Geology,mineralogy and geochemistry of the Kekem dyke swarm (Western Cameroon): Insights into Paleozoic–Mesozoic magmatism and geodynamic implications

The broadly N70°–90°E-trending dykes swarm at Kekem cut across the Paleoproterozoic-to-Achean terranes of West Cameroon remobilized during the Pan-African orogeny. They are picrite basalts and basalts with tholeiitic/transitional affinity, as shown by mineralogical and geochemical data, with variable major and trace element contents, MgO ranges from 7.3 to 12.4 wt.%, Cr from 190 to 411 ppm, Ni from 15 to 234 ppm. All the dykes are light REE enriched with La_N/Yb_N values of 5.3–8.1, suggesting a co-magmatic origin. They originated from a 2.8% partial melting of a spinel-mantle source with no or little crustal input. The geochemical features of Kekem dykes are similar to those of Paleozoic and Mesozoic dykes recorded in North and Central Africa, suggesting multiple reactivations of pre-existing fractures that resulted in the fragmentation of western Gondwana and the opening of Central and South Atlantic Oceans.     

Using genetic techniques to investigate the sources of the invasive alga Caulerpa taxifolia in three new locations in Australia

The invasive green alga Caulerpa taxifolia has gained a high profile due to 'outbreaks' in the Mediterranean and California. During the year 2000 three new discrete locations colonised by abundant C. taxifolia were discovered in New South Wales (NSW), Australia. Sequencing of the internal transcribed spacer (ITS) region of the ribosomal DNA was used to explore the source(s) of these new records, which is an important prerequisite for subsequent environmental management responses. Our results indicate that the NSW C. taxifolia originated from several sources and, hence, through different invasion events. For two of the new records (Port Hacking, Careel Bay) it can be excluded that they are derived from the so-called "aquarium strain" of C. taxifolia, closely related to the invasive Mediterranean populations. Port Hacking is likely to have originated from tropical native populations. However, samples from Lake Conjola cannot be sufficiently distinguished with the applied technique from native C. taxifolia in Moreton Bay and the Mediterranean/"aquarium strain".     

A note on John Playfair and the statistics of directional data

John Playfair recommended the resultant vector method of averaging directions as early as 1802.     

Quantifying, Understanding and Managing the Carbon Cycle in the Next Decades

The human perturbation of the carbon cycle via the release of fossil CO₂ and land use change is now well documented and agreed to be the principal cause of climate change. We address three fundamental research areas that require major development if we were to provide policy relevant knowledge for managing the carbon-climate system over the next few decades. The three research areas are: (i) carbon observations and multiple constraint data assimilation; (ii) vulnerability of the carbon-climate system; and (iii) carbon sequestration and sustainable development.     

Spatial variability of in situ microbial activity: biotracer tests

Biotracer tests have been proposed as a means by which to characterize the in situ biodegradation potential for field-scale systems. In this study, field experiments were conducted at two sites to evaluate the utility of the biotracer method for characterizing the spatial variability of microbial activity. The first site is a mixed waste-contaminated surficial aquifer in Utah, and the second site is a chlorinated solvent-contaminated regional aquifer in Tucson, Arizona. Mass recovery of the biotracer decreased approximately linearly with increasing residence time for the Tucson site. Similar behavior was observed at the Utah site, except in the region adjacent to the injection zone, where percent recoveries were much lower than those predicted using a correlation determined using data collected downgradient of the injection zone. First-order biodegradation rate coefficients obtained from model calibration of the tracer data varied between 0.2 and 0.5/day for the Tucson site. For the Utah site, the values varied between 0.1 and 0.6/day downgradient of the injection wells, and between 0.7 and 2.6/day near the injection wells. Considering the large range over which biodegradation rate coefficients can vary, the rate coefficient exhibited relatively minimal spatial variability (factor of 2.5) for the Tucson site. Conversely, the spatial variability of the rate coefficient was an order of magnitude greater for the Utah site. These differences in variability are consistent with conditions associated with the respective sites. For example, the greater microbial activity observed in the vicinity of the injection wells for the Utah site is consistent with the biomass distribution determined from analysis of core samples, which shows larger bacterial cell densities for the region near the injection wells. These results illustrate the utility of biotracer tests for in situ characterization of microbial activity (e.g., biodegradation potential), including evaluation of potential spatial variability.     

