Physico-chemical phenomena in comets—III: The continuum of comet Burnham (1960 II)

The new model of the cometary head proposed in papers I and II is developed and applied to comet Burnham. It takes into account the likely existence of a halo of large icy particles surrounding the nucleus. These particles are steadily stripped from the nucleus by evaporating gases. Their terminal velocity and their rate of evaporation set the size of the halo. The existence of the icy halo influences in two ways the photometric characteristics of the coma. This paper establishes the photometric shape of the continuum as reflected by the icy grains, and compares it to the observed continuum of comet Burnham. Paper IV will compare the predictions of the model with the photometric profile of the molecular emission bands of C₂, in the same comet.     

Past,present and future vegetation-cloud feedbacks in the Amazon Basin

To begin exploring the underlying mechanisms that couple vegetation to cloud formation processes, we derive the lifting condensation level (LCL) to estimate cumulus cloud base height. Using a fully coupled land–ocean–atmosphere general circulation model (HadCM3LC), we investigate Amazonian forest feedbacks on cloud formation over three geological periods; modern-day (a.d. 1970–1990), the last glacial maximum (LGM; 21 kya), and under a future climate scenario (IS92a; a.d. 2070–2090). Results indicate that for both past and future climate scenarios, LCL is higher relative to modern-day. Statistical analyses indicate that the 800 m increase in LCL during the LGM is related primarily to the drier atmosphere promoted by lower tropical sea surface temperatures. In contrast, the predicted 1,000 m increase in LCL in the future scenario is the result of a large increase in surface temperature and reduced vegetation cover.     

柴北缘鱼卡地区中侏罗统石门沟组含煤层段沉积有机相分析

柴达木盆地北缘鱼卡地区中侏罗统石门沟组含煤段中发育有油页岩、煤、碳质泥岩和泥岩等富有机质细粒沉积.为了研究其沉积有机相的类型及煤和油页岩形成的控制因素,本文通过岩心观察、工业分析结合有机地球化学测试分析等方法将含煤段细粒沉积物划分为了类型A(三角洲平原沉积环境)、B(三角洲前缘-浅湖沉积环境)、C1(湖沼非油页岩亚相)及C2(湖沼油页岩亚相)四种类型,其中类型C1沉积物中发育煤,类型C2中发育油页岩,且煤较油页岩具有更高的含油率、水分、挥发分以及发热量值.有机质类型特征方面,沉积有机相类型A、B及C1沉积物有机质类型均为Ⅱ2-Ⅲ型、有机质来源均以陆源和混合来源两种有机质来源为主,而类型C2中主要为Ⅱ2型,以混合有机质来源为主;类型A及C2其沉积物有机质均处于未成熟阶段,而类型B和C1中有机质均处于未成熟-低成熟阶段;有机质保存条件方面,类型A沉积物主要形成于缺氧的淡水环境,类型B主要形成于贫氧-缺氧的淡水-半咸水环境,类型C1形成于贫氧-缺氧的淡水-半咸水环境,而类型C2则主要形成于缺氧的淡水-半咸水环境.其中类型B较类型A,类型C2较类型C1,其沉积物均形成于更为还原且盐度更高的水体环境中.石门沟组含煤段是煤和油页岩的形成层位,稳定的沉积环境、丰富的湖泊有机质来源、良好的保存条件及较少的陆源碎屑的稀释共同促进了类型C2中油页岩的形成,而丰富的陆源植物供给及良好的保存条件则促进了类型C1中煤层的形成.     

Urban seismic stations: soil–structure interaction assessment by spectral ratio analyses

In this work we present and discuss the results of ambient seismic noise analyses computed at four sites where seismic stations, managed by the INGV (Italian Institute for Geophysics and Vulcanology) and the DPC (Italian Department of Civil Protection), are installed inside buildings. The experiments were performed considering different types of installation: sensor located at the bottom of a school, directly installed on rock (case 1); sensor located at the bottom of a medieval fortress, built on an isolate hill, directly installed on rock (case 2); sensor installed on the foundations of a medieval fortress, built on an isolate hill (case 3); sensor installed on the foundations of a school, built on alluvial deposits (case 4). Since recent works proposed the use of spectral ratio techniques to study the dynamic characterization of buildings, ambient seismic-noise measurements were performed for each site close to the stations (at the base of the structures), at the top of the structures and outside the buildings. In order to check the source of vibrations both horizontal to vertical spectral ratio (HVNR) and standard spectral ratio (SSR) techniques were applied. For all stations the results from ambient seismic noise were compared to those obtained from earthquakes (HVSR). In order to detect preferential directions of amplification, for each site average HVNRs and HVSRs were computed considering one azimuth for each set of 5°. We obtain different results for different types of installation: in cases 1 and 2, where the sensors are directly installed on rock, the vibrations of the structure do not affect the noise measures performed close to the stations, which show flat HVNR in the whole frequency range: in both cases the eigenfrequency of the building is given by the HVNR calculated from the measures performed at the top of the structure. In cases 3 and 4, where the sensors are installed on the foundations of the considered structures, both the amplification peaks between 5 and 9 Hz (case 3) and between 5.5 and 7 Hz (case 4) include the contribution of the free oscillations of the buildings. In particular, in case 4, HVNRs performed outside building highlight possible soil–structure resonance effects in case of an earthquake.     

