Algal flora associated with a Halophila stipulacea (Forsskål) Ascherson (Hydrocharitaceae,Helobiae) stand in the western Mediterranean     

《Oceanologica Acta》1999,22(4):421-429

Halophila stipulacea, a seagrass introduced into the Mediterranean Sea as a Lessepsian immigrant, is nowadays common in the eastern Mediterranean, and it was recently recorded in the western Mediterranean; very little information is available about the algal flora associated with this species. During a survey of a Halophila stipulacea stand at Vulcano Island (Eolian Islands, western Mediterranean), samples were collected at three depths (5 m, 15 m and 25 m) for identification of algal epiphytes. Thirty-six species of macroalgae were found. The epiflora of the leaves consisted of 20 species, the others being mixed with or entangled in the rhizomes. On the leaves, Ceramium tenerrimum, Dasya corymbifera, Polysiphonia cfr. tenerrima, Spyridia filamentosa, Chondria pygmaea and Laurencia sp. were the most common species; between the rhizomes, Dictyota linearis was abundant. A differentiation of the epiphytic assemblage between 5 m and the other depths was observed; the 5 m samples were characterized by the abundance of Ceramium tenerrimum, Chondria pygmaea and Polysiphonia cfr. tenerrima, while at 15 m and 25 m Laurencia sp., Dasya corymbifera and Spyridia filamentosa were the most common species. Epiphytic cover was generally very low. No rare species were found among the epiphytes. In comparison with other Mediterranean seagrasses, Halophila stipulacea has a qualitatively and quantitatively poor epiphytic flora. In particular, the virtual absence of encrusting corallines is noteworthy. A fast turnover rate of the leaves is hypothesized to be the main reason for this scarcity. Differences between this and other studies on epiphytes of Halophila stipulacea stands are discussed.  相似文献   

Oxygen fugacity dependence of Ni,Co, Mn,Cr, V,and Si partitioning between liquid metal and magnesiowüstite at 9–18 GPa and 2200°C     

《Geochimica et cosmochimica acta》1999,63(11-12):1853-1863

The oxidation states of Ni, Co, Mn, Cr, V and Si in magnesiowüstite have been determined in metal-oxide distribution experiments using a multi anvil apparatus at 9 and 18 GPa and 2200°C as a function of oxygen fugacity. Despite limitations to control oxygen fugacity by applying conventional buffering methods in high pressure experiments, a wide range of redox-conditions (3 log bar units) has been imposed to the metal-oxide partitioning experiments by varying the Si/O ratio of the starting material. The oxygen fugacity was calculated according to the Fe-FeO equilibrium between the run products. The ability to impose different oxygen fugacities by varying the starting material is confirmed by the large variation of element partitioning coefficients obtained at constant pressure and temperature. The calculated valences at both pressures investigated are divalent for Co, Mn, V and 4+ for Si. The results for Cr (∼2.5+) and Ni (∼1.5+) indicate non-ideal mixing of Ni and Cr in at least one of the product phases. Because the application of 1 bar activity coefficients for Ni and Cr in metal alloys does not change these valences, non-ideal mixing in magnesiowüstite or significantly larger non-ideal mixing properties of Ni and Cr in metal alloys at high pressure are likely to be responsible for the apparent valences. Omitting such non-ideal mixing properties when extrapolating high-pressure element partitioning data may be significant. The elements Cr, V and Mn become siderophile (D_M^met/ox > 1) at 9–18 GPa and 2200°C at oxygen fugacities below IW-2.7 to IW-3.7. Considering, in addition, the influence of temperature, the depletion of Cr, Mn and V in the Earth’s mantle may be due, at least partly, to siderophile behavior at high pressure and temperature.  相似文献   

Identification of aryl isoprenoids in source rocks and crude oils: Biological markers for the green sulphur bacteria     

《Geochimica et cosmochimica acta》1987,51(3):557-566

A series of C₁₃ to C₃₁ aryl isoprenoids (1-alkyl,2,3,6-trimethylbenzenes) have been identified in reef-hosted oils and their source rocks from the Middle and Upper Silurian of the Michigan Basin and Middle Devonian of the Alberta Basin, Canada. Their structure has been confirmed by unambiguous synthesis of the C₁₄ member of the series. Their structure and isotopic composition indicate that they are derived from isorenieratene from the Chlorobiaceae family of sulphur bacteria. These results are consistent with geological and geochemical studies that show that the source rocks were deposited under metahaline to hypersaline sulphate and sulphide rich water columns. The distribution of other biomarkers in these oils and source rocks indicates that a diverse biota contributed organic matter to the source environment. In conjunction with the aryl isoprenoids, they show that there is a remarkable similarity in composition between the two sets of oils and source rocks despite their great temporal and geographic separation. This reflects the similarity of their environments and emphasizes the importance of sedimentary facies in controlling the composition of organic matter in source rocks and their derived oils.  相似文献   

Cathodoluminescence microscopy and spectroscopy of forsterite from Kaba meteorite: An application to the study of hydrothermal alteration of parent body     

Arnold Gucsik  Taro Endo  Hirotsugu Nishido  Kiyotaka Ninagawa  Masahiro Kayama  Szaniszló Bérczi  Szabolcs Nagy  Péter Ábrahám  Yuki Kimura  Hitoshi Miura  Ildikó Gyollai  Irakli Simonia  Péter Rózsa  József Posta  Dániel Apai  Krisztián Mihályi  Mihály Nagy  Ulrich Ott 《Meteoritics & planetary science》2013,48(12):2577-2596

