Diffusion and partitioning of magnesium in quartz grain boundaries

Experiments were conducted to study Mg diffusion in quartz grain boundaries. A detector particle method was used to study grain-boundary diffusion because Mg was confined exclusively to the grain boundaries. Diffusion couples were assembled by placing a MgF₂ disk against a disk of quartzite, which was placed against a disk of quartzite that contained fayalite (Fe₂SiO₄) ‘detector particles.’ During diffusion experiments, Mg diffused along the grain boundaries of the central quartzite toward the fayalite quartzite where it was incorporated into fayalite detector particles. The only pathway for transport from the diffusant source to fayalite detector particles was through the grain boundaries in the central quartzite. The cross-sectional area of the grain boundaries that delivered Mg to the fayalite detector particles was determined from scanning electron microscope images. The Mg contents of the fayalite detector particles were used to calculate the mass of Mg that fluxed through the grain boundaries. During the diffusion experiments, pyroxene crystallites nucleated and grew in the central quartzites from Mg and Fe that was transported along quartz grain boundaries. The Mg contents of the crystallites vary linearly throughout the quartzites, suggesting that steady-state transport conditions were rapidly established in the quartz grain boundaries. Magnesium concentrations in the pyroxene crystallites are proportional to concentration gradients in the grain boundaries of the central quartzite. Grain-boundary fluxes and linear concentration gradients were used to calculate diffusion of Mg in grain boundaries of the central quartzite component in the diffusion couples.     

Radiocarbon Dating of Interdune Paleo‐Wetland Deposits to Constrain the Age of Mid‐to‐Late Holocene Microlithic Artifacts from the Zhongba site,Southwestern Qinghai‐Tibet Plateau

Microlithic artifacts, some found in situ, are abundant in the Zhongba archaeological site in southwestern Tibet. The site environment consists of extant wetlands and paleo‐wetland deposits found in depressions between sand dunes derived from the Yarlung Tsangpo floodplain. Constraining ¹⁴C dates from wetland vegetation and shell from one site fall between ca. 6600–2600 cal. yr B.P., while a second site is dated 3400–1200 cal. yr B.P. A significant and variable ¹⁴C reservoir effect—up to 1400 ¹⁴C yr—limits these ranges to terminus post quem constraints. The in situ artifacts are supplemented by surface collections fully characterizing raw material and typological variability for each site. Raw material found at Zhongba is chert and chalcedony likely sourced from Cretaceous bedrock near the site. Typologically, microblades are nongeometric and are derived from conical and wedge‐shaped cores similar to those identified in the Qinghai Lake Basin and the Chang Tang Nature Reserve of similar or greater age. The later occupation period at Zhongba is broadly contemporaneous with sites on the Qinghai‐Tibet Plateau containing bronze and iron artifacts, indicating microlithic technology remained an important tool‐making strategy in western Tibet late into the protohistoric period.     

Dynamic limits on planar libration-orbit coupling around an oblate primary

This paper explores the dynamic properties of the planar system of an ellipsoidal satellite in an equatorial orbit about an oblate primary. In particular, we investigate the conditions for which the satellite is bound in librational motion or when the satellite will circulate with respect to the primary. We find the existence of stable equilibrium points about which the satellite can librate, and explore both the linearized and non-linear dynamics around these points. Absolute bounds are placed on the phase space of the libration-orbit coupling through the use of zero-velocity curves that exist in the system. These zero-velocity curves are used to derive a sufficient condition for when the satellite’s libration is bound to less than $90^{\circ }$ . When this condition is not satisfied so that circulation of the satellite is possible, the initial conditions at zero libration angle are determined which lead to circulation of the satellite. Exact analytical conditions for circulation and the maximum libration angle are derived for the case of a small satellite in orbits of any eccentricity.     

Simultaneous Observations of the Chromosphere with TRACE and SUMER

Using mainly the 1600 Å continuum channel and also the 1216 Å Lyman-α channel (which includes some UV continuum and C iv emission) aboard the TRACE satellite, we observed the complete lifetime of a transient, bright chromospheric loop. Simultaneous observations with the SUMER instrument aboard the SOHO spacecraft revealed interesting material velocities through the Doppler effect existing above the chromospheric loop imaged with TRACE, possibly corresponding to extended nonvisible loops, or the base of an X-ray jet.     

