Modeling pN2 through Geological Time: Implications for Planetary Climates and Atmospheric Biosignatures

Nitrogen is a major nutrient for all life on Earth and could plausibly play a similar role in extraterrestrial biospheres. The major reservoir of nitrogen at Earth's surface is atmospheric N2, but recent studies have proposed that the size of this reservoir may have fluctuated significantly over the course of Earth's history with particularly low levels in the Neoarchean - presumably as a result of biological activity. We used a biogeochemical box model to test which conditions are necessary to cause large swings in atmospheric N2 pressure. Parameters for our model are constrained by observations of modern Earth and reconstructions of biomass burial and oxidative weathering in deep time. A 1-D climate model was used to model potential effects on atmospheric climate. In a second set of tests, we perturbed our box model to investigate which parameters have the greatest impact on the evolution of atmospheric pN2 and consider possible implications for nitrogen cycling on other planets. Our results suggest that (a) a high rate of biomass burial would have been needed in the Archean to draw down atmospheric pN2 to less than half modern levels, (b) the resulting effect on temperature could probably have been compensated by increasing solar luminosity and a mild increase in pCO2, and (c) atmospheric oxygenation could have initiated a stepwise pN2 rebound through oxidative weathering. In general, life appears to be necessary for significant atmospheric pN2 swings on Earth-like planets. Our results further support the idea that an exoplanetary atmosphere rich in both N2 and O2 is a signature of an oxygen-producing biosphere.Comment: 33 pages, 11 figures, 2 tables (includes appendix), published in Astrobiolog

Axial focusing of impact energy in the Earth's interior: Proof-of-principle tests of a new hypothesis

A causal link between major impact events and global processes would probably require a significant change in the thermal state of the Earth's interior, presumably brought about by coupling of impact energy. One possible mechanism for such energy coupling from the surface to the deep interior would be through focusing due to axial symmetry. Antipodal focusing of surface and body waves from earthquakes is a well-known phenomenon which has previously been exploited by seismologists in studies of the Earth's deep interior. Antipodal focusing from impacts on the Moon, Mercury, and icy satellites has also been invoked by planetary scientists to explain unusual surface features opposite some of the large impact structures on these bodies. For example, 'disrupted' terrains have been observed antipodal to the Caloris impact basis on Mercury and Imbrium Basin on the Moon. Very recently there have been speculations that antipodal focusing of impact energy within the mantle may lead to flood basalt and hotspot activity, but there has not yet been an attempt at a rigorous model. A new hypothesis was proposed and preliminary proof-of-principle tests for the coupling of energy from major impacts to the mantle by axial focusing of seismic waves was performed. Because of the axial symmetry of the explosive source, the phases and amplitudes are dependent only on ray parameter (or takeoff angle) and are independent of azimuthal angle. For a symmetric and homogeneous Earth, all the seismic energy radiated by the impact at a given takeoff angle will be refocused (minus attenuation) on the axis of symmetry, regardless of the number of reflections and refractions it has experienced. Mantle material near the axis of symmetry will experience more strain cycles with much greater amplitude than elsewhere and will therefore experience more irreversible heating. The situation is very different than for a giant earthquake, which in addition to having less energy, has an asymmetric focal mechanism and a larger area. Two independent proof-of-principle approaches were used. The first makes use of seismic simulations, which are being performed with a realistic Earth model to determine the degree of focusing along the axis and to estimate the volume of material, if any, that experiences significant irreversible heating. The second involves two-dimensional hydrodynamic code simulations to determine the stress history, internal energy, and temperature rise as a function of radius along the axis

Dynamic rock fragmentation: oil shale applications

Explosive rock fragmentation techniques used in many resource recovery operations have in the past relied heavily upon traditions of field experience for their design. As these resources, notably energy resources, become less accessible, it becomes increasingly important that fragmentation techniques be optimized and that methods be developed to effectively evaluate new or modified explosive deployment schemes. Computational procedures have significant potential in these areas, but practical applications must be preceded by a thorough understanding of the rock fracture phenomenon and the development of physically sound computational models. This paper presents some of the important features of a rock fragmentation model that was developed as part of a program directed at the preparation of subterranean beds for in situ processing of oil shale. The model, which has been implemented in a two-dimensional Lagrangian wavecode, employs a continuum damage concept to quantify the degree of fracturing and takes into account experimental observations that fracture strength and fragment dimensions depend on tensile strain rates. The basic premises of the model are considered in the paper as well as some comparisons between calculated results and observations from blasting experiments

Iron overload alters iron-regulatory genes and proteins, down-regulates osteoblastic phenotype, and is associated with apoptosis in fetal rat calvaria cultures

Iron overload has been implicated in decreased bone mineral density. However, the effect of iron overload on osteoblast lineage cells remains poorly understood. The purpose of this study was to examine osteoblast differentiation, function, and apoptosis in iron-loaded cells from fetal rat calvaria. Cells were incubated with media supplemented with 0–10 µM ferrous sulfate (FeSO4) during differentiation (days 6–20). Intracellular iron status was assessed by measuring iron content in cell layers and changes in transferrin receptor (TrfR) and ferritin gene and protein expression. Osteoblast differentiation and function were evaluated by measuring osteoblast phenotypic gene markers and capacity of cultures to form mineralized bone nodules. Apoptotic hallmarks were evaluated by microscopy. A 2.3-fold increase in media iron concentration resulted in saturable accumulation of iron in the cell layer 20-fold higher than control (p < 0.05) by mid-differentiation (day 15, D15). Iron accumulation resulted in rapid and sustained down-regulation of TrfR gene and protein levels (within 24 h) and up-regulation of light and heavy chain ferritin protein levels at late differentiation (day 20, D20). Concurrently, osteoblast phenotype gene markers were suppressed by D15 and a decreased number of mineralized nodules at D20 were observed. Apoptotic events were observed within 24 h of iron loading. These results provide evidence that iron overload alters iron metabolism and suppresses differentiation and function of cells in the osteoblast lineage associated with increased apoptosis

