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François Negro Romain Bousquet Flurin Vils Clara-Marine Pellet Jeanette Hänggi-Schaub 《Swiss Journal of Geoscience》2013,106(1):63-78
In the Western Alps, the Piemont-Ligurian oceanic domain records blueschist to eclogite metamorphic conditions during the Alpine orogeny. This domain is classically divided into two “zones” (Combin and Zermatt-Saas), with contrasting metamorphic evolution, and separated tectonically by the Combin fault. This study presents new metamorphic and temperature (RSCM thermometry) data obtained in Piemont-Ligurian metasediments and proposes a reevaluation of the P–T evolution of this domain. In the upper unit (or “Combin zone”) temperatures are in the range of 420–530 °C, with an increase of temperature from upper to lower structural levels. Petrological evidences show that these temperatures are related to the retrograde path and to deformation at greenschist metamorphic conditions. This highlights heating during exhumation of HP metamorphic rocks. In the lower unit (or “Zermatt-Saas zone”), temperatures are very homogeneous in the range of 500–540 °C. This shows almost continuous downward temperature increase in the Piemont-Ligurian domain. The observed thermal structure is interpreted as the result of the upper and lower unit juxtaposition along shear zones at a temperature of ~500 °C during the Middle Eocene. This juxtaposition probably occurred at shallow crustal levels (~15–20 km) within a subduction channel. We finally propose that the Piemont-Ligurian Domain should not be viewed as two distinct “zones”, but rather as a stack of several tectonic slices. 相似文献
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A new amber deposit from the Cretaceous (uppermost Albian-lowermost Cenomanian) of southwestern France 总被引：1，自引：0，他引：1
Didier Nraudeau Vincent Perrichot Jean-Paul Colin Vincent Girard Bernard Gomez Franois Guillocheau Edwige Masure Daniel Peyrot Florent Tostain Blaise Videt Romain Vullo 《Cretaceous Research》2008,29(5-6):925
A Cretaceous amber deposit has recently been discovered in a quarry of Charente-Maritime (southwestern France), at Cadeuil. This paper presents the sedimentary and palaeoenvironmental settings of the uppermost Albian-lowermost Cenomanian series including the amber deposit. A preliminary analysis of the amber samples reveals diverse fossil arthropods (a few mites and at least 20 insect families within 9 orders), as well as numerous micro-organisms, mainly algae and mycelia. A myceloid colony of bacteria, a flagellate algae and four especially well preserved insects are illustrated (Diptera Dolichopodidae, Diptera Chironomidae, Hymenoptera Parasitica, and Heteroptera Tingidae). The abundance of the limnic micro-organisms is discussed in terms of bloom events. Their relative scarcity in almost all the amber pieces containing fossil arthropods is attributed to differences in the origin of resin: production along trunk and branches for amber with arthropods; production by aquatic roots for amber rich in algae. The absence of pollen and spores in amber is attributed to differences in the respective periods of resin and palynomorph production, which may be related to a seasonal climate during the Albian-Cenomanian transition in Western Europe. 相似文献
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The Cretaceous blueschist belt, Tavşanlı Zone, representing the subducted and exhumed northern continental margin of the Anatolide–Tauride platform is exposed in Western Anatolia. The Sivrihisar area east of Tavşanlı is made up of tectonic units consisting of i) metaclastics and conformably overlying massive marbles (coherent blueschist unit), ii) blueschist-eclogite unit, iii) marble–calcschist intercalation and iv) metaperidotite slab. The metaclastics are composed of jadeite–lawsonite–glaucophane and jadeite–glaucophane–chloritoid schists, phengite phyllites, and calcschists with glaucophane–lawsonite metabasite layers. The blueschist-eclogite unit representing strongly sheared, deeply buried and imbricated tectonic slices of accreted uppermost levels of the oceanic crust with minor metamorphosed serpentinite bodies consists of lawsonite-bearing eclogitic metabasites (approximately 90% of the field), lawsonite eclogites, metagabbros, serpentinites, pelagic marbles, omphacite–glaucophane–lawsonite metapelites and metacherts. The mineral assemblage of the lawsonite eclogite (garnet + omphacite > 70%) is omphacite, garnet, lawsonite, glaucophane, phengite and rutile. Lawsonite eclogite lenses are enclosed by garnet–lawsonite blueschist envelopes.Textural evidence from lawsonite eclogites and country rocks reveals that they did not leave the stability field of lawsonite during subduction and exhumation. The widespread preservation of lawsonite in eclogitic metabasites and eclogites can be attributed to rapid subduction and subsequent exhumation in a low geothermal gradient of the oceanic crust material without experiencing a thermal relaxation. Peak P–T conditions of lawsonite eclogites are estimated at 24 ± 1 kbar and 460 ± 25 °C. These P–T conditions indicate a remarkably low geotherm of 6.2 °C/km corresponding to a burial depth of 74 km. 相似文献