ISO LWS Observations of H2O from R Cas: A Consistent Model For Its Circumstellar Envelope

We present an ISO LWS 43-197 μm grating spectrum of the oxygen-rich AGB star R Cas. The spectrum is rich of isolated and blended H2O lines. For their identification and in order to determine the physical parameters of the circumstellar envelope, we have constructed a model which treats radiative transfer, chemical exchange and photodissociation reactions, and various heating and cooling processes in a consistent manner. By fitting the observed line fluxes and using stellar parameters based upon the Hipparcos distance, we derived a mass-loss rate of M = 1 10-6 M⊙ yr-1 which is close to the value 6 10-7 M⊙ yr-1 previously derived for W Hya, another oxygen-rich AGB star. This revised version was published online in August 2006 with corrections to the Cover Date.     

A granular flow model for dense planetary rings

We study the viscosity of a differentially rotating particle disk in the limiting case where the particles are densely packed and their collective behavior resembles that of a liquid. The pressure tensor is derived from the equations of hydrodynamics and from a simple kinetic model of collisions described by Haff (1983). We find that density waves and narrow circular rings are unstable if the liquid approximation applies. The resulting development of nonlinear perturbations may give rise to “splashing” of the ring material in the vertical direction. These results may help in understanding the origin of the ellipticities of ringlets, the nonaxisymmetric features near the outer edge of the Saturnian B ring, and the unexplained residuals in kinematic models of the Saturnian and Uranian rings.     

The range of validity of the two-body approximation in models of terrestrial planet accumulation: II. Gravitational cross sections and runaway accretion

The validity of the two-body approximation in calculating encounters between planetesimals has been evaluated as a function of the ratio of unperturbed planetesimal velocity (with respect to a circular orbit) to mutual escape velocity when their surfaces are in contact (V/V_e). Impact rates as a function of V/V_e are calculated to within ～20% by numerical integration of the equations of motion. It is found that when V/V_e > 0.4, the two-body approximation is a good one. At low velocities (V/V_e < 0.1) two-body “collision-course” trajectories fail to lead to impacts. On the other hand, at these low velocities many impacts result from encounter trajectories with unperturbed separation distances far beyond the two-body gravitational radius. These two effects tend to cancel, and the resulting impact rates remain within a factor of ～3 of the two-body value in spite of these major differences in the nature of the impact trajectories. Therefore, on the average, the two-body approximation is useful well below the value of V/V_e for which it fails to describe individual encounters, and the required corrections are not large. As a consequence of this “anomalous gravitational focusing” planetesimals will continue to interact even when their orbits are noncrossing. This reduces the difficulty with premature isolation of planetesimal embryos during accumulation. Quantitatively, when 0.06 ? V/V_e ? 0.2, the impact rate varies approximately with the fifth power of the radius of the larger body, and is about a factor of 3 above that predicted using the conventional two-body gravitational cross-section formula. At lower values of V/V_e , the impact rate increases less rapidly. Finally, at the lowest values of V/V_e (<.02), the impact rate increases only in proportion to the geometric cross section, as a consequence of the swarm being essentially two dimensional for large unperturbed encounter distances. The gravitational enhancement in effective cross section is thereby limited to a value of about 3000. This leads to an optimal size for growth of planetesimals from a swarm of given eccentricity, and places a limit on the extent of runaway accretion.     

Some terms of nutation derived from the BIH data

Summary. The z -term of latitude and the w -term of UT, as computed by the BIH, are suitable for investigating imperfections in the representation of the real nutation in space. They have been previously used for deriving only the amplitude of the principal term of nutation (Feissel & Guinot). In this study they are used more generally for deriving the amplitudes of two terms of nutation and searching for a possible nutation in space due to the nearly-diurnal wobble.
Two kinds of data are used for this purpose: the values of z and w from 1962.0 to 1980.0 at 0.05 yr intervals and the values of z from 1967.0 to 1980.0 at 5 day intervals. The first ones have been used previously by Feissel & Guinot (using the values up to 1974.0 and 1976.95 respectively). The second ones have not previously been used for such studies.
The derived amplitudes of the principal and the fortnightly terms of nutation are in good agreement with other values deduced from observations, and with the theoretical ones corresponding to the best models of a non-rigid earth. The values obtained are not yet sufficiently precise to distinguish between these models, but could become so in the near future.
The least-squares fits, performed among the z and w data in order to look for any error in the representation of nutation, show the existence of a retrograde term, of a 434 day period for z and 444 day for w and an amplitude of the order of 0.01 arcsec. It may be due to the nearly-diurnal wobble.