductions in inflow during excavation are predictable based on the flow dimension, which can be estimated using pre-excavation borehole investigations or from the initial changes in inflow during the first several days. This approach may be helpful for improving the efficiency of excavations.

     

Carbon cycle and climate change during the Cretaceous inferred from a biogeochemical carbon cycle model

The carbon cycle and climate change during the Cretaceous are reconstructed by using a carbon cycle model, and discussed. The model takes into account the effects of the enhanced magma eruption and organic carbon burial rates, both of which characterize the carbon cycle during the Cretaceous. The result for the CO₂ variation is roughly consistent with the pattern of paleoclimate change inferred from the geological record. The CO₂ level during the mid-Cretaceous is estimated to be 4–5 times the present atmospheric level, corresponding to a surface temperature of 20–21°C. The warm, equable Cretaceous resulted from the effects of tectonic forcing such as enhanced CO₂ degassing, although the enhanced organic carbon burial has a tendency to decrease the CO₂ level. The organic carbon burial rate during the Cretaceous is generally larger than those for the Cenozoic, and is characterized by three major peaks (~ 1.5–1.8 times the present-day value) corresponding to the major oceanic anoxic events. In the case for the extensive mantle plume degassing, although the CO₂ levels are only 10% higher than those for the standard case during 120–100 Ma, the causes for the enhanced CO₂ levels would be quite different. If the globally averaged surface temperature had increased due to paleogeographic forcing effects, the greenhouse effect of CO₂ (and thus the CO₂ level) should be lower than the values estimated for the standard case. If the CO₂ levels are similar to, but the surface temperature is higher than, those for the standard case, either the parameter β (an influence of the Himalayas–Tibetan Plateau on the global weathering today) may be unreasonably large or the dependence of the silicate weathering rate on the CO₂ partial pressure and the surface temperature should be much weaker than those previously proposed.     

Age estimation of the Solomon Sea based on heat flow data

Several heat flow measurements were made during the NAT83 cruise in the central part of the Solomon Sea Basin. The average value of 87 mW/m² (2.08 HFU) calculated from these and other data indicates that the age of the Solomon Sea Basin may range from 24 to 44 Ma. This is supported by the water depth, of approximately 4,500 m, versus age relationship. There is a possibility that the Solomon Sea Basin is not a back-arc basin associated with an arc but was formerly a relatively large oceanic plate. The agreement in age from both heat flow and water depth data favors the latter hypothesis.     

High Resolution, Deep-Tow Seismic Survey to Investigate the Methane Hydrate Stability Zone in the Nankai Trough

Abstract. A high frequency deep-tow seismic survey was carried out in the Nankai Trough area in 1996. The objective of the survey was to obtain high resolution seismic sections and velocity profiles of the methane hydrate zone, inferred from the strong BSR events seen on conventional seismic data in the area. A special feature of the survey is that both the source and the streamer cable are towed close to the seabed. This special acquisition geometry requires special data processing to handle the varying source and receiver depths. A CMP floating datum processing sequence was designed which led to high quality sections of the shallow geology. A key step in the processing was devising a residual statics technique to compensate for errors in the measured depths.
The processing sequence was applied to a number of lines, totaling 200 km. The final data quality was highly variable. Some lines produced high quality sections and others, much poorer sections with few interpretable events. Conventional seismic data in the area also shows variation in the data quality so part of the reason is a variation in the sub-sea geology, but the deep-tow data is much more sensitive to change in conditions than conventional data. With the current acquired data and processing sequence the deep-tow system offers most advantages when 1) the water depth is around 1 km or greater, 2) the seabed and underlying geology is not too complex, and 3) the acquisition proceeds smoothly with regular shotpoints, slowly varying depths, and with accurate positioning.     

Stick–slip behavior of Indian gabbro as studied using a NIED large-scale biaxial friction apparatus

This paper reports stick–slip behaviors of Indian gabbro as studied using a new large-scale biaxial friction apparatus, built in the National Research Institute for Earth Science and Disaster Prevention(NIED), Tsukuba, Japan. The apparatus consists of the existing shaking table as the shear-loading device up to 3,600 k N, the main frame for holding two large rectangular prismatic specimens with a sliding area of 0.75 m2 and for applying normal stresses rnup to 1.33 MPa, and a reaction force unit holding the stationary specimen to the ground. The shaking table can produce loading rates v up to 1.0 m/s,accelerations up to 9.4 m/s2, and displacements d up to0.44 m, using four servocontrolled actuators. We report results from eight preliminary experiments conducted with room humidity on the same gabbro specimens at v = 0.1–100 mm/s and rn= 0.66–1.33 MPa, and with d of about 0.39 m. The peak and steady-state friction coefficients were about 0.8 and 0.6, respectively, consistent with the Byerlee friction. The axial force drop or shearstress drop during an abrupt slip is linearly proportional to the amount of displacement, and the slope of this relationship determines the stiffness of the apparatus as1.15 9 108N/m or 153 MPa/m for the specimens we used.This low stiffness makes fault motion very unstable and the overshooting of shear stress to a negative value was recognized in some violent stick–slip events. An abrupt slip occurred in a constant rise time of 16–18 ms despite wide variation of the stress drop, and an average velocity during an abrupt slip is linearly proportional to the stress drop.The use of a large-scale shaking table has a great potential in increasing the slip rate and total displacement in biaxial friction experiments with large specimens.