Variación secular geomagnética en Sudamérica y alrededores con relación a todo el Hemisferio Sur

Resumen Se aplica el análisis de energía de espectro a los datos de la variación secular geomagnética para tres observatorios del Hemisferio Sur: Pilar, Amberley, Mauritius. Las figuras 2–10 indican generalmente tres tipos de picos: a) de periodos de menos de 10 años; b) del periodo aproximado de la actividad solar de 10–11 años; c) de una fluctuación de 40–60 años. Los coeficientes de la expansión esférico-armónica de la variación secular muestran también una fluctuación del último tipo.

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Summary The power spectrum analysis is applied to geomagnetic secular variation data of three southern magnetic observatories: Pilar, Amberley, Mauritius. The figures 2–10 indicate in general three types of peaks: a) of periods less than 10 years; b) of approximately solar activity cycle; c) of 40–60 years fluctuation. Also the coefficients of spherical harmonic expansion of SV show the last type fluctuation.

     

Cloud processes associated with past and future climate changes

 To investigate the cloud response during cold and warm periods, we have performed simulations of the Last Glacial Maximum (LGM-21ky BP) and of double CO₂ concentration using the LMD AGCM model. We observe that the thermal characteristics of these two climates are opposite, but the cloud response is more complex and does not display the same symmetry When doubling the CO₂, the warming of the troposphere and the cooling of the stratosphere are clearly linked with a reduction in low-level clouds and an increase of high-level clouds associated with relative humidity changes. For the LGM, the cloud response is more complex. In the inter tropical region, we show that the Hadley cell is reinforced during LGM (+20%) whereas it is reduced (−10%) for the double CO₂ experiments. The most important feature is that we observe an enlarged Hadley cell for LGM climate which strongly modifies the atmospheric dynamics and water transport. For LGM conditions, the cloud response is then mostly driven by these dynamical changes at low latitudes though at high latitudes the thermal changes explain a large part of the cloud response. Two different versions of the model, using different parametrizations for the precipitation show that cloud feedbacks may act differently for cold and warm climates; and that the cloud response may be more complex that previously expected, but also indicate that the details of these effects are model dependent.     

白云鄂博REE-Nb-Fe矿床成因的氧、碳和锶同位素及微量元素地球化学证据

根据白云鄂博赋矿白云石大理岩的岩石学特征及地质产状将其分为两类：粗粒和细粒白云石大理岩．它们的氧、碳和锶同位素及微量元素地球化学特征显然有别于分布在宽沟背斜以北典型的沉积石灰岩和白云岩,而和幢源火成碳酸岩十分相似．与矿床进行对比研究说明,成矿流体和矿质主要起源于碳酸岩浆的分异作用,其放射性成因同位素和微量元素保持了地但指纹,而氧和碳同位素组成却向壳源方向漂移,证实碳酸岩浆侵位过程中受大陆地壳的混染作用非常微弱,但是由碳酸岩浆活动所引起的成矿热液体系中却有一定的地表水混人认为白云鄂博REE-Nb-Fe超大型矿床的成因应归属于火成碳酸岩型矿床．     

The conditions for biodegradation of petroleum hydrocarbons at sea

The hydrocarbon degradation potential of seawater appears higher in zones chronically polluted by these materials; they are characterized by an abundant bacterial development. The salinity of the effluent from a refinery and also its hydrocarbon content are factors affecting the speed of degradation.     

The fit of the continents around the North Atlantic Ocean

A new reassembly of the continents around the North Atlantic Ocean is presented. The first criterion used for this reassembly is the identification of the structural framework related to the opening which consists of marginal fracture zones generated by offsets of the Rift. The Africa—North America, Eurasia—Greenland, Greenland—North America and Eurasia—North America adjustments are successively discussed. It is argued that the adjustments are best made at the 3000-meters isobath between Africa and North America and at the 2000-meters isobath for the younger rifts. The difference is attributed to subsidence and modification of continental margins with time. The importance of the Late Paleozoic tectonic phase in determining the subsequent pattern of Mesozoic rifting is emphasized.     

Etude des solutions solides des néphélines (Na,K)AlSiO4 et (Na,Rb)AlSiO4

Ion exchange equilibrium of nepheline solid solutions (Na, K)AlSiO₄ and (Na, Rb)AlSiO₄ with hydrothermal solutions has been studied at 600°C and 2000 bars. The behaviour of dilute solid solutions was specially investigated.Na-Rb ion exchange data can be represented satisfactorily by a model taking into account the existence of two different sites in the structure of nepheline. At 600°C Rb atoms substitute almost exclusively for Na atoms situated in the larger sites. On the other hand, this model only partially applies to Na-K ion exchange equilibrium.Finally, the importance of the ion exchange data concerning extremely dilute solutions to calculate activity-composition diagrams is emphasized with special reference to the nepheline solid solutions.     

