Measurements of the limb darkening in the forbidden Mgi line at 4571.1 Å

We report high resolution measurements of the center-to-limb variation of the Mgi line at 4571.1 Å. This forbidden line is of interest because it should be formed in LTE. Comparison of our measurements with the Harvard-Smithsonian Reference Atmosphere show that the line center radiation originates in the temperature minimum region from 330 to 550 km above the point where _continuum = 1. Observations near the limb confirm that the temperature minimum is 4200K.The National Center for Atmospheric Research and Kitt Peak National Observatory are sponsored by the National Science Foundation.     

Resolution of the Hα double-limb controversy

The discussion of the H double limb had reached the point where the question of its existence as a real solar phenomenon could not be resolved without new observations made with the Lockheed filter and the Mount Wilson spectroheliograph. A study of the instrumental profiles had indicated that there was sufficient off-band light to produce the observed inner limb step in the Mount Wilson instrument, but this analysis was not completely satisfactory because of limitations inherent in the measurement of instrument functions with a Hg-198 source. The instrumental profile work did indicate, however, that the spectral purity of the instruments in question could be substantially improved by the use of narrow-band interference filters. An experimental program was thus launched to determine the effect of such a blocking filter on the appearance of the H limb. The results of these observations with three Halle filter systems and the Mount Wilson spectroheliograph are that the inner limb completely disappears at the center of H when a blocking filter is used to reduce unwanted light, which originates at wavelengths beyond ±0.8 Å. In addition, the contrast and visibility of the chromospheric fine structure is increased by eliminating the off-band light. Thus the experiment conclusively demonstrates that the apparent inner limb is not a solar feature but is due entirely to instrumental parasitic light.     

An asymmetrically rotating fluid disc with applications

The rotation of a compressible inviscid fluid disc of (1) slowly varying density or (2) nonuniform density (cold gas approximation) or (3) nonuniform density (hot, but tenuous) is considered. Perturbation methods for solving the basic equation for conservation of vorticity are used. It is found that steady state conditions are realized when vortex waves and differential rotation (jet streams) coexist; special solutions for these vortex waves are obtained. For one of these solutions, a given jet stream and its associated vortex (only one vortex per jet allowed) wave can exist only at certain discrete orbital distances, given by a geometric progressionA ⁿ wheren is an integer andA is a constant. This progression is a good representation for the distances of planets and satellites, with small orbital inclinations, from their respective parent bodies. Certain other solutions for the vortex wave yield streamlines that are logarithmic spirals. Some justifications are given for applying the model to the dynamics of hurricanes and spiral galaxies. Comparisons with observations are surprisingly favorable.The possible role of the jet streams and the steady state long vortex waves (a cooperative-vortex phenomenon) in the formation and evolution of the solar system is also discussed. Comparisons are made with the von Weizsäcker (1944 and Chandrasekhar, 1946) model of turbulent eddies in the solar nebula and with the particle (asteroidal) jet streams of Alfvén and Arrhenius (1970a, b).     

A mantle plume origin for the Siberian traps: uplift and extension in the West Siberian Basin, Russia

The West Siberian Basin (WSB) records a detailed history of Permo-Triassic rifting, extension and volcanism, followed by Mesozoic and Cenozoic sedimentation in a thermally subsiding basin. Sedimentary deposits of Permian age are absent from much of the basin, suggesting that large areas of the nascent basin were elevated and exposed at that time. Industrial seismic and well log data from the basin have enabled extension and subsidence modelling of parts of the basin. Crustal extension (β) factors are calculated to be in excess of 1.6 in the northern part of the basin across the deep Urengoy graben. 1-D backstripping of the Triassic to Cenozoic sedimentary sequences in this region indicates a period of delayed subsidence during the early Mesozoic. The combination of elevation, rifting and volcanism is consistent with sublithospheric support, such as a hot mantle plume.

This interpretation accords with the geochemical data for basalts from the Siberian Traps and the West Siberian Basin, which are considered to be part of the same large igneous province. Whilst early suites from Noril'sk indicate moderate pressures of melting (mostly within the garnet stability field), later suites (and those from the West Siberian Basin) indicate shallow average depths of melting. The main region of magma production was therefore beneath the relatively thin (ca. 50–100 km) lithosphere of the basin, and not the craton on which the present-day exposure of the Traps occurs. The indicated uplift, widespread occurrence of basalts, and short duration of the volcanic province as a whole are entirely consistent with published models involving a mantle plume. The main argument against the plume model, namely lack of any associated uplift, appears to be untenable.     

