Molecular hydrogen in damped Lyα systems: clues to interstellar physics at high redshift

In order to interpret H₂ quasar absorption-line observations of damped Lyα systems (DLAs) and subDLAs, we model their H₂ abundance as a function of dust-to-gas ratio, including H₂ self-shielding and dust extinction against dissociating photons. Then, we constrain the physical state of the gas by using H₂ data. Using H₂ excitation data for DLAs with H₂ detections, we derive a gas density  1.5 ≲ log n (cm⁻³) ≲ 2.5  , temperature  1.5 ≲ log T (K) ≲ 3  , and an internal ultraviolet (UV) radiation field (in units of the Galactic value)  0.5 ≲ log χ≲ 1.5  . We then find that the observed relation between the molecular fraction and the dust-to-gas ratio of the sample is naturally explained by the above conditions. However, it is still possible that H₂ deficient DLAs and subDLAs with H₂ fractions less than  ∼10⁻⁶  are in a more diffuse and warmer state. The efficient photodissociation by the internal UV radiation field explains the extremely small H₂ fraction  (≲10⁻⁶)  observed for  κ≲ 1/30  (κ is the dust-to-gas ratio in units of the Galactic value); H₂ self-shielding causes a rapid increase in, and large variations of, H₂ abundance for  κ≳ 1/30  . We finally propose an independent method to estimate the star formation rates of DLAs from H₂ abundances; such rates are then critically compared with those derived from other proposed methods. The implications for the contribution of DLAs to the cosmic star formation history are briefly discussed.     

Evidence for crustal assimilation,mixing of magmas,and a87Sr-rich upper mantle

¹⁸O/¹⁶O,⁸⁷Sr/⁸⁶Sr and chemical analyses were made on 39 lavas and ignimbrites from M. Vulsini, the most northerly district of the K-rich Quaternary Roman Province of Italy. These rocks belong mainly to the undersaturated, leucite-bearing (High-K) series, but also included are samples from the less abundant, SiO₂-saturated, hypersthene-(quartz)-normative (Low-K) series. The effects of post-eruption alteration on the ¹⁸O of these lavas were taken into account by analyzing phenocrysts or by using the extrapolation procedure developed for the nearby Alban Hills center. Because of the high Sr contents (500–2400 ppm), the⁸⁷Sr/⁸⁶Sr ratios of these rocks were little affected by such alteration processes. The M. Vulsini volcanics have Sr- and O-isotopic ratios much less uniform, and on the average much higher, than at any of the other volcanic centers of the province:⁸⁷Sr/⁸⁶Sr=0.7097 to 0.7168; ¹⁸O=6.5 to 13.8. This is attributable to the fact that M. Vulsini is one of the sites of greatest crustal assimilation and hybridism between K-rich Roman magmas and SiO₂-rich Tuscan anatectic magmas. The High-K series parent magmas at M. Vulsini had a very high and uniform⁸⁷Sr/⁸⁶Sr=0.7102 to 0.7104, and a somewhat more variable ¹⁸O=+5.5 to +7.5; they must have come from an upper mantle source region previously metasomatically enriched in⁸⁷Sr and LIL elements. These¹⁸O/¹⁶O and⁸⁷Sr/⁸⁶Sr ratios are identical to the parent magma at the Alban Hills, 120 km to the south, where Low-K lavas are absent. Low-K series magmas at M. Vulsini originated from a lower-⁸⁷Sr source region than the High-K series (<0.7097); a similar relationship is observed in all of the other localities in Italy where the two magma series coexist.Contribution No. 4167, Division of Geological and Planetary Sciences, California Institute of Technology     

A Physically Consistent Speckle Model for Marine SLC SAR Images

A new physically based speckle model for marine single-look complex (SLC) synthetic aperture radar (SAR) images is here presented and investigated. The model allows using full-resolution SAR images instead of multilook SAR images, in which, at the expense of a coarser spatial resolution, the speckle is mitigated. The model is based on the three-parameters generalized-K (GK) probability density function (pdf). GK pdf is a suitable physically-based speckle model for marine SAR images ensuring a continuous and physically consistent transition among different scattering scenarios. This speckle model embodies Rayleigh, K, and Rice scattering scenes which are typical of marine scenes. The use of the three parameters, related to the GK pdf ones, is able to highlight the presence of both low backscattering areas and areas in which a small dominant scatterer is present. This is operationally interesting in SAR oil spill detection procedures.     

Mercury levels in the dissolved and particulate fractions of the Tyrrhenian sea

Determination of the actual mercury concentration in Mediterranean basin seawater was achieved by means of an instrument based on fluorescence spectrometry developed for this purpose, during a field study aboard the oceanographic ship “L.F. Marsili”, between August 1980 and May 1982.Dissolved ·total’ and ·reactive’ mercury and mercury associated with particulate matter were determined on surface and subsurface waters in the Tyrrhenian Sea from La Spezia to Sicily.Concentrations in the range 1.4–19.7 ng l⁻¹ for ·total dissolved mercury’, 0.5–5.9 ng l⁻¹ for ·reactive dissolved mercury’ and 0.3–8.0 ng l⁻¹ for mercury associated with the particulate matter, were measured on surface and subsurface waters in the Tyrrhenian Sea from La Spezia to Sicily.Even if the mean value of the total mercury concentration (dissolved + particulate) was found to be about twice as high as those observed for the oceans, the difference does not seem to be as high as predicted by the model proposed by Buffoni and co-workers to explain the large difference of mercury levels between tunas caught, respectively, in the Mediterranean and in the Oceans.     

