Late evolution of the southern European Variscan belt: Exhumation of the lower crust in a context of oblique convergence

The south-eastern part of the European Variscan belt forms a zone composed of several crystalline segments: the External Crystalline Massifs of the western Alps, the Maures-Tanneron massif, also Corsica and Sardinia, which are mutually displaced due to the Alpine deformation of the European Paleozoic lithosphere. All these crystalline fragments record similar structural, metamorphic, geochronological and magmatic histories during Paleozoic times. In particular, the Late Carboniferous period (～320–300 Ma) is characterized by crustal-scale folding associated with strike-slip faulting and intracontinental basin formation. In this transpressive context, dome structures exhume partially-molten crust in convergent setting, which is in contradiction with generally accepted models of late orogenic collapse of the Variscan belt. It is suggested that this particular transpressive–obliquely convergent template, exemplified by tectonometamorphic evolution of the Maures-Tanneron massif, is valid for the whole eastern European Variscan margin.     

A pollen-based record of late glacial–Holocene climatic variability in the southern lake district, Chile

A pollen record from Puyehue area (40°S; 72°W) in the southern Lake District, Chile, indicates that prior to 13,410 ¹⁴C yr BP (ca. 16,500–15,200 cal yr BP), cold resistant and hygrophilous vegetation, particularly Nothofagus forest and myricaceous vegetation, covered the area. From ca. 15,000 cal yr BP onward, the forest became increasingly dense. Between 10,010 and 7450 ¹⁴C yr BP (ca. 11,000–8000 cal yr BP), the expansion of Nothofagus obliqua and the spread of grasses suggests the climate became warmer and semi-arid. Lowland deciduous forest (Nothofagus obliqua, Aextoxicon punctatum, Laurelia sempervirens) and Valdivian rainforest (Nothofagus dombeyi, Eucryphia cordifolia, Caldcluvia paniculata, Aextoxicon punctatum, Laureliopsis philippiana) were abundant. During the next two thousand years, stable warm climatic conditions prevailed, and the diversity of the vegetation increased. From 5760 to 1040 ¹⁴C yr BP (ca. 6500–900 cal yr BP), the North Patagonian rainforest expanded. The presence of Pilgerodendron/Fitzroya, together with Nothofagus forest, suggests that humid conditions prevailed. During the last millennium, human impact intensified and regional vegetation was disturbed, particularly the lowland deciduous forest and Valdivian rainforest. North-Patagonian and subantartic taxa, such as Podocarpus nubigena, Pilgerodendron/Fitzroya, Nothofagus dombeyi type, Austrocedrus chilensis and Drimys winteri, occupied the low and high-altitude parts of the Cordillera. Five hundred years ago, shrub and grasses expanded in the Nothofagus forest, suggesting that forest became more open under cool–cold, and humid climatic conditions. These conditions prevail to the present day. This is the fourth in a series of eight papers published in this special issue dedicated to the 17,900 year multi-proxy lacustrine record of Lago Puyehue, Chilean Lake District. The papers in this special issue were collected by M. De Batist, N. Fagel, M.-F. Loutre and E. Chapron.     

西北东部夏季极端干旱事件机理分析

利用国家气候中心提供的1951—2012年逐日降水、温度、综合气象干旱指数、逐月NCEP/NCAR再分析资料等,采用REOF分析、动力诊断、相关分析以及合成分析等方法,从大气环流异常特征、高空急流与季风异常等方面揭示西北东部夏季极端干旱事件的可能机理。研究发现西北东部夏季发生极端干旱时,副热带急流轴"倾斜",且急流与东亚夏季风强度均处于相对偏弱阶段。极端干旱的成因研究表明:急流轴"倾斜"及其强度减弱导致西北东部地区高层大范围的异常辐合;该地区为水汽源区,对流层整层水汽收支显著亏损;此外,该地区低层盛行来自内陆干旱区的异常西南风,东亚夏季风强度偏弱,高低层配置及大尺度环流形势不利于降水产生。     

Mechanisms controlling the air–sea flux in the North Sea

The mechanisms driving the air–sea exchange of carbon dioxide (CO₂) in the North Sea are investigated using the three-dimensional coupled physical–biogeochemical model ECOHAM (ECOlogical-model, HAMburg). We validate our simulations using field data for the years 2001–2002 and identify the controls of the air–sea CO₂ flux for two locations representative for the North Sea's biogeochemical provinces. In the seasonally stratified northern region, net CO₂ uptake is high () due to high net community production (NCP) in the surface water. Overflow production releasing semi-labile dissolved organic carbon needs to be considered for a realistic simulation of the low dissolved inorganic carbon (DIC) concentrations observed during summer. This biologically driven carbon drawdown outcompetes the temperature-driven rise in CO₂ partial pressure (pCO₂) during the productive season. In contrast, the permanently mixed southern region is a weak net CO₂ source (). NCP is generally low except for the spring bloom because remineralization parallels primary production. Here, the pCO₂ appears to be controlled by temperature.     

