Estimates of Trends of Climatically Important Atmospheric Gases Near St. Petersburg

Izvestiya, Atmospheric and Oceanic Physics - Total column amounts (TCAs) of various climatically important atmospheric gases have been determined in Peterhof for the period between 2009 and 2018....     

The oxidation state of subcontinental mantle: oxygen thermobarometry of mantle xenoliths from central Asia

The oxygen fugacities of 48 mantle xenoliths from 5 localities in southern Siberia (USSR) and Mongolia have been determined. Ferric iron contents of spinels were measured by ⁵⁷Fe Mössbauer spectroscopy and oxygen fugacities calculated from spinel-olivineorthopyroxene equilibrium. The samples studied represent the major types of upper mantle lithologies including spinel and garnet peridotites and pyroxenites, fertile and depleted peridotites and anhydrous and metasomatized samples which come from diverse tectonic settings. Extensive geochemical and isotope data are also available for these samples. Oxygen fugacity values for most central Asian xenoliths fall within the range observed in peridotite xenoliths from other continental regions at or slightly below the FMQ buffer. However, xenoliths from the Baikal rift zone are the most reduced among xenoliths for which Mössbauer data on spinels are available. They yield fO₂ values similar to those in oceanic peridotites and MORBs, while xenoliths in other occurrences have higher fO₂s. In general, the continental lithosperic mantle is more oxidized than MORB-like oceanic mantle. This difference seems to be due to incorporation of oxidized material into some parts of the subcontinental mantle as a result of subduction of oceanic crust. Garnet- and garnet-spinel lherzolites from the Baikal rift area have slightly higher oxygen fugacities than shallower spinel lherzolites. Oxygen fugacity does not appear to be correlated with the degree of depletion of peridotites, and its values in peridotites and pyroxenites are very much alike, suggesting that partial melting (at least at moderate degrees) takes place at essentially the same fO₂s that are now recorded by the residual material. Modally (amphibole- and phlogopitebearing) and cryptically metasomatized xenoliths from the Baikal rift zone give the same fO₂ values as depleted anhydrous peridotites, suggesting that solid-melt-fluid reactions in the continental rift mantle also take place without substantial change in redox state. This is in contrast to other tectonic environments where metasomatism appears to be associated with oxidation.     

Analytical expressions for the borehole seismic signatures excited by an explosive source buried in an elastic half‐space

Borehole guided waves that are excited by explosive sources outside of the borehole are important for interpreting borehole seismic surveys and for rock property inversion workflows. Borehole seismograms are typically modelled using numerical methods of wave propagation. In order to benchmark such numerical algorithms and partially to interpret the results of modelling, an analytical methodology is presented here to compute synthetic seismograms. The specific setup is a wavefield emanating from a monopole point source embedded within a homogeneous elastic medium that interacts with a fluid‐filled borehole and a free surface. The methodology assumes that the wavelength of the seismic signal is much larger than the borehole radius. In this paper, it is supposed that there is no poroelastic coupling between the formation and the borehole. The total wavefield solution consists of P, PP, and PS body waves; the surface Rayleigh wave; and the low‐frequency guided Stoneley wave (often referred as the tube wave) within the borehole. In its turn, the tube wave consists of the partial responses generated by the incident P‐wave and the reflected PP and PS body waves at the borehole mouth and by the Rayleigh wave, as well as the Stoneley wave eigenmode. The Mach tube wave, which is a conic tube wave, additionally appears in the Mach cone in a slow formation with the tube‐wave velocity greater than the shear one. The conditions of appearance of the Mach wave in a slow formation are formulated. It is shown that the amplitude of the Mach tube wave strongly depends on Poisson's ratio of the slow surrounding formation. The amplitude of the Mach tube wave exponentially decreases when the source depth grows for weakly compressible elastic media with Poisson's ratio close to 0.5 (i.e., saturated clays and saturated clay soils). Asymptotic expressions are also provided to compute the wavefield amplitudes for different combinations of source depth and source‐well offset. These expressions allow an approximate solution of the wavefield to be computed much faster (within several seconds) than directly computing the implicit integrals arising from the analytical formulation.     