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Christophe Lécuyer Vincent Balter François Fourel Romain Amiot Olga Otero Gérard Panczer Rossana Martini 《Geochimica et cosmochimica acta》2010,74(7):2072-584
The oxygen isotope fractionation between the structural carbonate of inorganically precipitated hydroxyapatite (HAP) and water was determined in the range 10-37 °C. Values of 1000 ln α() are linearly correlated with inverse temperature (K) according to the following equation: 1000 ln α() = 25.19 (±0.53)·T−1 − 56.47 (±1.81) (R2 = 0.998). This fractionation equation has a slightly steeper slope than those already established between calcite and water (
[O’Neil et al., 1969] and [Kim and O’Neil, 1997]) even though measured fractionations are of comparable amplitude in the temperature range of these experimental studies. It is consequently observed that the oxygen isotope fractionation between apatite carbonate and phosphate increases from about 7.5‰ up to 9.1‰ with decreasing temperature from 37 °C to 10 °C. A compilation of δ18O values of both phosphate and carbonate from modern mammal teeth and bones confirms that both variables are linearly correlated, despite a significant scattering up to 3.5‰, with a slope close to 1 and an intercept corresponding to a 1000 ln α() value of 8.1‰. This apparent fractionation factor is slightly higher or close to the fractionation factor expected to be in the range 7-8‰ at the body temperature of mammals. 相似文献
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We report Li isotopic compositions, for river waters and suspended sediments, of about 40 rivers sampled within the Mackenzie River Basin in northwestern Canada. The aim of this study is to characterize the behaviour of Li and its isotopes during weathering at the scale of a large mixed lithology basin. The Mackenzie River waters display systematically heavier Li isotopic compositions relative to source rocks and suspended sediments. The range in δ7Li is larger in dissolved load (from +9.3‰ to +29.0‰) compared to suspended sediments (from −1.7‰ to +3.2‰), which are not significantly different from δ7Li values in bedrocks. Our study shows that dissolved Li is essentially derived from the weathering of silicates and that its isotopic composition in the dissolved load is inversely correlated with its relative mobility when compared to Na. The highest enrichment of 7Li in the dissolved load is reported when Li is not or poorly incorporated in secondary phases after its release into solution by mineral dissolution. This counterintuitive observation is interpreted by the mixing of water types derived from two different weathering regimes producing different Li isotopic compositions within the Mackenzie River Basin. The incipient weathering regime characterizing the Rocky Mountains and the Shield areas produces 7Li enrichment in the fluid phase that is most simply explained by the precipitation of oxyhydroxide phases fractionating Li isotopes. The second weathering regime is found in the lowland area and produces the lower δ7Li waters (but still enriched in 7Li compared to bedrocks) and the most Li-depleted waters (compared to Na). Fractionation factors suggest that the incorporation of Li in clay minerals is the mechanism that explains the isotopic composition of the lowland rivers. The correlation of boron and lithium concentrations found in the dissolved load of the Mackenzie Rivers suggests that precipitation of clay minerals is favoured by the relatively high residence time of water in groundwater. In the Shield and Rocky Mountains, Li isotopes suggest that clay minerals are not forming and that secondary minerals with stronger affinity for 7Li appear.Although the weathering mechanisms operating in the Mackenzie Basin need to be characterized more precisely, the Li isotope data reported here clearly show the control of Li isotopes by the weathering intensity. The spatial diversity of weathering regimes, resulting from a complex combination of factors such as topography, geology, climate and hydrology explains, in fine, the spatial distribution of Li isotopic ratios in the large drainage basin of the Mackenzie River. There is no simple relationship between Li isotopic composition and chemical denudation fluxes in the Mackenzie River Basin. 相似文献