Study of water tension differences in heterogeneous sandy soils using surface ERT

Herbaceous vegetation in the Sahel grows almost exclusively on sandy soils which preferentially retain water through infiltration and storage. The hydrological functioning of these sandy soils during rain cycles is unknown. One way to tackle this issue is to spatialize variations in water content but these are difficult to measure in the vadose zone. We investigated the use of Electrical Resistivity Tomography (ERT) as a technique for spatializing resistivity in a non-destructive manner in order to improve our knowledge of relevant hydrological processes. To achieve this, two approaches were examined. First, we focused on a possible link between water tension (which is much easier to measure in the field by point measurements than water content), and resistivity (spatialized with ERT). Second, because ERT is affected by solution non-uniqueness and reconstruction smoothing, we improved the accuracy of ERT inversion by comparing calculated solutions with in-situ resistivity measurements. We studied a natural microdune during a controlled field experiment with artificial sprinkling which reproduced typical rainfall cycles. We recorded temperature, water tension and resistivity within the microdune and applied surface ERT before and after the 3 rainfall cycles. Soil samples were collected after the experiment to determine soil physical characteristics. An experimental relationship between water tension and water content was also investigated. Our results showed that the raw relationship between calculated ERT resistivity and water tension measurements in sand is highly scattered because of significant spatial variations in porosity. An improved correlation was achieved by using resistivity ratio and water tension differences. The slope of the relationship depends on the soil solution conductivity, as predicted by Archie's law when salted water was used for the rain simulation. We found that determining the variations in electrical resistivity is a sensitive method for spatializing the differences in water tension which are directly linked with infiltration and evaporation/drainage processes in the vadose zone. However, three factors complicate the use of this approach. Firstly, the relation between water tension and water content is generally non-linear and dependent on the water content range. This could limit the use of our site-specific relations for spatializing water content with ERT through tension. Secondly, to achieve the necessary optimization of ERT inversion, we used destructive resistivity measurements in the soil, which renders ERT less attractive. Thirdly, we found that the calculated resistivity is not always accurate because of the smoothing involved in surface ERT data inversion. We conclude that further developments are needed into ERT image reconstruction before water tension (and water content) can be spatialized in heterogeneous sandy soils with the accuracy needed to routinely study their hydrological functioning.     

Inhomogeneous distribution of REE in scheelite and dynamics of Archaean hydrothermal systems (Mt. Charlotte and Drysdale gold deposits, Western Australia)

Scheelite is a widespread accessory mineral in hydrothermal gold deposits, and its rare earth element (REE) patterns and Nd and Sr isotopic compositions can be used to constrain the path and origin of the mineralising fluids and the age of the hydrothermal activity. Micro-analyses by laser ablation high resolution inductively coupled mass spectroscopy and cathodoluminescence imaging reveal a very inhomogeneous distribution of the REE in single scheelite grains from the Mt. Charlotte and Drysdale Archaean gold deposits in Western Australia. Two end-member REE patterns are distinguished: type I is middle REE (MREE)-enriched, with no or minor positive Eu-anomaly, whereas type II is flat or MREE-depleted with a strong positive Eu-anomaly. The chemical inhomogeneity of these scheelites is related to oscillatory zoning involving type I and type II patterns, with zone widths varying from below 1 to 200 μm. Intra-sectorial growth discontinuities, syn-crystallisation brittle deformation, and variations in the relative growth velocities of crystallographically equivalent faces suggest a complex crystallisation history under dynamic hydraulic conditions. The co-existence of MREE-enriched and MREE-depleted patterns within single scheelite crystals can be explained by the precipitation of a mineral which strongly partitions MREE relative to light and heavy REE. Scheelite itself has such characteristics, as does fluorapatite, which is locally abundant and has REE contents similar to that of scheelite. In this context, the systematic increase of the Eu-anomaly between type I and type II patterns is produced by the difference between the partition coefficients of Eu²⁺ and Eu³⁺, and not by fluid mixing or redox reactions. Consequently, the high positive Eu-anomaly typical of scheelite from gold ores may not necessarily be inherited from the hydrothermal fluid, but may reflect processes occurring during ore deposition. This case study demonstrates that in hydrothermal systems characterised by low REE concentrations in the fluid, and by the precipitation of a REE-rich mineral which strongly fractionates the REE, the REE patterns of such a mineral will be highly sensitive to the dynamics of the hydrothermal system. Received: 1 November 1999 / Accepted: 4 February 2000     

Strong intrusions of the Northern Mediterranean Current on the eastern Gulf of Lion: insights from in-situ observations and high resolution numerical modelling

The Northern Mediterranean Current is the return branch of the cyclonic circulation of the northwestern Mediterranean Sea. Because of geostrophic constraints, this warm and oligotrophic current is forced to flow westward along the continental slope of the Gulf of Lion. But, occasionally, it penetrates on the shelf and strongly impacts the local biogeochemistry and in turn the primary production. By combining in situ observations and high-resolution modelling, it is shown that intrusions on the eastern part of the gulf are mainly forced by easterly or northwesterly wind events, through physical mechanisms that are very different in nature. Easterlies induce a piling of water along the Gulf of Lion coast that drives, through geostrophy, an alongshore shelf-intruding current. This intrusive current occurs independently of the stratification and is concomitant with the wind forcing. On the other hand, intrusions due to northwesterlies only occur during stratified conditions and are related to the development of upwellings along the Gulf of Lion coasts. When the upwelling develops, a northwestward alongshore pressure force balances the Coriolis force associated with the onshore flow at depth. When the winds drop, the upwelling relaxes and the onshore flow weakens. Consequently, the Coriolis force no longer counterbalances the pressure force that ultimately dominates the momentum balance, causing the displacement of the Northern Current on the Gulf of Lion shelf approximately 1 day after the wind relaxation. This time lag between the northwesterlies decrease and the intrusions permits to anticipate possible changes in the biogeochemistry of the Gulf of Lion.