Statistical biases and errors inherent in photoclinometric surface slope estimation with natural light

Photoclinometry is the most common method used to obtain high-resolution topographic maps of planetary terrain. We derive the likelihood function of photoclinometric surface slope from (1) the probability distribution of the measured photon count of natural sunlight through a Charge-Coupled Device (CCD) including uncertainty due to camera shot noise, camera read noise, small-scale albedo fluctuation and atmospheric haze, and (2) a photometric model relating photocount to surface orientation. We then use classical estimation theory to determine the theoretically exact biases and errors inherent in photoclinometric surface slope and show when they may be approximated by asymptotic expressions for sufficiently high sample size. We show how small-scale albedo variability often dominates biases and errors, which may become an order of magnitude larger than surface slopes when surface reflectance has a weak dependence on surface tilt. We provide bounds on the minimum possible error of any unbiased photoclinometric surface slope estimate, and compute the sample sizes necessary to constrain errors within desired design thresholds.     

Thermodynamic controls on element partitioning between titanomagnetite and andesitic–dacitic silicate melts

Titanomagnetite–melt partitioning of Mg, Mn, Al, Ti, Sc, V, Co, Ni, Cu, Zn, Ga, Zr, Nb, Mo, Hf and Ta was investigated experimentally as a function of oxygen fugacity (fO₂) and temperature (T) in an andesitic–dacitic bulk-chemical compositional range. In these bulk systems, at constant T, there are strong increases in the titanomagnetite–melt partitioning of the divalent cations (Mg²⁺, Mn²⁺, Co²⁺, Ni²⁺, Zn²⁺) and Cu²⁺/Cu⁺ with increasing fO₂ between 0.2 and 3.7 log units above the fayalite–magnetite–quartz buffer. This is attributed to a coupling between magnetite crystallisation and melt composition. Although melt structure has been invoked to explain the patterns of mineral–melt partitioning of divalent cations, a more rigorous justification of magnetite–melt partitioning can be derived from thermodynamic principles, which accounts for much of the supposed influence ascribed to melt structure. The presence of magnetite-rich spinel in equilibrium with melt over a range of fO₂ implies a reciprocal relationship between a(Fe²⁺O) and a(Fe³⁺O_1.5) in the melt. We show that this relationship accounts for the observed dependence of titanomagnetite–melt partitioning of divalent cations with fO₂ in magnetite-rich spinel. As a result of this, titanomagnetite–melt partitioning of divalent cations is indirectly sensitive to changes in fO₂ in silicic, but less so in mafic bulk systems.     

AIRFLOW PATTERNS FROM PHOTOGRAMMETRIC MEASUREMENTS OF WINDBLOWN TIMBER

Aerial photography may be used to quickly locate and measure the extent and severity of forest damage due to windblow. This paper describes such an application in a small area of semi-natural woodland in central Scotland after the “Great Gale” of 14th and 15th January, 1968. A 1 : 2000 scale map, showing the location and direction of fall of each windblown tree, suggests certain relationships between topography and the severity of woodland damage and gives considerable information on the airflow pattern existing over the area during the gale.     

BHαBAR: big Hα kinematical sample of barred spiral galaxies – I. Fabry–Perot observations of 21 galaxies

We present the Hα gas kinematics of 21 representative barred spiral galaxies belonging to the BHαBAR sample. The galaxies were observed with FaNTOmM, a Fabry–Perot integral-field spectrometer, on three different telescopes. The three-dimensional data cubes were processed through a robust pipeline with the aim of providing the most homogeneous and accurate data set possible useful for further analysis. The data cubes were spatially binned to a constant signal-to-noise ratio, typically around 7. Maps of the monochromatic Hα emission line and of the velocity field were generated and the kinematical parameters were derived for the whole sample using tilted-ring models. The photometrical and kinematical parameters (position angle of the major axis, inclination, systemic velocity and kinematical centre) are in relative good agreement, except perhaps for the later-type spirals.     

Diffusive equilibrium in thin films provides evidence of suppression of hyporheic exchange and large‐scale nitrate transformation in a groundwater‐fed river

The hyporheic zone of riverbed sediments has the potential to attenuate nitrate from upwelling, polluted groundwater. However, the coarse‐scale (5–10 cm) measurement of nitrogen biogeochemistry in the hyporheic zone can often mask fine‐scale (<1 cm) biogeochemical patterns, especially in near‐surface sediments, leading to incomplete or inaccurate representation of the capacity of the hyporheic zone to transform upwelling NO₃^?. In this study, we utilised diffusive equilibrium in thin‐films samplers to capture high resolution (cm‐scale) vertical concentration profiles of NO₃^?, SO₄^2?, Fe and Mn in the upper 15 cm of armoured and permeable riverbed sediments. The goal was to test whether nitrate attenuation was occurring in a sub‐reach characterised by strong vertical (upwelling) water fluxes. The vertical concentration profiles obtained from diffusive equilibrium in thin‐films samplers indicate considerable cm‐scale variability in NO₃^? (4.4 ± 2.9 mg N/L), SO₄^2? (9.9 ± 3.1 mg/l) and dissolved Fe (1.6 ± 2.1 mg/l) and Mn (0.2 ± 0.2 mg/l). However, the overall trend suggests the absence of substantial net chemical transformations and surface‐subsurface water mixing in the shallow sediments of our sub‐reach under baseflow conditions. The significance of this is that upwelling NO₃^?‐rich groundwater does not appear to be attenuated in the riverbed sediments at <15 cm depth as might occur where hyporheic exchange flows deliver organic matter to the sediments for metabolic processes. It would appear that the chemical patterns observed in the shallow sediments of our sub‐reach are not controlled exclusively by redox processes and/or hyporheic exchange flows. Deeper‐seated groundwater fluxes and hydro‐stratigraphy may be additional important drivers of chemical patterns in the shallow sediments of our study sub‐reach. © 2015 The Authors. Hydrological Processes Published by John Wiley & Sons Ltd.