Indoor Air Background Levels of Volatile Organic Compounds and Air-Phase Petroleum Hydrocarbons in Office Buildings and Schools

A background indoor air study has been completed which includes the collection of indoor air samples from office buildings and schools. The anonymous study was designed with input from the U.S. Environmental Protection Agency and the Massachusetts Department of Environmental Protection. The sampling was implemented in 2013, 2014, and 2015 and included the collection of 25 school building samples and 61 office building samples. The study generated 14,668 new indoor air background data points, with samples collected from buildings located in 26 cities in 18 states, including Arizona, California, Connecticut, Indiana, Kansas, Maine, Massachusetts, Minnesota, Montana, New Hampshire, New Jersey, New York, Nevada, North Carolina, Ohio, Texas, Utah, and Washington. Indoor air background concentrations of target compound volatile organic compounds (VOCs) ranged from less than the laboratory method reporting limit of 0.044 μg/m³ to concentrations up to 1190 μg/m³, with hydrocarbon ranges from less than the reporting method limit of 10 μg/m³ to concentrations up to 3000 μg/m³. Some VOCs were identified ubiquitously in indoor air background, and some were identified at concentrations which exceeded risk-based regulatory screening levels. These study results provide useful and updated information on indoor air background and air quality in offices and schools and can be used in future regulatory guidance update considerations, for further examination of relationships between these data and residential study data, in human health risk assessments and risk communication, and in planning future studies.     

Ganymede crater dimensions – Implications for central peak and central pit formation and development

The morphology of impact craters on the icy Galilean satellites differs from craters on rocky bodies. The differences are thought due to the relative weakness of ice and the possible presence of sub-surface water layers. Digital elevation models constructed from Galileo images were used to measure a range of dimensions of craters on the dark and bright terrains of Ganymede. Measurements were made from multiple profiles across each crater, so that natural variation in crater dimensions could be assessed and averaged scaling trends constructed. The additional depth, slope and volume information reported in this work has enabled study of central peak formation and development, and allowed a quantitative assessment of the various theories for central pit formation. We note a possible difference in the size-morphology progression between small craters on icy and silicate bodies, where central peaks occur in small craters before there is any slumping of the crater rim, which is the opposite to the observed sequence on the Moon. Conversely, our crater dimension analyses suggest that the size-morphology progression of large lunar craters from central peak to peak-ring is mirrored on Ganymede, but that the peak-ring is subsequently modified to a central pit morphology. Pit formation may occur via the collapse of surface material into a void left by the gradual release of impact-induced volatiles or the drainage of impact melt into sub-crater fractures.     

Winter Cyclones and Circulation Patterns on the Western Great Lakes

The climatology of winter cyclones crossing the western Great Lakes was investigated. Data concerning storm strength, place of origin, and surface and 500 mb level synoptic characteristics were obtained from the months of October through February, 1955–1976, for 469 cyclones. November cyclones had the lowest mean pressure and strongest pressure gradient of the 5 months examined, but cyclonic frequencies were greatest in December and January. In all months, cyclones originating in the southwestern United States were significantly deeper than storms of northern origins, and these cyclones crossed the western Lakes most frequently in November. In November, cyclogenesis in the southwest was related to departures of the observed flow from the mean monthly mid-tropospheric circulation with trough development over the west. Yearly variations in the number of severe November cyclones were related to differences of mean monthly 500 mb flow.     

THE INFLUENCE OF RELATIVE HUMIDITY ON THE DEVELOPMENT OF TROPICAL CYCLONES

Traditionally, the development of tropical cyclones has been attributed to the coincidence of warm water, weak upper level divergence, and weak upper level wind shear. In this study, numerical simulations clearly demonstrate the importance of moisture on the intensification of these systems. Simulated tropical cyclones develop more quickly and become more intense when a moister initial sounding is used. Thus, attempts to explain the spatial and temporal distributions of tropical cyclones should include humidity in the list of variables to be examined.