Theoretical and experimental evidence for a post-perovskite phase of MgSiO3 in Earth's D" layer

The Earth's lower mantle is believed to be composed mainly of (Mg,Fe)SiO3 perovskite, with lesser amounts of (Mg,Fe)O and CaSiO3). But it has not been possible to explain many unusual properties of the lowermost 150 km of the mantle (the D" layer) with this mineralogy. Here, using ab initio simulations and high-pressure experiments, we show that at pressures and temperatures of the D" layer, MgSiO3 transforms from perovskite into a layered CaIrO3-type post-perovskite phase. The elastic properties of the post-perovskite phase and its stability field explain several observed puzzling properties of the D" layer: its seismic anisotropy, the strongly undulating shear-wave discontinuity at its top and possibly the anticorrelation between shear and bulk sound velocities.Comment: PUBLISHED IN Nature 430, 445-448 (2004

Vertical jump performance after 90 days bed rest with and without flywheel resistive exercise, including a 180 days follow-up

Muscle atrophy and neuromuscular de-conditioning occur in response to space flight and bed-rest. In this study, we investigated the efficacy of flywheel training to conserve jumping power and height during 90 days bed rest. Twenty-four young healthy men underwent strict bed-rest (-6° head down tilt) for 90 days. Eight participants were assigned to a flywheel group (FW) and 16 to a control group (Ctrl). The ground reaction force was measured during vertical jump tests twice during baseline data collection, and on day 4, 7, 14, 90 and 180 of recovery. In half of the participants, jump tests were also performed within minutes after re-ambulation and on four more occasions during the first 2 days of recovery. Jump height was reduced from 40.6 cm (SD 6.1 cm) during the first baseline measurement to 27.6 cm (SD 5.6 cm) on day 4 of recovery in Ctrl, but only from 38.6 cm (SD 3.9 cm) to 34.4 cm (SD 6.5 cm) in FW (P < 0.001). At the same time, peak power was reduced from 47.4 W/kg (SD 8.0 W/kg) to 34.5 W/kg in Ctrl, but only from 46.2 W/kg (6.0 W/kg) to 42.2 W/kg SD 4.6 W/kg) in FW (P < 0.001). Jump height and peak power were completely recovered after 163 and 140 days in Ctrl, respectively, and after 72 and 18 days in FW (regression analysis). In conclusion, flywheel exercise could effectively offset neuromuscular de-conditioning during bed-rest, and led to full recovery at an earlier stage. These findings nourish the hope that adequate training paradigms can fully sustain neuromuscular function under microgravity conditions

First-principles constraints on diffusion in lower-mantle minerals and a weak D'' layer

Post-perovskite MgSiO3 is believed to be present in the D '' region of the Earth's lower most mantle(1-4). Its existence has been used to explain a number of seismic observations, such as the D '' reflector and the high degree of seismic anisotropy within the D '' layer(5-8). Ionic diffusion in post-perovskite controls its viscosity, which in turn controls the thermal and chemical coupling between the core and the mantle, the development of plumes and the stability of deep chemical reservoirs(9). Here we report the use of first-principles methods to calculate absolute diffusion rates in post-perovskite under the conditions found in the Earth's lower mantle. We find that the diffusion of Mg2+ and Si4+ in post-perovskite is extremely anisotropic, with almost eight orders of magnitude difference between the fast and slow directions. If post-perovskite in the D '' layer shows significant lattice-preferred orientation, the fast diffusion direction will render post-perovskite up to four orders of magnitude weaker than perovskite. The presence of weak postperovskite strongly increases the heat flux across the core-mantle boundary and alters the geotherm(9). It also provides an explanation for laterally varying viscosity in the lowermost mantle, as required by long-period geoid models(10). Moreover, the behaviour of very weak post-perovskite can reconcile seismic observation of a D '' reflector with recent experiments showing that the width of the perovskite-to-post-perovskite transition is too wide to cause sharp reflectors(11). We suggest that the observed sharp D '' reflector is caused by a rapid change in seismic anisotropy. Once sufficient perovskite has transformed into post-perovskite, post-perovskite becomes interconnected and strain is partitioned into this weaker phase. At this point, the weaker post-perovskite will start to deform rapidly, thereby developing a strong crystallographic texture. We show that the expected seismic contrast between the deformed perovskite-plus-post-perovskite assemblage and the overlying isotropic perovskite-plus-post-perovskite assemblage is consistent with seismic observations