Geochemistry and petrogenesis of basalts from the West Siberian Basin: an extension of the Permo–Triassic Siberian Traps, Russia

New major and trace element data for the Permo–Triassic basalts from the West Siberian Basin (WSB) indicate that they are strikingly similar to the Nadezhdinsky suite of the Siberian Trap basalts. The WSB basalts exhibit low Ti/Zr (50) and low high-field-strength element abundances combined with other elemental characteristics (e.g., low Mg#, and negative Nb and Ti anomalies on mantle-normalised plots) typical of fractionated, crustally contaminated continental flood basalts (CFBs). The major and trace element data are consistent with a process of fractional crystallisation coupled with assimilation of incompatible-element-enriched lower crust. Relatively low rates of assimilation to fractional crystallisation (0.2) are required to generate the elemental distribution observed in the WSB basalts. The magmas parental to the basalts may have been derived from source regions similar to primitive mantle (OIB source) or to the Ontong Java Plateau source. Trace element modelling suggests that the majority of the analysed WSB basalts were derived by large degrees of partial melting at pressures less than 3 GPa, and therefore within the garnet-spinel transition zone or the spinel stability field.

It seems unlikely that large-scale melting in the WSB was induced through lithospheric extension alone, and additional heating, probably from a mantle plume, would have been required. We argue that the WSB basalts are chemically and therefore genetically related to the Siberian Traps basalts, especially the Nadezhdinsky suite found at Noril'sk. This suite immediately preceded the main pulse of volcanism that extruded lava over large areas of the Siberian Craton. Magma volume and timing constraints strongly suggest that a mantle plume was involved in the formation of the Earth's largest continental flood basalt province.     

A record of Holocene climate change from lake geochemical analyses in southeastern Arabia

Lacustrine sediments from southeastern Arabia reveal variations in lake level corresponding to changes in the strength and duration of Indian Ocean Monsoon (IOM) summer rainfall and winter cyclonic rainfall. The late glacial/Holocene transition of the region was characterised by the development of mega-linear dunes. These dunes became stabilised and vegetated during the early Holocene and interdunal lakes formed in response to the incursion of the IOM at approximately 8500 cal yr BP with the development of C3 dominated savanna grasslands. The IOM weakened ca. 6000 cal yr BP with the onset of regional aridity, aeolian sedimentation and dune reactivation and accretion. Despite this reduction in precipitation, the lake was maintained by winter dominated rainfall. There was a shift to drier adapted C4 grasslands across the dune field. Lake sediment geochemical analyses record precipitation minima at 8200, 5000 and 4200 cal yr BP that coincide with Bond events in the North Atlantic. A number of these events correspond with changes in cultural periods, suggesting that climate was a key mechanism affecting human occupation and exploitation of this region.     

Coupling chemical weathering with soil production across soil‐mantled landscapes

Soil‐covered upland landscapes constitute a critical part of the habitable world. Our understanding of how they evolve as a function of different climatic, tectonic and geological regimes is important across a wide range of disciplines and depends, in part, on understanding the links between chemical and physical weathering processes. Extensive previous work has shown that soil production rates decrease with increasing soil column thickness, but chemical weathering rates were not measured. Here we examine a granitic, soil‐mantled hillslope at Point Reyes, California, where soil production rates were determined using in situ produced cosmogenic nuclides (¹⁰Be and ²⁶Al), and we quantify the extent as well as the rates of chemical weathering of the saprolite from beneath soil from across the landscape. We collected saprolite samples from the base of soil pits and analysed them for abrasion pH as well as for major and trace elements by X‐ray fluorescence spectroscopy, and for clay mineralogy by X‐ray diffraction spectroscopy. Our results show for the first time that chemical weathering rates decrease with increasing soil thickness and account for 13 to 51 per cent of total denudation. We also show that spatial variation in chemical weathering appears to be topographically controlled: weathering rate decreases with slope across the divergent ridge and increases with upslope contributing area in the convergent swale. Furthermore, to determine the best measure for the extent of saprolite weathering, we compared four different chemical weathering indices – the Vogt ratio, the chemical index of alteration (CIA), Parker's index, and the silicon–aluminium ratio – with saprolite pH. Measurements of the CIA were the most closely correlated with saprolite pH, showing that weathering intensity decreases linearly with an increase in saprolite pH from 4·7 to almost 7. Data presented here are among the first to couple directly rates of soil production and chemical weathering with how topography is likely to control weathering at a hillslope scale. Copyright © 2006 John Wiley & Sons, Ltd.