Stacking for the frequencies and Qs of 0S0 and 1S0

Summary. Using nine IDA records for the Indonesian earthquake of 1977 August 19, we have formed an optimal linear combination of the records and have measured the frequency and Q of ₀ S ₀ and ₁ S ₀. The frequency was measured using the moment ratio method. The attenuation was measured by the minimum width method and by the time-lapse method. The frequency and attenuation were measured simultaneously by varying them to obtain a best fit to the data. A 2000-hr stack, the sum of nine individual records, for ₀ S ₀ gave a frequency of 0.814664 mHz±4 ppm. The values for the Q of ₀ S ₀ for the three different methods of measurement were 5600,5833 and 5700, respectively. The error in the estimates of Q ^-1 is about 5 per cent for the minimum power method. For ₁ S ₀ a 300-hr stack yielded a frequency of 1.63151 mHz±30 ppm. The values of Q for this mode were 1960, 1800 and 1850, respectively, with an error in Q ^-1 of about 12 per cent for the minimum power method.     

Fluxes of CO2 and CH4 in high latitude wetlands: measuring, modelling and predicting response to climate change

This review covers selected aspects of recent international efforts to measure and model greenhouse gas emission from northern wetlands, to identify the environmental factors that control gas emission, and to investigate wetlands'responses (particularly with respect to gas emission) to global change. Both bottom-up and top-to-bottom approaches, based respectively on local observations plus inventory of gas fluxes and inverse modelling of global circulation, agree on the size of the high latitude (>60_°N) contribution to global methane, which should be about 13% or 70 Tg/year. It has been shown that winter and spring fluxes are an essential part in the annual budget of CH₄ and especially CO₂ exchange (varying from 5 to 50%). Soil micro-organisms were shown to be able to respire during winter even at-16_°C. In comparison to aerobically respiring organisms, anaerobic methanogenic bacteria were less active in frozen soil, although they are subjected to significant stimulation by soil freeze-thaw cycles. The absence of immediate coupling of methanogenesis with plant photosynthesis implies that substrates for methane formation are derived from peat decomposition rather than from root exudation.     

Gyromagnetic remanence acquired by greigite (Fe3S4) during static three-axis alternating field demagnetization

A magnetic study was carried out on lacustrine sediments from the Zoigê basin, Tibetan Plateau, in order to obtain a better understanding of palaeoclimatic changes there. Gyromagnetic remanence (GRM) acquisition is unexpectedly observed during static three-axis alternating field (AF) demagnetization in about 20 per cent of a large number of samples. X-ray diffraction (XRD) analysis on a magnetic extract clearly shows that greigite is the dominant magnetic mineral carrier. Scanning electron microscopy (SEM) reveals that the greigite particles are in the grain size range of 200–300  nm, possibly in the single-domain state. Greigite clumps of about 3  μm size are sealed by silicates. Fitting of XRD peaks yields a crystalline coherence length of about 15  nm, indicating that the particles seen in the SEM are polycrystalline.
  GRM intensities of most samples are of the same order as the NRM, while others show much stronger GRM although their magnetic properties are similar. Variation of the demagnetization sequence confirms that GRM is mainly produced perpendicular to the AF direction. The anisotropy direction can be derived from GRM, but more systematic studies are needed for detailed conclusions. An attempt to correct for GRM failed due to high GRM intensities and because smaller GRM acquisition was also found along the demagnetization axis. Behaviours of acquisition and AF demagnetization of GRM are comparable with those of NRM, ARM, IRM, indicating fine grain sizes of remanence carriers.     

40Ar/39Ar dates of eastern Iceland lavas

Summary. Twenty-two plateau ages obtained by the ⁴⁰Ar/³⁹Ar step-heating method span most of the time of formation of eastern Iceland. With one exception the dates are consistent with the stratigraphy, the exception having a very unusual argon release pattern. It is concluded that the ⁴⁰Ar/³⁹Ar step-heating method can give correct dates on hydrothermally altered samples the total fusion ages of which are not correct; however, it is not possible to conclude from the results obtained that the method is inherently more accurate than the conventional method using carefully selected samples.
Though it is known from geological evidence that minor irregularities of spreading have occurred the results show that, within the resolution of the dates, spreading has been steady and symmetrical; in particular, neither a large eastward jump nor repetitive westward jumps can have occurred. The average half spreading rate for the period 2Myr to 12 Myr ago was 0.78 ± 0.16 cm yr-¹.     