First evidence for large earthquakes on the Deshir Fault,Central Iran Plateau

Although sliced by several strike slip faults, a large part of Central Iran remained aseismic during the period of time covered by the instrumental and historical seismic records. Stating the existence of earthquakes in the Holocene is therefore important for the assessment of the regional seismic hazard. A palaeoseismic study of the Deshir fault demonstrates that Central Iran hosted large earthquakes during latest Pleistocene and Holocene. The last event corresponds to 1 m‐deep fissures, which sandy infilling yielded an optically stimulated luminescence (OSL) age of 2.8 ± 1.4 ka. At least two previous events, outlined by older fissures and/or colluvial wedges, have been recorded over the last 10–30 ka. The magnitudes are difficult to assess because the actual slips per event are unknown. The size of the fissures and the significant vertical displacement associated with a colluvial wedge are nevertheless compatible with M ≈ 7 events along a primary strike‐slip surface break.     

Accumulation,solid partitioning and remobilisation of 99Tc in subtidal and intertidal sediments in the Irish Sea

Recent studies have shown that accumulation of ⁹⁹Tc in seabed sediments labelled by authorised radioactive liquid discharges into the NE Irish Sea from the Sellafield reprocessing complex is greater than previously thought. In this paper, new data on ⁹⁹Tc concentration profiles in subtidal and intertidal sediments from the eastern and western Irish Sea are provided with a view to elucidating the processes responsible for the incorporation and retention of ⁹⁹Tc in the seabed. The data show that substantial amounts of ⁹⁹Tc have accumulated in the fine-grained subtidal sediments off the Cumbrian coast, particularly after increased releases from Sellafield following the commissioning of the Enhanced Actinide Removal Plant (EARP) in 1994. In all the cores taken in this area, ⁹⁹Tc has been found to be present to depths in excess of 30 cm. Analysis of ¹³⁷Cs and ²⁴¹Am profiles, together with other supporting geochemical data, show a high degree of homogenisation of the sediments down to these depths as a result of physical and biological processes, and confirm that incorporation of ⁹⁹Tc into the sediment compartment is actually the result of mixing and reworking, rather than active sediment accumulation. In contrast, active deposition of material transported from this mixed pool of sediment appears to be the dominant mechanism controlling ⁹⁹Tc profiles in intertidal areas close to the Sellafield discharge outfall.Data obtained from the analysis of subtidal sediment cores from the western Irish Sea mud basin suggest that similar mixing processes to those occurring in the subtidal sediments of the eastern Irish Sea are also active in this area. Time-series data on ⁹⁹Tc concentrations in surficial sediments from this basin, gathered in the period 1988–2004, inclusive, show a clear increase in concentrations, by a factor of ～2, between samples collected pre-EARP and post-EARP. The constancy of ⁹⁹Tc concentrations in surface sediments throughout the 1980s and the early-1990s suggests that little redissolution and export of ⁹⁹Tc occurred over this extended period. A similar observation applies to the post-EARP period, when concentrations remained relatively constant despite the reported steady decrease in ⁹⁹Tc concentrations in the overlying waters.This apparent lack of remobilisation is consistent with data from sequential extraction analyses, which indicate that the bulk of the ⁹⁹Tc is strongly bound to non-labile geochemical phases, with only a small proportion associated with exchangeable and acido-soluble phases. Further, these analyses show that ⁹⁹Tc is not associated with oxygen-sensitive and highly-reactive acid-volatile sulphides (AVS) to any significant extent.     

CHEMINI: A new in situ CHEmical MINIaturized analyzer

“CHEMINI” is a new instrument developed for the measurement of seawater chemical parameters. It is a mono-parameter in situ chemical analyzer based on flow injection analysis and colorimetric detection. The deep-sea version of CHEMINI combines two modules to perform the analysis of dissolved iron [Fe (II) or Fe (II+III)] and total sulphide (H₂S+HS^?+S^2?) up to 6000 m depth. Detection limits are, respectively, 0.3 and 0.1 μM for iron and sulphide. The system proved highly reliable during the MoMARETO cruise on the Mid-Atlantic Ridge. The two CHEMINIs were used to describe the chemical environment in 12 mussel beds on the Tour Eiffel hydrothermal edifice.