Osmium isotopes in the aerosols of the mantle volcano Mauna Loa

Aerosols and reactive gases from the spring 1984 eruption of Mauna Loa Volcano on Hawaii were collected and analyzed for osmium and its isotopic composition. The measured¹⁸⁷Os/¹⁸⁶Os ratio of1.14 ± 0.03 is close to the ratio in matter with solar systemRe/Os abundance. This result shows that the aerosols from Mauna Loa originated in the mantle and that their composition was not or only slightly influenced by their contact with the crust.     

Human impacts and eutrophication patterns during the past ~200 years at Lago Grande di Avigliana (N. Italy)

A short sediment core from Lago Grande di Avigliana (Piedmont, Italy), the second most eutrophied lake in Italy, was analysed for pollen and diatoms to reconstruct land-use changes and to estimate baseline conditions for total phosphorus (TP) in the water column. Varve counts on sediment thin-sections and ²¹⁰Pb, ²²⁶Ra, and ¹³⁷Cs dating provided a reliable chronology for the past ~200 years. The main pollen-inferred land-use changes showed a sharp decrease of hemp retting around AD 1900, as well as a gradual change to less intensive agriculture and increasing abundance of exotic plants since AD ~1970. Diatom-inferred TP reconstructions indicated stable TP concentrations until AD ~1950, revealing baseline mesotrophic conditions (TP <25 μg l⁻¹). After AD ~1950, TP values increased distinctly and continuously, culminating in the late 1960s with concentrations of 150 μg l⁻¹. Subsequently, diatoms implied a linear decrease of TP, with an inferred value of 40 μg l⁻¹ in the surface sediment sample. Comparison with instrumental TP measurements from the water column since AD 1980 showed a rapid recovery and allowed a direct validation of the diatom TP inference. However, although the TP concentration has decreased considerably, baseline conditions have not yet been reached. When compared to the limnological effects of sewage discharges on inferred-TP concentration, our results indicated that agricultural land use played a minor role in the lake’s eutrophication.     

First alkaline magmatism during Iberia–Newfoundland rifting

The age and origin of alkaline rocks emplaced into the sediments of the rift‐related continental Lusitanian basin were investigated to constrain earliest magmatic activity occurring prior to oceanic plate formation between Iberia and Newfoundland. The U–Pb titanite ages are 146.5 ± 1.6 (2σSTERR), 145.3 ± 1.4 and 142.3 ± 1.0 Ma, and initial Pb isotopic ratios of feldspars lie at 18.418–18.978 for ²⁰⁶Pb/²⁰⁴Pb, at 15.594–15.925 for ²⁰⁷Pb/²⁰⁴Pb and at 37.105–39.216 for ²⁰⁸Pb/²⁰⁴Pb. Initial ⁸⁷Sr/⁸⁶Sr ratios measured in the same feldspar fractions lie at 0.705409–0.706462. This episode of magmatic activity lasting for at least 4.2 ± 2.6 Myr most likely marks a phase of maximum lithospheric thinning during which zones of weakness were created to allow deep magmas to reach the surface. Such zones are preferentially re‐activated Palaeozoic faults of the Iberian plate. The isotope data suggest that the dominant volume of alkaline magmas was generated by partial melting of the metasomatized subcontinental Iberian mantle.     

大别山的构造变形期次和超高压岩石折返的动力学

从构造关系上看,大别山可分为南部,中部和北部3个构造系统,大别山南部构造系统为一套（原地）构造堆叠系统（相对的）,从上到下依次为：（1）未变质的震旦纪一早三叠世沉积盖层,它被造山后的侏罗－白垩纪砂－砾岩不整合覆盖;（2）弱变质的新元古代板岩;（3）高压变质岩系（宿松群）,这些变质岩系大多退变质为绿片岩相;（4）含柯石英榴辉岩超高压变质单元的异地系统;（5）未经历超高压变质作用的原地（相对的）片麻岩系。北西－南东向的拉伸线理在各个地质单元均有表现。从动力学上看,上盘指向NW的剪切运动被后期的褶皱所改变,大别山中部为（混合岩）热穿窿改造系统,系经历了超高压变质作用的地体叠加了混合岩化作用的产物,它与超高压变质地体的界限为一拆离断层,而正是这个拆离断层使超高压变质地体向地表折返并经历了角闪岩相的退变质作用。大别山北部为构造堆叠系统,早期的面理和南北向的挤压线理被轴面北倾的褶皱所改造,这种褶皱对应于大别山中部同折返期的韧性变形的隆升构造。同时建立了地球运力学模型并讨论了逆冲作用和正断层的作用。     

A Possible Photochemical Link Between Stratospheric and Near-Surface Ozone on Swiss Mountain Sites in Late Winter

Record high near-surface ozone concentrations at two elevated sites (Chaumont, 1140 m asl, and Rigi, 1030 m asl) in Switzerland were observed simultaneously with extremely low total ozone during a fair weather period in mid-February 1993. An analysis of ozone, temperature, humidity, and wind profiles suggests that the surface ozone peaks were most possibly generated within the region in a layer between about 1000 and 1500 m asl. Mean diurnal cycles of ozone concentration during the period shows a strong increase from late morning to late afternoon at Chaumont and at the same time a decrease at the high alpine site Jungfraujoch (3580 m asl). The different diurnal ozone cycles can both be explained photochemically by taking into account the large difference in NOx concentrations (about two orders of magnitude) between the sites. Photochemical processes are also indicated by the diurnal cycles of NO2 and NO concentration. As a strong photochemical activity is not expected in mid-February at 47°N, we hypothesize that the extremely low total ozone played a role. Total ozone controls the amount of UV-B radiation reaching the troposphere and thus influences photochemical processes. Using a radiation model, we calculated an increase in ozone photolysis at Chaumont and Jungfraujoch of 73% and 83%, respectively, on the day with the lowest total ozone (243 DU) compared to average February conditions (335 DU). It is suggested that total ozone changes have the potential to stimulate photochemistry sufficiently to produce the observed surface ozone peaks at Chaumont and Rigi of 61 and 64 ppbv, respectively. A fog layer just below Chaumont during these days probably also influenced photochemistry, but on a smaller spatial scale. Our empirical results on the influence of changing UV radiation on tropospheric photochemistry are in close agreement with model studies of other groups. Although this case study represents unique conditions, a distinct anticorrelation between near-surface ozone at Chaumont and total ozone also appears in other years (1992–1997) when selecting fair weather days in mid-February. However, other influences cannot be excluded. The selected days provide evidence of a significant photochemical source of ozone in the mid-latitude lower troposphere in late winter.