Nitrogen recycling in subducted mantle rocks and implications for the global nitrogen cycle

The nitrogen concentrations [N] and isotopic compositions of ultramafic mantle rocks that represent various dehydration stages and metamorphic conditions during the subduction cycle were investigated to assess the role of such rocks in deep-Earth N cycling. The samples analyzed record low-grade serpentinization on the seafloor and/or in the forearc wedge (low-grade serpentinites from Monte Nero/Italy and Erro Tobbio/Italy) and two successive stages of metamorphic dehydration at increasing pressures and temperatures (high-pressure (HP) serpentinites from Erro Tobbio/Italy and chlorite harzburgites from Cerro del Almirez/Spain) to allow for the determination of dehydration effects in ultramafic rocks on the N budget. In low-grade serpentinites, δ¹⁵N_air values (?3.8 to +3.5 ‰) and [N] (1.3–4.5 μg/g) are elevated compared to the pristine depleted MORB mantle (δ¹⁵N_air ~ ?5 ‰, [N] = 0.27 ± 0.16 μg/g), indicating input from sedimentary organic sources, at the outer rise during slab bending and/or in the forearc mantle wedge during hydration by slab-derived fluids. Both HP serpentinites and chlorite harzburgites have δ¹⁵N_air values and [N] overlapping with low-grade serpentinites, indicating no significant loss of N during metamorphic dehydration and retention of N to depths of 60–70 km. The best estimate for the δ¹⁵N_air of ultramafic rocks recycled into the mantle is +3 ± 2 ‰. The global N subduction input flux in serpentinized oceanic mantle rocks was calculated as 2.3 × 10⁸ mol N₂/year, assuming a thickness of serpentinized slab mantle of 500 m. This is at least one order of magnitude smaller than the N fluxes calculated for sediments and altered oceanic crust. Calculated global input fluxes for a range of representative subducting sections of unmetamorphosed and HP-metamorphosed slabs, all incorporating serpentinized slab mantle, range from 1.1 × 10¹⁰ to 3.9 × 10¹⁰ mol N₂/year. The best estimate for the δ¹⁵N_air of the subducting slab is +4 ± 1 ‰, supporting models that invoke recycling of subducted N in mantle plumes and consistent with general models for the volatile evolution on Earth. Estimates of the efficiency of arc return of subducted N are complicated further by the possibility that mantle wedge hydrated in forearcs, then dragged to beneath volcanic fronts, is capable of conveying significant amounts of N to subarc depths.     

Platinum group elements in impactites of the ICDP Chicxulub drill core Yaxcopoil‐1: Are there traces of the projectile?

Abstract— This study presents results of platinum group element (PGE) analyses of impactites from the Yaxcopoil‐1 (Yax‐1) and Yucatán 6 drill cores of the 180 km‐diameter Chicxulub crater. These are the main elements used for projectile identification. They were determined by nickel sulfide fire assay combined with inductively coupled plasma mass spectrometry. The concentration of PGE in the samples are low. The concentration patterns of the suevite samples resemble the pattern of the continental crust. We conclude that any meteoritic fraction in these samples is below 0.05%. A syn‐ and post‐impact modification of the PGE pattern from meteoritic toward a continental crust pattern is very unlikely. The globally distributed fallout at the Cretaceous‐Tertiary (K/T) boundary, however, has high PGE concentrations. Therefore, the lack of a significant meteoritic PGE signature in the crater is not an argument for a PGE‐poor impactor. Taking the results of three‐dimensional numerical simulations of the Chicxulub event into account, the following conclusions are drawn: 1) The main fraction of the impactor was ejected into and beyond the stratosphere, distributed globally, and deposited in the K/T boundary clay; and 2) the low amount of projectile contamination in the Yax‐1 lithologies may reflect an oblique impact. However, the role of volatiles in the mixing process between projectile and target is not well‐understood and may also have played a fundamental role.     

Implications of the fault scaling law for the growth of topography: mountain ranges in the broken foreland of north-east Tibet

A fault scaling law suggests that, over eight orders of magnitude, fault length L is linearly related to maximum displacement D. Individual faults may therefore retain a constant ratio of D/L as they grow. If erosion is minor compared with tectonic uplift, the length and along‐strike relief of young mountain ranges should thus reflect fault growth. Topographic profiles along the crests of mountain ranges in the actively deforming foreland of north‐east Tibet exhibit a characteristic shape with maximum height near their centre and decreasing elevation toward the tips. We interpret the along‐strike relief of these ranges to reflect the slip distribution on high‐angle reverse faults. A geometric model illustrates that the lateral propagation rate of such mountain ranges may be deciphered if their length‐to‐height ratio has remained constant. As an application of the model, we reconstruct the growth of the Heli Shan using a long‐term uplift rate of ～1.3 mm yr^?1 derived from ²¹Ne and ¹⁰Be exposure dating.     

Giga-Pixels and Sky Surveys

We describe a project to build a new type of astronomical CCD that should significantly decrease the cost per pixel of detectors. This device should also provide very fast readout, autoguiding capability, image motion compensation, and good red sensitivity. This revised version was published online in July 2006 with corrections to the Cover Date.     

Experimental evidence for the potential impact ejection of viable microorganisms from Mars and Mars-like planets

Bacterial spores (Bacillus subtilis), cyanobacteria (Chroococcidiopsis sp.), and lichen (Xanthoria elegans) embedded in martian analogue rock (gabbro) were exposed to shock pressures between 5 and 50 GPa which is the range of pressures observed in martian meteorites. The survival of Bacillus subtilis and Xanthoria elegans up to 45 GPa and of Chroococcidiopsis sp. up to 10 GPa supports the possibility of transfer of life inside meteoroids between Mars and Earth and it implies the potential for the transfer of life from any Mars-like planet to other habitable planets in the same stellar system.     

Is GRS 1915+105 A Microquasar?

The large mechanical luminosity of the jets of GRS 1915+105 should give rise to luminous emission regions, similar to those observed in radio galaxies, where the jets interact with the gas surrounding the source. However, no radio synchrotron emission of the expected morphology has been found. Here we present the results of a study suggesting that radio bremsstrahlung from the compressed and heated ISM in front of the jets should be detectable, while the synchrotron lobes may be too faint. We identify these jet impact sites with two well-known IRAS regions. This identification suggests a distance of GRS 1915+105 of 6.5± 1.6 kpc, significantly closer than the usually assumed distance of 11–12 kpc. We discuss the implications of this reduced distance estimate. The non-detection of the synchrotron radio lobes implies a significant fraction of non-radiating particles, possibly protons, in the jets. The apparent motion of small-scale jet components is not superluminal, so if superluminal motion is required for an object to be termed a microquasar, GRS 1915+105 actually does not qualify. The mass of the black hole in the system is increased to 21± 9 M_⊙, while the mechanical luminosity of the jets is reduced to 14% of the Eddington luminosity.