Century-Scale Change in Water Availability: CO2-Quadrupling Experiment

It has been suggested that, unless a major effort is made, the atmospheric concentration of carbon dioxide may rise above four times the pre-industrial level in a few centuries. Here we use a coupled atmosphere-ocean-land model to explore the response of the global water cycle to such a large increase in carbon dioxide, focusing on river discharge and soil moisture. Our results suggest that water is going to be more plentiful in those regions of the world that are already `water-rich'. However, water stresses will increase significantly in regions and seasons that are already relatively dry. This could pose a very challenging problem for water-resource management around the world. For soil moisture, our results indicate reductions during much of the year in many semi-arid regions of the world, such as the southwestern region of North America, the northeastern region of China, the Mediterranean coast of Europe, and the grasslands of Australia and Africa. In some of these regions, soil moisture values are reduced by almost a factor of two during the dry season. The drying in semi-arid regions is likely to induce the outward expansion of deserts to the surrounding regions. Over extensive regions of both the Eurasian and North American continents in high and middle latitudes, soil moisture decreases in summer but increases in winter, in contrast to the situation in semi-arid regions. For river discharge, our results indicate an average increase of ～ 15% during the next few centuries. The discharges from Arctic rivers such as the Mackenzie and Ob' increase by much larger fractions. In the tropics, the discharges from the Amazonas and Ganga-Brahmaputra also increase considerably. However, the percentage changes in runoff from other tropical and many mid-latitude rivers are smaller.     

The use of sintered diamond anvils in the MA8 type high-pressure apparatus

A new multi-anvil type high-presure apparatus has been developed using sintered diamond anvils to generate pressures over 30 GPa and temperatures up to about 2000°C. A maximum sample volume of about 1 mm³ is available in this system. The pressure was confirmed by dissociation of forsterite into Mg-perovskite and periclase. The basic techniques and problems in utilizing sintered diamond in the MA8 type high-pressure apparatus are discussed with an emphasis on the future prospect of incorporating simultancous X-ray diffraction observation.     

Equilibrium interface segregation in the diopside-forsterite system I: Analytical techniques, thermodynamics, and segregation characteristics

After syntheses of partially molten diopside-forsterite polycrystalline aggregates doped with various solutes, we analyzed the equilibrium segregation of Ni, Mn, Sr, Al, Yb, Y, Nd, La, and Ti at interfaces between diopside/diopside, diopside/forsterite and, forsterite/forsterite grains based on STEM/EDX (scanning transmission electron microscopy/energy dispersive X-ray spectrometry) to examine the effects of ionic size, valence state, co-segregation, and interface type on interface chemistry. We derive relationships between two quantities describing interface segregation and X-ray intensities acquired both from areas that include an interface and from areas that do not. These segregation quantities are (i) interface excess density and (ii) interface enrichment factor, which rely on Gibbsian thermodynamics and the Langmuir-McLean segregation model, respectively. Interface excess densities, which vary from −0.5 to 10 atoms/nm², indicate that the level of interface excess density depends on solutes and sample assemblage. Interface enrichment factors, which range from almost 1 to 130, reveal that the ionic size of the solutes affects their segregation via production of misfit lattice strain due to the difference between the size of a solute ion and that of the ideal strain-free lattice site. The ionic sizes of Yb and Y are almost identical to the size of the strain-free site; however, their segregation is significant indicating that a difference in valence state between the host elements (i.e., Ca and Mg) and the solutes also drives segregation. In contrast to other solutes, segregation characteristics of Al differ from these simple segregation rules. Segregation quantities do not change with interface type, indicating that the number of sites available for segregants and the driving force for segregation are similar among type of interfaces. We compare the element partitioning between diopside-melt and diopside-interfaces within the same sample assemblages. These two partition coefficients coincide if we approximate the number of segregation sites at interfaces as equivalent to 2 mono-atomic layers. Examination of the energetics in crystal-melt partitioning reveals that the interface segregation energy is essentially equal to the solute solution energy in a crystal.     