Highly forsteritic olivine (Fo: 99.2–99.7) in the Kaba meteorite emits bright cathodoluminescence (CL). CL spectra of red luminescent forsterite grains have two broad emission bands at approximately 630 nm (impurity center of divalent Mn ions) in the red region and above 700 nm (trivalent Cr ions) in the red–IR region. The cores of the grains show CL blue luminescence giving a characteristic broad band emission at 400 nm, also associated with minor red emissions related to Mn and Cr ions. CL color variation of Kaba forsterite is attributed to structural defects. Electron probe microanalyzer (EPMA) analysis shows concentrations of Ca, Al, and Ti in the center of the forsterite grain. The migration of diffusible ions of Mn, Cr, and Fe to the rim of the Kaba meteoritic forsterite was controlled by the hydrothermal alteration at relatively low temperature (estimated at about 250 °C), while Ca and Al ions might still lie in the core. A very unusual phase of FeO (wüstite) was also observed, which may be a terrestrial alteration product of FeNi‐metal.  相似文献   

Seawater–freshwater mixing and resulting calcite dissolution: an example from a coastal alluvial aquifer in eastern South Korea     

Gi-Tak Chae  Sung-Moon Yun  Kyoung-Ho Kim  Chil-Sup So 《水文科学杂志》2013,58(8):1672-1683

Abstract

In order to evaluate groundwater quality and geochemical reactions arising from mixing between seawater and dilute groundwater, we performed a hydrochemical investigation of alluvial groundwater in a limestone-rich coastal area of eastern South Korea. Two sites were chosen for comparison: an upstream site and a downstream site. Data of major ion chemistry and ratios of oxygen–hydrogen isotopes (δ¹⁸O, δD) revealed different major sources of groundwater salinity: recharge by sea-spray-affected precipitation in the upstream site, and seawater intrusion and diffusion zone fluctuation in the downstream site. The results of geochemical modelling showed that Ca²⁺ enrichment in the downstream area is caused by calcite dissolution enhanced by the ionic strength increase, as a result of seawater–groundwater mixing under open system conditions with a constant P_CO2 value (about 10^?1.5 atm). The results show that, for coastal alluvial groundwater residing on limestone, significant hydrochemical change (especially increased hardness) due to calcite dissolution enhanced by seawater mixing should be taken into account for better groundwater management. This process can be effectively evaluated using geochemical modelling.

Editor D. Koutsoyiannis; Associate editor Y. Guttman

Citation Chae, G.-T., Yun, S.-T., Yun, S.-M., Kim, K.-H., and So, C.-S., 2012. Seawater–freshwater mixing and resulting calcite dissolution: an example from a coastal alluvial aquifer in eastern South Korea. Hydrological Sciences Journal, 57 (8),1–12.  相似文献   

Effects of the inclusion of bending-to-stretching transitions in the non-LTE modeling of ozone vibrational temperatures     

Rafael P. Fernandez  Martin Kaufmann  Beatriz M. Toselli 《Journal of Atmospheric and Solar》2010,72(11-12):890-899

  相似文献   

Association of dissolved radionuclides released by the Chernobyl accident with colloidal materials in surface water     

《Applied Geochemistry》2004,19(10):1581-1599

The association of dissolved ⁹⁰Sr, ^239,240Pu and ²⁴¹Am with natural colloids was investigated in surface waters in the Chernobyl nuclear accident area. A 4-step ultrafiltration (UF) study (<1 kilodaltons (Da), 1–10 kDa, 10–100 kDa, 100 kDa<) showed that 49–83% of ^239,240Pu and 76% of ²⁴¹Am are distributed in colloids of the two size fractions larger than 10 kDa (nominal molecular weight limit of the filter, NMWL), while ⁹⁰Sr was found exclusively (85–88%) in the lowest molecular size fraction below 1 kDa (NMWL) for the Sahan River water at the highly contaminated area close to the Chernobyl Nuclear Power Plant (ChNPP). Consistent results were obtained by 2-step fractionation (larger than and smaller than 10 kDa (NMWL)) for river and lake waters including other locations within about 30 km away from ChNPP. It is likely that Pu and Am isotopes were preferentially associated with dissolved organic matter of high molecular size, as suggested by the fact that (i) only a few inorganic elements (Mg, Ca, Sr, Si, Mn, Al) were found in the colloidal size ranges, and (ii) the positive correlation between dissolved organic C (DOC) concentrations and UV absorbance at 280 nm, a broad absorption peak characteristic of humic substances (HS) was found. A model calculation on the complexation of Pu and Am with HS as an organic ligand suggests that the complexed form could be dominant at a low DOC concentration of 1 mgC L⁻¹, that is commonly encountered as a lower limit in fresh surface water. The present results suggest the general importance of natural organic colloids in dictating the chemical form of actinides in the surface aquatic environment.  相似文献   

Experimental studies of oxygen isotope fractionation between rhodochrosite (MnCO₃) and water at low temperatures     

Sang-Tae Kim  Jung Ok Kang  James R. O’Neil 《Geochimica et cosmochimica acta》2009,73(15):4400-1828

Rhodochrosite crystals were precipitated from Na-Mn-Cl-HCO₃ parent solutions following passive, forced and combined passive-to-forced CO₂ degassing methods. Forced and combined passive-to-forced CO₂ degassing produced rhodochrosite crystals with a small non-equilibrium oxygen isotope effect whereas passive CO₂ degassing protocols yielded rhodochrosite in apparent isotopic equilibrium with water. On the basis of the apparent equilibrium isotopic data, a new temperature-dependent relation is proposed for the oxygen isotope fractionation between rhodochrosite and water between 10 and 40 °C: