Geo-located Twitter as proxy for global mobility patterns

Pervasive presence of location-sharing services made it possible for researchers to gain an unprecedented access to the direct records of human activity in space and time. This article analyses geo-located Twitter messages in order to uncover global patterns of human mobility. Based on a dataset of almost a billion tweets recorded in 2012, we estimate the volume of international travelers by country of residence. Mobility profiles of different nations were examined based on such characteristics as mobility rate, radius of gyration, diversity of destinations, and inflow–outflow balance. Temporal patterns disclose the universally valid seasons of increased international mobility and the particular character of international travels of different nations. Our analysis of the community structure of the Twitter mobility network reveals spatially cohesive regions that follow the regional division of the world. We validate our result using global tourism statistics and mobility models provided by other authors and argue that Twitter is exceptionally useful for understanding and quantifying global mobility patterns.     

Geochemical and stable isotopic variability within two rivers rising under the same mountain,but belonging to two distant watersheds

Complementary geochemical and stable isotope investigations of the Gorski Kotar karst aquifer system in western Croatia were obtained for the first time, to answer the question whether both studied rivers drain the same aquifer system or not. The two main rivers, the Kupa and the Rje?ina, rise under the same mountain range, but belong to two different watersheds (Black Sea and Adriatic Sea). The karst aquifer of Gorski Kotar is a potentially important source of drinking water for two neighboring countries, Croatia and Slovenia (Central and South Europe), and is strongly influenced by both Mediterranean and continental weather conditions. It is a part of the Dinaric karst, which is “locus typicus” for karst worldwide and one of the most typical karst areas in the world.     

Magnetic properties of high-Ti basaltic rocks from the Krušné hory/Erzgebirge MTS. (Bohemia/Saxony), and their relation to mineral chemistry

This study provides new thermomagnetic and petrographic data on specific basaltic rock association from the broader vicinity of the Lou?ná-Oberwiesenthal volcanic centre, western Bohemia/Saxony. Two types of volcanic rocks were recognized there: (i) high-Ti types (3.5–5.2 wt% TiO₂) represented by (mela)nephelinite s.s., and sporadically present (ii) medium-Ti types (2.5–3.5 wt% TiO₂) of olivine nephelinite, nepheline basanite and phonotephrite compositions. In order to examine the rock-magnetic behaviour, they were studied for their variations in the Curie temperature (T_C) and field-dependent susceptibility, spinel group minerals, chemistry and petrology. Magnetic susceptibility of ulvöspinel-rich titanomagnetite, as a dominant magnetic carrier, depends on the amplitude of measured magnetic field, whereas pure magnetite is field-independent. Field dependence parameter ^kHD of the studied basaltic rocks ranges from 0.8 to 18.7%, TiO₂ contents in titanomagnetite range from 12.7 to 20.1 wt.%. TiO₂ content in titanomagnetite does not correlate with whole-rock TiO₂ content (2.8 to 5.6 wt.%). The content of substituted titanium in the sublattice of magnetite is also sensitively reflected in the Curie temperature, ranging from 200 to 580°C. The spinel group minerals are designated as titanomagnetite with the dominance of ulvöspinel, magnetite and magnesioferrite components, or titanomagnetite with the magnetite, ulvöspinel and magnesioferrite components. Only two samples are characterized by a significant presence of Cr-spinel and magnesiochromite components forming cores of titanomagnetites. The titanomagnetite-bearing rocks in the studied area, likewise the low- to medium-Ti basaltic rocks from the ?eské st?edoho?í Mts., provide similar thermomagnetic curves.     

Effect of micro‐inhomogeneity on the effective stress coefficients and undrained bulk modulus of a poroelastic medium: a double spherical shell model

Although most rocks are complex multi‐mineralic aggregates, quantitative interpretation workflows usually ignore this complexity and employ Gassmann equation and effective stress laws that assume a micro‐homogeneous (mono‐mineralic) rock. Even though the Gassmann theory and effective stress concepts have been generalized to micro‐inhomogeneous rocks, they are seldom if at all used in practice because they require a greater number of parameters, which are difficult to measure or infer from data. Furthermore, the magnitude of the effect of micro‐heterogeneity on fluid substitution and on effective stress coefficients is poorly understood. In particular, it is an open question whether deviations of the experimentally measurements of the effective stress coefficients for drained and undrained elastic moduli from theoretical predictions can be explained by the effect of micro‐heterogeneity. In an attempt to bridge this gap, we consider an idealized model of a micro‐inhomogeneous medium: a Hashin assemblage of double spherical shells. Each shell consists of a spherical pore surrounded by two concentric spherical layers of two different isotropic minerals. By analyzing the exact solution of this problem, we show that the results are exactly consistent with the equations of Brown and Korringa (which represent an extension of Gassmann's equation to micro‐inhomogeneous media). We also show that the effective stress coefficients for bulk volume α, for porosity n_? and for drained and undrained moduli are quite sensitive to the degree of heterogeneity (contrast between the moduli of the two mineral components). For instance, while for micro‐homogeneous rocks the theory gives n_? = 1, for strongly micro‐inhomogenous rocks, n_? may span a range of values from –∞ to ∞ (depending on the contrast between moduli of inner and outer shells). Furthermore, the effective stress coefficient for pore volume (Biot–Willis coefficient) α can be smaller than the porosity ?. Further studies are required to understand the applicability of the results to realistic rock geometries.     

Elasticity of pyrope and majorite-pyrope solid solutions to high temperatures

Majorite-garnet solid solutions are major mineral phases in the Earth’s upper mantle and transition zone. Here we present the first Brillouin scattering measurements of the elasticity of majorite (Mj, Mg₄Si₄O₁₂)-pyrope (Py, Mg₃Al₂Si₃O₁₂) solid solutions (Mj₅₀Py₅₀ and Mj₈₀Py₂₀) and single-crystal elasticity of pure synthetic pyrope at temperatures up to 800°C. The temperature derivatives of the adiabatic bulk (K_S) and shear (μ) moduli for all compositions along the Mj-Py join are the same within the experimental uncertainties (−∂K_S/∂T=14.0-14.5(20) MPa/K, −∂μ/∂T=8.3-9.2(10) MPa/K). The temperature dependence of the acoustic velocities for Mj-Py solid solutions is about half that of other major transition zone minerals. This implies that temperature variations in the transition zone, inferred from lateral velocity heterogeneity, can be significantly underestimated if the properties of majoritic garnet are not taken into account.     

Kornerupine parageneses in whiteschists and other magnesian rocks: is kornerupine + talc a high-pressure assemblage equivalent to tourmaline + orthoamphibole?

Kornerupine, (□,Fe,Mg)(Mg,Fe,Al)₉(Si,Al,B)₅ (O,OH,F)₂₂, has been reported with talc in rocks from six localities worldwide, but only at Chilapila Hill in the Lufilian Arc, Zambia do textural relationships imply that kornerupine (Krn) equilibrated with talc (Tlc) during a prograde metamorphic event at T≈ 640 °C, P≈ 13 kbar; a prograde Krn + Tlc assemblage has also been reported from Mautia Hill, Tanzania (P ≤ 13 kbar). In order to estimate possible constraints on the stability range for the kornerupine + talc paragenesis in nature, we constructed a P-T diagram in the model system MgO-Al₂O₃-SiO₂-H₂O (MASH) for seven phases quartz (Qtz), B-free kornerupine sensu stricto, anthophyllite (Ath), chlorite (Chl), cordierite (Crd), kyanite (Ky), and talc. The minimum pressure for Krn + Tlc + Ky stability in MASH is close to that for Ky + Tlc stability, i.e., 6–8 kbar, at T≤ 780 °C. However, in the natural system, B₂O₃ and Na₂O are major constituents in Krn and orthoamphibole (Oam), respectively, and dravitic tourmaline (Tur) is widespread. The critical assemblage alternative to Krn + Tlc in nature is Tur + Oam. The upper pressure limit of Tur + Ath is determined by the upper pressure for anthophyllite: 7.7–10.5 kbar at 682–794 °C in the MgO-SiO₂-H₂O system (Chernosky et al. 1985, Am Mineral 70:223–236), and is undoubtedly higher in the presence of Na₂O, CaO, and Al₂O₃. At three of the six localities, talc is a retrograde phase; nonetheless, it possibly equilibrated with kornerupine on the retrograde path or during a later metamorphic event at P-T conditions appropriate for Ky + Tlc. At the sixth locality (Mulvoj, southwestern Pamir Mountains, Tajikistan), Krn is found in the same thin section as talc and kyanite and all three minerals formed during a prograde metamorphic event at T≥ 650 °C, P near 7 kbar. However, Krn is restricted to a lens 4 to 6 mm thick of phlogopite + anthophyllite + Tur and it does not touch either talc or kyanite. A reaction relating the Mulvoj and Chilapila Hill (Krn + Tlc + Ky + Qtz + Tur) parageneses is calculated from compositions in the Mulvoj rock to be 0.40Tur + 2.55Ath + 1.33H₂O + 0.27F = Krn + 2.16Tlc + 0.36B₂O₃ + 0.02Rutile + 0.19Na₂O + 0.17CaO. Given the difference in metamorphic pressures estimated for Mulvoj and Chilapila Hill, Krn + Tlc is inferred to be favored by increasing pressure as well as by low Na₂O and CaO contents. Some FeO, F, Fe₂O₃, and BeO are present in measurable amounts in at least one of the phases in the Mulvoj and Chilapila Hill whiteschists (e.g., Krn contains 0.24–0.67 wt% BeO), but the effect of these constituents is subordinate to that of Na₂O, CaO and B₂O₃. The Krn + Tlc could be a more important assemblage in B-bearing whiteschists than has been reported to date, particularly at pressures where orthoamphibole is no longer stable. Received: 21 April 1997 / Accepted: 13 October 1997     