Effect of hydrostatic pressure on the lattice parameters of Fe2SiO4 olivine up to 70 kbar

The pressure dependence of the three lattice parameters and unit cell volume of fayalite (Fe₂SiO₄ olivine) was determined by X-ray diffraction under hydrostatic pressures up to 70 kbar. In order to eliminate stress inhomogeneity within a composite material consisting of a specimen mixed with an internal-pressure standard, a liquid (1 : 1 mixture of ethanol and methanol) was used as a pressure-transmitting medium. The isothermal bulk modulus calculated on the basis of the second-order Birch-Murnaghan equation of state gives the values K₀ = 1.19 ± 0.10 Mbar and K₀′ = 7 ± 4, and if we assume K₀′ = 5: K₀ = 1.24 ± 0.02 Mbar. Three axes of fayalite were found to be compressible in the following order, b >c >a. Comparisons with the results obtained under non-hydrostatic compression are made.     

Reversed magnetozone in the late Pleistocene sediments from the Pacific coast of Odaka, northeast Japan

Paleomagnetic measurements have been carried out on 103 specimens taken at about 15 cm intervals in a sea cliff exposing the marine terrace formation. Secondary components were removed by partial demagnetization in a peak field of 200 Oe. Two short reversed magnetozones are recognized. Geological and biostratigraphical evidence indicates that they are equivalent to the Blake event, which was first reported by Smith and Foster (1969) (Science 163, 565–567) from the North Atlantic deep-sea cores. The present result suggests a split nature for the Blake Event.     

Synthesis of highly dense and fine-grained aggregates of mantle composites by vacuum sintering of nano-sized mineral powders

Synthesized mineral powders with particle size of <100 nm are vacuum sintered to obtain highly dense and fine-grained polycrystalline mantle composites: single phase aggregates of forsterite (iron-free), olivine (iron containing), enstatite and diopside; two-phase composites of forsterite + spinel and forsterite + periclase; and, three-phase composites of forsterite + enstatite + diopside. Nano-sized powders of colloidal SiO₂ and highly dispersed Mg(OH)₂ with particle size of ≤50 nm are used as chemical sources for MgO and SiO₂, which are common components for all of the aggregates. These powders are mixed with powders of CaCO₃, MgAl₂O_4, and Fe(CO₂CH₃)₂ to introduce mineral phases of diopside, spinel, and olivine to the aggregates, respectively. To synthesize highly dense composites through pressureless sintering, we find that calcined powders should be composed of particles that have: (1) fully or partially reacted to the desired minerals, (2) a size of <100 nm and (3) less propensity to coalesce. Such calcined powders are cold isostatically pressed and then vacuum sintered. The temperature and duration of the sintering process are tuned to achieve a balance between high density and fine grain size. Highly dense (i.e., porosity ≤1 vol%) polycrystalline mantle mineral composites with grain size of 0.3–1.1 μm are successfully synthesized with this method.     

Geochemical study of arsenic and other trace elements in groundwater and sediments of the Old Brahmaputra River Plain,Bangladesh

The geochemical study of groundwaters and core sediments from the Old Brahmaputra plain of Bangladesh was conducted to investigate the distribution of arsenic and related trace elements. Groundwaters from tube wells are characterized by pH of 6.4–7.4, dissolved oxygen (DO) of 0.8–1.8 mg/l, Ca contents of 5–50 mg/l, and Fe contents of 0.2–12.9 mg/l. Arsenic concentrations ranged from 8 to 251 μg/l, with an average value of 63 μg/l. A strong positive correlation exists between As and Fe (r ² = 0.802; p = 0.001) concentrations in groundwater. The stratigraphic sequences in the cores consist of yellowish silty clays at top, passing downward into grayish to yellowish clays and sands. The uppermost 3 m and lower parts (from 13 to 31 m) of the core sediments are oxidized (average oxidation reduction potential (ORP) +170 and +220 mV, respectively), and the ORP values gradually become negative from 3 to 13 m depths (−35 to −180 mV), indicating that anoxic conditions prevail in the shallow aquifers of the Brahmaputra plain. Age determinations suggest that clay horizons at ~10 m depth were deposited at around 2,000 and 5,000 years BP (¹⁴C ages) during the transgressive phase of sea-level change. Elevated concentrations of As, Pb, Zn, Cu, Ni, Cr, and V are present in the silts and clays, probably due to adsorption onto clay particles. Significant concentrations of As occur in black peat and peaty sediments at depths between 9 and 13 m. A strong positive correlation between As and Fe was found in the sediments, indicating As may be adsorbed onto Fe oxides in aquifer sediments.