Magnetic properties of hydrothermally synthesized greigite (Fe3 S4 )—II. High- and low-temperature characteristics

The magnetic behaviour of hydrothermally synthesized greigite was analysed in the temperature range from 4 K to 700 °C. Below room temperature, hysteresis parameters were determined as a function of temperature, with emphasis on the temperature range below 50 K. Saturation magnetization and initial susceptibility were studied above room temperature, along with X-ray diffraction analysis of material heated to various temperatures. The magnetic behaviour of synthetic greigite on heating is determined by chemical alteration rather than by magnetic unblocking. Heating in air yields more discriminative behaviour than heating in argon. When heated in air, the amount of oxygen available for reaction with greigite determines the products and magnetic behaviour. In systems open to contact with air, haematite is the final reaction product. When the contact with air is restricted, magnetite is the final reaction product. When air is excluded, pyrrhotite and magnetite are the final reaction products; the amount of magnetite formed is determined by the purity of the starting greigite and the degree of its surficial oxidation. The saturation magnetization of synthetic greigite is virtually independent of temperature from room temperature down to 4 K. The saturation remanent magnetization increases slowly by 20–30 per cent on cooling from room temperature to 4 K. A broad maximum is observed at ~10 K which may be diagnostic of greigite. The coercive and remanent coercive force both increase smoothly with decreasing temperature to 4 K. The coercive force increases from ~50 mT at room temperature to approximately 100–120 mT at 4 K, and the remanent coercive force increases from approximately 50–80 mT at room temperature to approximately 110–180 mT at 4 K.     

Numerical computation of the three-dimensional normal ellipsoidal field with a 10−2 m and 10−2 mgal accuracy

Summary. A new computation of the relevant geometrical and mechanical quantities pertaining to the three-dimensional normal ellipsoidal field (Bocchio) has been worked out with reference to a spatial grid ranging from 0 to 9000 m in dynamic height and from 0 to 90° in latitude, with a mesh size of 500 m and 1° respectively giving a better accuracy. An estimate of the latter at the upper end of the vertical interval gives 10⁻² m for the radii of curvature of the equipotential surfaces and 10⁻²mgal for normal gravity. Twenty-six functions of interest have been numerically computed including the metric tensor, Christoffel's symbols and the tensor of gravity gradients giving about 45 000 tabulated data.     

Temperature Dependence of Single-Crystal Spinel (MgAl2O4) Elastic Constants from 293 to 423°K Measured by Light-Sound Scattering in the Raman-Nath Region

Summary. The temperature dependence of single-crystal elastic constants of synthetic stoichiometric MgAl₂O₄ spinel has been measured by the light-sound scattering technique in the Raman-Nath region. The crystal is set into forced vibration by a single crystal LiNbO₃ transducer coupled to one crystal face. A He-Ne Laser beam is diffracted by the stress-induced birefringence inside the crystal. The diffraction angle is determined from the distance between two spots exposed on a photographic plate by the first order diffracted beams as measured by a microdensitometer. The sound wavelength inside the crystal is then inferred from the laser diffraction angle. Combining the sound wavelength with the measured transducer frequency, the velocity inside the crystal is determined typically to a precision of 0·05 per cent. In this method, the measurement of velocity is not dependent on either the determination of sample length or on phase shifts at sample-transducer interface. Velocities of four pure modes, L //[001], T //[001], L //[110], and T //[110]( P //[1 1 0] are measured in the temperature range between 293 and 423 °K. A linear temperature dependence is fit to the data by a least square method. Values obtained at 25 °C from this linear fit are
The temperature dependence of the adiabatic elastic constants and bulk and shear (VRH average) moduli is computed using the density and literature value of thermal expansion coefficient. Values obtained are:
A comparison with previous measurements by pulse superposition and ultrasonic interferometry methods is made. Disagreement, when present, is discussed in terms of the separate measuring techniques. Finally, the present method, with its possibility for further improvement, is evaluated as a new method to measure temperature and pressure dependence of elastic constants.