Meteorite concentration mechanisms in Antarctica

The location of most Antarctic meteorite finds is on stagnant, highly ablative surfaces known as blue ice. The role of blue ice as transporter, concentrator, and preserver of specimens from the time of fall until find is discussed

The Allan Hills Meteorite Icefield-An alternative view

More than 1300 meteorites have been found on the surface of blue icefields in North Victorialand, Antarctica. The Allan Hills Icefield (about 100 km^2) has exceptionally high meteorite concentrations while other icefields to the West have much smaller concentrations. Measurements of the rates of ablation and of horizontal displacement of the ice surface near the Allan Hills have been conducted since 1978 at a 20 station triangulation network. These data show that the horizontal ice velocity at the most active stations is about one m/yr and less at the high meteorite concentration site. Ablation of the ice surface averages about 4.2 em/yr. Measurements of the oxygen isotopic composition of surface ice along the triangulation network show a rather large scatter of about 8δ^(18)O‰. This indicates that the ice comes from different areas or possibly is different in age. The terrestrial ages of Allan Hills meteorites are between 0 and 700,000 years, with only a few older than 400,000 years. A model for the appearance of meteorites on blue ice surfaces in Antarctica is that specimens are carried within the moving ice sheet to stagnant areas where they are uncovered by the ablation process (see e.g. Bull and Lipschutz, 1982). These areas ("emergent zones") are fed by ice that originally accumulated as snow at the source regions of the ice. This model accounts for the general occurrence of meteorites on blue ice fields but an additional mechanism is needed to explain the high concentrations found at the Allan Hills. It is suggested that this icefield has concentrated meteorites primarily by horizontal movement of the ice from the emergent zones located to the west of the concentration zone. These meteorites are transported by surface compressive flow of the ice into the Allan Hills Meteorite Icefield and left stranded in an area where ice is only lost by the ablation process. This model seems to be in agreement with all field and laboratory observations

International Workshop on Antarctic Meteorites

Topics addressed include: meteorite concentration mechanisms; meteorites and the Antarctic ice sheet; iron meteorites; iodine overabundance in meteorites; entrainment, transport, and concentration of meteorites in polar ice sheets; weathering of stony meteorites; cosmic ray records; radiocarbon dating; element distribution and noble gas isotopic abundances in lunar meteorites; thermoanalytical characterization; trace elements; thermoluminescence; parent sources; and meteorite ablation and fusion spherules in Antarctic ice

The role of delayed bone age in the evaluation of stature and bone health in glucocorticoid treated patients with Duchenne muscular dystrophy

Background Low bone mineral density and an increased risk of appendicular and vertebral fractures are well-established consequences of Duchenne muscular dystrophy (DMD) and the risk of fractures is exacerbated by long-term glucocorticoid treatment. Monitoring of endocrine and skeletal health and timely intervention in at-risk patients is important in the management of children with DMD. Methods As part of the Norwegian Duchenne muscular dystrophy cohort study, we examined the skeletal maturation of 62 boys less than 18 years old, both currently glucocorticoid treated (n = 44), previously treated (n = 6) and naïve (n = 12). The relationship between bone age, height and bone mineral density (BMD) Z-scores was explored. Results The participants in the glucocorticoid treated group were short in stature and puberty was delayed. Bone age was significantly delayed, and the delay increased with age and duration of treatment. The difference in height between glucocorticoid treated and naïve boys was no longer significant when height was corrected for delayed skeletal maturation. Mean BMD Z-scores fell below − 2 before 12 years of age in the glucocorticoid treated group, with scores significantly correlated with age, duration of treatment and pubertal development. When BMD Z-scores were corrected for by retarded bone age, the increase in BMD Z-scores was significant for all age groups. Conclusion Our results suggest that skeletal maturation should be assessed in the evaluation of short stature and bone health in GC treated boys with DMD, as failing to consider delayed bone age leads to underestimation of BMD Z-scores and potentially overestimation of fracture risk

Ground-based and Polar spacecraft observations of a giant (Pg) pulsation and its associated source mechanism

Multi-instrument observations of a Pg pulsation, which occurred on the morning of May 16, 1998, are reported. The wave signature was observed simultaneously on the ground, by the International Monitor for Auroral Geomagnetic Effects (IMAGE) magnetometer network and in the ionosphere by the Doppler Pulsation Experiment (DOPE) high resolution HF Doppler sounder. The wave occurred in the morning sector and possessed an azimuthal wave number, m, of 30±5 with a westward phase propagation. Shortly before the Pg commenced, energetic particle instruments on board the Polar spacecraft detected protons with a non-Maxwellian energy distribution drifting westward toward the location of IMAGE and DOPE. An investigation has been undertaken to determine whether these particles were involved in the wave-particle interaction considered responsible for generating the Pg pulsation. Proton energies of around 7 keV, which occur at the low-energy edge of the unstable distribution (where ∂ƒ/∂W>0), satisfy the drift-bounce resonance relation, ω − mω[subscript d] = Nω[subscript b], for N=1. This result indicates that this particular wave is likely to be the result of a drift-bounce resonance mechanism and that it has an even mode standing wave structure in the magnetosphere. This result is discussed in terms of previous observations of Pgs