The origin of coarse garnet peridotites in cratonic lithosphere: new data on xenoliths from the Udachnaya kimberlite, central Siberia

We report new textural and chemical data for 10 garnet peridotite xenoliths from the Udachnaya kimberlite and examine them together with recent data on another 21 xenoliths from the 80–220 km depth range. The samples are very fresh (LOI near zero), modally homogeneous and large (>100 g). Some coarse-grained peridotites show incipient stages of deformation with <10 % neoblasts at grain boundaries of coarse olivine. Such microstructures can only be recognized in very fresh rocks, because fine-grained interstitial olivine is strongly affected by alteration, and may have been overlooked in previous studies of altered peridotite xenoliths in the Siberian and other cratons. Some of the garnet peridotites are similar in composition to low-opx Udachnaya spinel harzburgites (previously interpreted as pristine melt extraction residues), but the majority show post-melting enrichments in Fe and Ti. The least metasomatized coarse peridotites were formed by 30–38 % of polybaric fractional melting between 7 and 4 GPa and ≤1–3 GPa. Our data together with experimental results suggest that garnet in these rocks, as well as in some other cratonic peridotites elsewhere, may be a residual mineral, which has survived partial melting together with olivine and opx. Many coarse and all deformed garnet peridotites from Udachnaya underwent modal metasomatism through interaction of the melting residues with Fe-, Al-, Si-, Ti-, REE-rich melts, which precipitated cpx, less commonly additional garnet. The xenoliths define a complex geotherm probably affected by thermal perturbations shortly before the intrusion of the host kimberlite magmas. The deformation in the lower lithosphere may be linked to metasomatism.     

Redox state of deep off-craton lithospheric mantle: new data from garnet and spinel peridotites from Vitim, southern Siberia

Oxygen fugacity (fO₂) affects melting, metasomatism, speciation of C–O–H fluids and carbon-rich phases in the upper mantle. fO₂ of deep off-craton mantle is poorly known because garnet-peridotite xenoliths are rare in alkali basalts. We examine the redox and thermal state of the lithospheric mantle between the Siberian and North China cratons using new Fe³⁺/ΣFe ratios in garnet and spinel obtained by M?ssbauer spectroscopy, major element data and P–T estimates for 22 peridotite xenoliths as well as published data for 15 xenoliths from Vitim, Russia. Shallow spinel-facies mantle is more oxidized than deep garnet peridotites (average, ?0.1 vs. ?2.5 ΔlogfO₂(FMQ)). For intermediate garnet–spinel peridotites, fO₂ estimates from spinel-based oxybarometers are 1.5–3.2 ΔlogfO₂(FMQ) lower than those from garnet-based oxybarometers. These rocks may be out of phase and chemical inter-mineral equilibrium because the spinel–garnet reaction and concomitant changes in mineral chemistry do not keep up with P–T changes (e.g., lithospheric heating by recent volcanism) due to slow diffusion of trivalent cations and because gar-, gar-spl and spl-facies rocks may coexist on centimeter–meter scale. The spinel-based fO₂ estimates may not be correct while garnet-based fO₂ values provide conditions before the heating. The T (780–1,100?°C) and fO₂ ranges of the Vitim xenoliths overlap those of coarse garnet and spinel cratonic peridotites. However, because of a higher geothermal gradient, the deepest Vitim garnet peridotites are more reduced (by 0.5–2.0 ΔlogfO₂(FMQ)) than cratonic garnet peridotites at similar depths, and the “water maximum” conditions (>80?% H₂O) in the off-craton mantle exist in a more shallow and narrow depth range (60–85?km) than in cratonic roots (100–170?km). The base of the off-craton lithospheric mantle (≥90?km) at 2.5?GPa and 1,150?°C has fO₂ of ?3.0 ?logfO₂(FMQ), with dominant CH₄ and H₂O and minor H₂ in the fluid. Melting near the base of off-craton mantle lithosphere may be induced by increasing water share in migrating fluids due to oxidation of methane.     

NO<Subscript>2</Subscript> content variations near St. Petersburg as inferred from ground-based and satellite measurements of scattered solar radiation

An automatic spectral complex developed at the Institute of Physics, St. Petersburg State University, is described. This complex is used for regular ground-based spectroscopic measurements of the total NO₂ content in the vertical column of the atmosphere during the twilight and daylight hours of the day near St. Petersburg (Petrodvorets). In 2004–2006, a number of ground-based twilight measurements of the total NO₂ content were obtained near St. Petersburg, and variations in the NO₂ content in the troposphere were estimated from the results of daytime ground-based measurements. An example of the spatial annual mean distribution of the NO₂ content (central and northern Europe, northwestern Russia) based on the data of satellite measurements over the period 2003–2005 is presented. This example demonstrates the main sources of anthropogenic pollution. An increase in the mean annual contents of tropospheric NO₂ near Moscow and St. Petersburg is preliminarily estimated for the entire period of satellite observations with the GOME instrument at about 30–40% over ten years.     