Computation of the heat function connected with short-wave radiation fluxes

Summary A report is presented on the use of the method, suggested by H. and K. Lettau, for the purposes of computing short-wave radiation fluxes and the heat function, connected with them, in atmospheric models, used for short-range numerical weather forecasting.Dedicated to 90th Birthday of Professor Frantiek Fiala     

Weathering fluxes of arsenic from a small catchment in Slovak Republic

Inputs of As to a small catchment due to chemical weathering of bedrock, mechanical weathering of bedrock, and atmospheric precipitation were 71.53, 23.98 and 0.02 g ha⁻¹ year⁻¹, respectively. The output fluxes of As due to mechanical erosion of soil, biological uptake, stream discharge, and groundwater flow were 6.32, 4.77, 0.37 and 0.02 g ha⁻¹ year⁻¹, respectively. The results indicate that arsenic accumulates in soil and regolith with a very high rate. This is attributed to the selective weathering and erosion with respect to arsenic and fixation of arsenic in the secondary solids produced by weathering. The output fluxes of As in stream and groundwater in Vydrica catchment in Slovak Republic (0.39 g ha⁻¹ year⁻¹) based on muscovite–biotite granites and granodiorites were much lower compared to catchments in a gold district in the Czech Republic. These results may be ascribed to the low levels of arsenic pollution measured in Vydrica catchment. The arsenic fluxes were estimated by calculation of mechanical and chemical weathering rates of the bedrocks in Vydrica catchment from mass balance data on sodium and silica. The justification of the steady state of Na and Si is that neither of the elements is appreciably accumulated in plants and in exchangeable pool of ions in soil.     

Modeling the electric structures of two thunderstorms and their contributions to the global circuit

This study examines the electricity in two thunderstorms, typical for their respective locales (the Great Plains and the New Mexico mountains), by modeling them as a set of steady-state horizontal layers of external currents. The model electric sources, corresponding to the charge separation processes in the thundercloud, are embedded in an exponential conducting atmosphere. The source parameters are determined by fitting the model electric field to measured profiles. The resulting currents to the ionosphere (i.e., the Wilson current) from the two storms are 0.53 A and 0.16 A, while the calculated electrical energies of the storms are 2.3 × 10¹⁰ J and 2.8 × 10⁹ J, respectively. The more vigorous storm is estimated to transfer 16 000 C in the global circuit during 8.5 h of its lifetime, while the weaker mountain storm transferred about 1200 C in its entire 2-h lifetime. Removal of the screening charge layer from above the updraft region in one modeled storm leads to only a small increase in the net Wilson current of less than 3%, while it provides a substantial local disturbance of the electric field. Overall, the model findings indicate that differences in the Wilson currents and electrical energies of the two storms result from differences in their internal dynamical and electrical structures as well as their geographical locations.