Intramolecular carbon isotopic analysis of acetic acid by direct injection of aqueous solution

We report an improved method for determining the intramolecular carbon isotopic composition of acetate using direct injection of aqueous samples. The system builds upon prior work that established pyrolytic conditions for online analysis and represents a significant advance in that it requires minimal preparation for samples containing as little as 1 mM sodium acetate in aqueous solution. The technique is applicable for analysis of oilfield brines, culture samples, biological samples and natural porewaters. We demonstrate its accuracy by use of a stable isotope dilution series. We also show that addition of a base and cryogenic preconcentration may induce an isotopic effect on the carboxyl carbon. This isotopic fractionation does not appear to extend to the measured methyl carbon isotope value although it can significantly alter the measured isotopic composition of the whole molecule. Our preconcentration experiments demonstrate that the method is suitable for carbon isotopic measurements of acetate methyl carbon in natural samples at concentrations as low as 90 μM, considerably broadening potential applications.     

Early-Type Stars

Away from the young disk, several classes of early type stars are found. They include (i) the old, metal-poor blue horizontal branch stars of the halo and the metal-poor tail of the thick disk; (ii) metal-rich young A stars in a rapidly rotating subsystem but with a much higher velocity dispersion than the A stars of the young disk, and (iii) a newly discovered class of metal-poor young main sequence A stars in a subsystem of intermediate galactic rotation (V_rot ≈ 120 km s⁻¹). The existence and kinematics of these various classes of early type stars provide insight into the formation of the metal-poor stellar halo of the Galaxy and into the continuing accretion events suffered by our Galaxy. This revised version was published online in July 2006 with corrections to the Cover Date.     

Autoregressive estimation of complex eigenfrequencies in low frequency seismic spectra

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The fact that a seismogram can be represented as an autoregressive (AR) time series and the use of Prony's method enable us to obtain the complex frequencies from the poles of the corresponding AR process. A frequency-domain formulation which employs a time-domain tapering technique is devised so that spectral peaks can be analysed individually, or in small groups. Statistical analysis of the estimators and several examples from two recent earthquakes illustrate the method.     

A photoautotrophic source for lycopane in marine water columns

Suspended particulate matter and recent sediments from diverse oceanic sites have been investigated for their contents of lycopane. Lycopane was present in all samples, including both oxic and anoxic water column and sediments. The highest concentrations in the water column were found in surface waters of the central Pacific gyre (1.5 ng/L) and in the anoxic waters of the Cariaco Trench (1.1 ng/L) and the Black Sea (0.3 ng/L). Vertical concentration profiles suggest that lycopane is probably algal in origin. Moreover, biogeochemical conditions in anoxic zones apparently result in a secondary production of lycopane from an as yet unidentified precursor. Compound-specific carbon isotopic analyses have been carried out on lycopane from water column and sediment samples. Isotopic compositions of lycopane range between -23.6% and -32.9% and are consistent with a photoautotrophic origin. We postulate that some lycopane is produced in surface waters of the ocean, while additional lycopane is produced in anoxic zones by anaerobic microbial action on an algal precursor.     

The upper atmosphere of Neptune: An analysis of occultation observations

An analysis of available observations of the April 7, 1968 occultation of BD ?17° 4388 by Neptune yields upper atmosphere temperatures of ～140°K near the 5 × 10¹⁴cm^?3 level. The temperature structure of the atmosphere at these levels is complicated and nonisothermal. Diurnal temperature variations are certainly less than 10°K and may be absent. The average temperature decreases by less than 15°K between 0° and 55° latitude.     

Lunar electric fields,surface Potential and Associated Plasma Sheaths

This paper reviews the electric field environment of the Moon. Lunar surface electric potentials are reported as follows: Solar Wind - Dayside: ø_o + 10 to + 18 V Solar Wind - Terminator: ø_o ç ? 10 to ? 100 V Electron and ion densities in the plasma sheath adjacent to each surface potential regime are evaluated and the corresponding Debye length estimated. The electric fields are then approximated by the surface potential over the Debye length. The results are: Solar Wind - Dayside: E_o ? 10 V m^?1 outward Solar Wind - Terminator: E_o ç 1 to 10 V m^?1 inward These fields are all at least 3 orders of magnitude higher than the pervasive solar wind electric field; however they are confined to within a few tens of meters of the lunar surface.     

The Resolving Power of Gross Earth Data

A gross Earth datum is a single measurable number describing some property of the whole Earth, such as mass, moment of interia, or the frequency of oscillation of some identified elastic-gravitational normal mode. We show how to determine whether a given finite set of gross Earth data can be used to specify an Earth structure uniquely except for fine-scale detail; and how to determine the shortest length scale which the given data can resolve at any particular depth. We apply the general theory to the linear problem of finding the depth-variation of a frequency-independent local Q from the observed quality factors Q of a finite number of normal modes. We also apply the theory to the non-linear problem of finding density vs depth from the total mass, moment, and normal-mode frequencies, in case the compressional and shear velocities are known.     

Size-dependent distribution, abundance and diurnal rhythmicity patterns in the short-spined sea urchin Anthocidaris crassispina

Little is known about the ecology and behaviour of the sea urchin Anthocidaris crassispina, particularly in relation to changes in its surrounding environment. In Cape d'Aguilar Marine Reserve, Hong Kong Island, urchins ranged from a high abundance of 16 ind m⁻² on steeply inclined rocky outcrops, which were exposed to strong onshore waves surges, to a complete absence on gravel and sandy substrata. Using a generalized additive model (GAM) the abundance and spatial extent of A. crassispina was predicted within the Bay. Predictions were strongly associated with the surface complexity of the habitat. On steep rocky slopes A. crassispina exhibited a size-dependent gradient where the mean test diameter increased in a down-shore direction as water depth increased. Since the mechanisms for maintaining this size-dependent distribution are unknown, a translocation experiment was conducted on two different size-classes of urchins. Following translocation, both large and small size-classes of A. crassispina were able to re-establish their original size-gradient within 3–5 days. Size-dependent distribution in A. crassispina may indicate resource partitioning, although the influence of hydrodynamic conditions on test size may also mediate this segregation. Anthocidaris crassispina was predominately nocturnal with almost 100% of the population moving between dusk and dawn. Locomotory activity patterns of urchins were strongly correlated with changes in seawater depth and changes in the direction of water-flow during tidal cycles. Anthocidaris crassispina exhibited an endogenously controlled locomotory activity pattern that was synchronized with changes in the tidal cycle, and which remained free-running for 13 h under constant laboratory conditions.