Centre for Baltic and arctic studies at St. Petersburg state University as a naional node of “University GIS network” in Russia

The possibilities of recent advantages in Geographical Information Systems (GIS) technology as a tool for multidisciplinary coastal zone research are discussed. International education in the new high technologies is one of the most urgent tasks for the University GIS Network.     

New Olivine Reference Material for In Situ Microanalysis

A new olivine reference material – MongOL Sh11‐2 – for in situ analysis has been prepared from the central portion of a large (20 × 20 × 10 cm) mantle peridotite xenolith from a ~ 0.5 My old basaltic breccia at Shavaryn‐Tsaram, Tariat region, central Mongolia. The xenolith is a fertile mantle lherzolite with minimal signs of alteration. Approximately 10 g of 0.5–2 mm gem quality olivine fragments were separated under binocular microscope and analysed by EPMA, LA‐ICP‐MS, SIMS and bulk analytical methods (ID‐ICP‐MS for Mg and Fe, XRF, ICP‐MS) for major, minor and trace elements at six institutions world‐wide. The results show that the olivine fragments are sufficiently homogeneous with respect to major (Mg, Fe, Si), minor and trace elements. Significant inhomogeneity was revealed only for phosphorus (homogeneity index of 12.4), whereas Li, Na, Al, Sc, Ti and Cr show minor inhomogeneity (homogeneity index of 1–2). The presence of some mineral and fluid‐melt micro‐inclusions may be responsible for the inconsistency in mass fractions obtained by in situ and bulk analytical methods for Al, Cu, Sr, Zr, Ga, Dy and Ho. Here we report reference and information values for twenty‐seven major, minor and trace elements.     

Nitrogen dioxide in the air basin of St. Petersburg: Remote measurements and numerical simulation

The results of ground-based and satellite spectroscopic measurements of the tropospheric NO₂ content near St. Petersburg in January–March 2006 are presented. It is shown that the increased concentrations of NO₂ observed in St. Petersburg and its vicinities in this period were caused by NO₂ accumulation due to unfavorable weather conditions, which is confirmed by an analysis of meteorological data and the results of a numerical simulation of the dispersion of urban air pollutants. Data from satellite and ground-based measurements agree with each other satisfactorily (a correlation coefficient of 0.5) and with model calculations of tropospheric NO₂ conducted for the coordinates of a station of ground-based measurements (a correlation coefficient of 0.6). The HYSPLIT dispersion model also made it possible to estimate the scale of the NO₂ spatial-temporal variability in the near-surface layer in the vicinities of St. Petersburg.     

Ionization fraction in the thermosphere of the exoplanet HD 209458b

Dissociation and ionization of hydrogen molecules and ionization of hydrogen atoms due to extreme UV radiation from the parent star are accompanied by the formation of a concurrent photoelectron flux with excess kinetic energy. These dissociation and ionization processes are the main source of atomic and molecular ions in the thermospheres of extrasolar planets, such as the “hot Jupiter” HD 209458b. The ionization processes are the most important part of contemporary aeronomic models of planetary atmospheres in the Solar System and extrasolar systems (Johnson et al., 2008; Yelle et al., 2008). We estimate the contribution of the dissociation and ionization processes due to the stellar UV radiation and the concurrent photoelectron flux to the formation of extended ionospheres around extrasolar giant planets. As opposed to models of other researchers, we calculated the ionization rates due to the concurrent photo-electron flux for the first time. It is established that, in contrast to a widely used parametrization of the photoelectron contribution (Cecchi-Pestellini et al., 2006; 2009), the rate of secondary ionization due to the photoelectrons depends appreciably on the altitude, approaching the photoionization rate in the lower layers of the thermosphere. The calculated ionization rate in the thermosphere of the extrasolar giant planet (EGP) orbiting close to its parent star is a necessary link when modeling an aeronomic model and estimating the rate of the EGP atmospheric loss.