Factors influencing the daily behaviour of dissolved gaseous mercury concentration in the Mediterranean Sea

Mercury evasional fluxes from the sea surface into the atmosphere play an important role in the Hg biogeochemical cycle, especially in the Mediterranean basin, which is characterized by the presence of large cinnabar deposits, intense solar radiation and high temperatures for many months of the year. Since the available experimental methodologies to measure mercury flux can be used only in good weather conditions, at present it is necessary to make use of exchange models that require the knowledge of the dissolved gaseous mercury (DGM) concentration in seawater. In this paper, the main factors affecting DGM levels are discussed considering the determination of the DGM daily behaviour in different meteo-marine and weather conditions at coastal and offshore locations of the Mediterranean basin. A fully automatic device for continuous analyses of DGM concentration with a high time resolution was used. Results show that the daily trend of DGM concentration tracks that of the solar radiation intensity, often mapping the movement of the clouds. DGM levels can be decreased by the presence of high winds that increase mercury evasion from the water surface, as well as by the mixing of the surface water layer. The presence of high levels of dissolved organic matter favours the photo-induced reduction of mercury as observed by the measurements performed in a lagoon water.     

The Palaeocene–Middle Eocene deposits of Sardinia (Italy) and their palaeoclimatic significance

The Palaeocene of southern Sardinia includes a continental ferruginous sedimentation, with a high content of Al and Fe, indicative of a subhumid tropical climate. The subsequent microcodium carbonated detrital microcodium levels, containing an abundant quartzose fraction, rather suggest semiarid conditions. In SW Sardinia, the marine deposits, referred to Late Thanetian–Lower Ypresian (=Ilerdian) times, are limestones including larger foraminifers and contain significant amounts of quartz. A lower content of detrital kaolinite points a sediment source from the surrounding areas with a semiarid climate. The occurrence, at the top of this unit, of small trochospiral rotaliids and larger amount of detrital kaolinite suggests a transition to a rainy tropical climate in the adjacent areas. These limestones pass gradually to carbonate sediments characterized by large amounts of detrital kaolinite and intercalated coal layers, with pollen of tropical palms, attributed to the Late Ypresian (=Cuisian)–Early Lutetian and referred to a humid-subhumid tropical climate.     

Intercomparison of Boron Isotope and Concentration Measurements. Part II: Evaluation of Results

The Istituto di Geoscienze e Georisorse (IGG), on behalf and with the support of the International Atomic Energy Agency (IAEA), prepared eight geological materials (three natural waters and five rocks and minerals), intended for a blind interlaboratory comparison of measurements of boron isotopic composition and concentration. The materials were distributed to twenty seven laboratories - virtually all those performing geochemical boron isotope analyses in the world -which agreed to participate in the intercomparison exercise. Only fifteen laboratories, however, ultimately submitted the isotopic and/or concentration results they obtained on the intercomparison materials. The results demonstrate that interlaboratory reproducibility is not well reflected by the precision values reported by the individual laboratories and this observation holds true for both boron concentration and isotopic composition. The reasons for the discrepancies include fractionations due to the chemical matrix of materials, relative shift of the zero position on the δ¹¹B scale and a lack of well characterized materials for calibrating absolute boron content measurements. The intercomparison materials are now available at the IAEA (solid materials) and IGG (waters) for future distribution.     

Effect of degassing on sulfur contents and δ34S values in Somma-Vesuvius magmas

Sulfur contents and δ³⁴S values of Somma-Vesuvius magmas are consistent with syneruptive, open-system degassing at temperatures of 800–850°C for Plinian pumices and 1100–1200°C for lavas. The extent of degassing appears to be greater in lavas than in pumices. The key parameter controlling the ³⁴S/³²S ratio of Somma-Vesuvius volcanics is the average magma oxidation state, which generally varies from 0.85 to 1.20 Δ NNO units for lavas and from 1.20 to 1.40 Δ NNO units for pumices. Consequently, S contents and δ³⁴S values of magmas constitute a potentially valuable tool in estimating their average redox conditions. The results of this study may help in risk mitigation when the Vesuvius magmatic system evolves toward eruptive conditions. Received: 20 January 1998 / Accepted: 5 May 1998     

A geotechnical perspective of raw earth building

Much research has been devoted over the past 30 years to the development of construction materials that can lower the environmental and economic costs of buildings over their entire life by reducing embodied energy, minimizing air conditioning needs and cutting down demolition waste. In this respect, raw earth is an attractive material because it is natural and largely available. In its simplest form, this material consists of a compacted mixture of soil and water which is put in place with the least possible transformation. Raw earth construction has been practised in ancient times but has only recently been rediscovered thanks to modern technology, which has improved fabrication efficiency. If properly manufactured, raw earth exhibits comparable mechanical characteristics and better hygro-thermal properties than concrete or fired bricks. After a brief historical overview, we discuss the advantages of raw earth construction in terms of environmental impact, energy consumption and indoor air quality together with the main obstacles to its wider dissemination. We also review the hydro-thermo-mechanical behaviour of raw earth in the context of the recent geotechnical literature, by examining the dependency of key parameters such as strength, stiffness and moisture retention on: (a) material variables (e.g. particle size and mineralogy), (b) manufacturing variables (e.g. density and stabilization) and (c) environmental variables (e.g. pore suction, ambient humidity and temperature).