Towards assessing the violence of reaction during cookoff of confined energetic materials

An analysis of post-ignition events in a variable confinement cookoff test (VCCT) geometry is presented aimed toward predicting the level of violence during cookoff of confined thermally-degraded energetic materials. This study focuses on the dynamic events following thermal initiation whereby accelerated combustion interacts with confinement. Numerical simulations, based on a model of reactive multiphase mixtures, indicate that the response of energetic material is highly dependent upon thermal/mechanical damage states prior to ignition. These damaged states affect the rate of pressurization, dynamic compaction behavior and subsequent growth to detonation. Variations of the specific surface area and porosity produced by decomposition of the energetic material causes different responses ranging from pressure burst to detonation. Calculated stress histories are used in estimating breakup of the VCCT confinement based on Grady-Kipp fragmentation theory

Neuromuscular Electrical Stimulation Effects on Skeletal Muscle Fatigue in Older Adults

Neuromuscular electrical stimulation (NMES) is often used as a rehabilitative modality and evidence has suggested that high frequencies of NMES may elicit increases in muscle strength. However, little is known regarding the effects of a high-frequency NMES intervention on voluntary skeletal muscle fatigue. PURPOSE: The aim of this study was to determine the effect of a 4-week high-frequency NMES intervention on voluntary muscular fatigue and changes in neuromuscular activation patterns of the quadriceps during voluntary fatiguing muscle contractions in older adults. METHODS: Seventeen healthy, older adults (68.8 ± 1.8 years old) participated in the study (NMES: n = 12; SHAM: n = 5). Each participant was seated on an isokinetic dynamometer, and a 40-min NMES treatment was applied to the quadriceps muscles of each leg 3x/week for 4 weeks with the stimulation frequency set at 60 Hz. Stimulation intensity was set to achieve 15% of knee extension maximal voluntary contraction (MVC). Those in the SHAM group underwent the same treatment procedures but did not receive the NMES treatment. All subjects performed maximal voluntary contractions (MVC) and an intermittent knee extension isometric submaximal voluntary fatigue task at 50% MVC until the fatigue criteria were met for pre-post testing. Surface electromyography (sEMG) of the vastus lateralis (VL) and vastus medialis (VM) muscles were recorded during the fatigue task to examine changes in muscle activation. EMG data were quantified for root mean square (RMS) EMG and reported as a percent rate of change over the duration of the fatigue task and median frequency (MF) is reported as the average MF during the fatigue task. Repeated measures ANOVAs were used to determine differences pre-post NMES for muscular endurance time, MVC and EMG measures. Statistical significance was set at p \u3c 0.05. RESULTS: MVC increased pre-post NMES in the NMES group (117.1 ± 8.7 Nm vs 127.6 ± 11.1 Nm, p = 0.049; pre- and post-training, respectively) with no change in SHAM (p = 0.96). Muscular endurance time did not change pre-post NMES (NMES: 159.3 ± 20.1s vs 141.9 ± 21.2s, p = 0.29; SHAM: 242.2 ± 43.3s vs 202.9 ± 23.3s, p = 0.13; pre- and post-training, respectively). RMS EMG rate of change did not change following NMES treatment (NMES: VL: 16.6 ± 3.6% vs 18.8 ± 10.4%, p = 0.84; VM: 11.4 ± 2.1% vs 19.6 ± 5.5%, p = 0.15; SHAM: VL: 7.8 ± 1.6% vs 7.1 ± 3.0%, p = 0.81; VM: 7.1 ± 3.3% vs 5.9 ± 2.2%, p = 0.55; pre- and post-training, respectively). Also, there was no difference in MF EMG with NMES training (NMES: VL: 77.6 ± 4.1 Hz vs 74.9 ± 3.6 Hz, p = 0.13; VM: 72.5 ± 2.4 Hz vs 72.6 ± 2.2 Hz, p = 0.97; SHAM: VL: 79.3 ± 3.4 Hz vs 80.2 ± 4.9 Hz, p = 0.85; VM: 76.9 ± 3.7 Hz vs 83.9 ± 5.1 Hz, p = 0.12; pre- and post-training, respectively). CONCLUSION: Treatment with high-frequency NMES did not improve muscle endurance or related EMG parameters. It is possible that NMES induced adaptations may be frequency-specific and that high-frequency NMES may not be efficacious when the goal is to improve skeletal muscle endurance

Energetic and spatial bonding properties from angular distributions of ultraviolet photoelectrons: application to the GaAs(110) surface

Angle-resolved ultraviolet photoemission spectra are interpreted by combining the energetics and spatial properties of the contributing states. One-step calculations are in excellent agreement with new azimuthal experimental data for GaAs(110). Strong variations caused by the dispersion of the surface bands permit an accurate mapping of the electronic structure. The delocalization of the valence states is discussed analogous to photoelectron diffraction. The spatial origin of the electrons is determined, and found to be strongly energy dependent, with uv excitation probing the bonding region.Comment: 5 pages, 3 figures, submitted for publicatio