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Constraining a Flow Model with Field Measurements to Assess Water Transit Time Through a Vadose Zone
The modeling of thick vadose zones is particularly challenging because of difficulties in collecting a variety of measured sediment properties, which are required for parameterizing the model. Some models rely on synthetic data, whereas others are simplified by running as homogeneous sediment domains and relying on a single set of sediment properties. Few studies have simulated flow processes through a thick vadose zone using real and comprehensive data sets comprising multiple measurements. Here, we develop a flow model for a 7-m-thick vadose zone. This model, combining the numerical codes CTRAN/W with SEEP/W, includes the measured sediment hydraulic properties of the investigated vadose zone and incorporates the actual climate and subsurface conditions of the study site (precipitations, water-table elevations, and stable isotope data). The model is calibrated by fitting the simulated and measured vertical profiles of water content. Our flow model calculates a transit time of 1 year for the travel of water through the 7-m vadose zone; this estimate matches stable isotope-based results obtained previously for this site. A homogeneous sediment domain flow model, which considers only a single set of sediment properties, produces a transit time that is approximately half the duration of that of the heterogeneous flow model. This difference highlights the importance of assuming heterogeneous material within models of thick vadose zones and testifies to the advantage gained when using real sediment hydraulic properties to parametrize a flow model. 相似文献
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The regional climate model (MAR) is used to perform a simulation of the year 1992 over West Africa. It is shown that MAR is
able to simulate the main features of the rainy regime over West Africa and especially the discontinuous seasonal progression
of the West African Monsoon along the year. One particular feature that is reasonably well reproduced is the abrupt shift
of the rain band from 5° to 10°N at the end of June (also called “monsoon jump”). This study suggests that such a phenomenon
is associated with the shift of the Saharan heat low between two favourite positions: one being over the Sahelian area (10–15°N)
and the other over the Saharan area (20–25°N). These two favourite locations of the heat low are linked to the spatial distribution
of surface albedo over West Africa that drives the spatio-temporal location of the surface temperature maxima. A detailed
analysis of this “monsoon jump” is performed and the causes of the strong decrease in precipitation that precedes the northward
shift of the rain band are also investigated. 相似文献
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Christophe Messager Hubert Gallée Olivier Brasseur Bernard Cappelaere Christophe Peugeot Luc Séguis Michel Vauclin Romain Ramel Gilles Grasseau Laurent Léger Denis Girou 《Climate Dynamics》2006,27(2-3):199-214
The forcing of a hydrologic model (ABC) by both observed and simulated precipitation from a regional climate model (MAR) has been performed over the Sirba watershed (39,000 km2) located in the Sahelian region. Two aspects have been more specifically examined: the spatial and temporal representations of precipitation. The comparison between simulated and observed discharges—using observed rainfall datasets as forcing of the hydrologic model—has shown that the representation of daily precipitation (which is mainly convective in the Sahelian region) was not sufficiently accurate to correctly simulate the hydrologic response of the watershed. Since this response drives the soil water budget and consequently the amount of evaporation in forthcoming coupling experiments, it is thus necessary to develop more realistic infra-daily precipitation associated with convective events. A new temporal disaggregation scheme has been then developed. Considering observed as well as simulated precipitation fields, this method has significantly improved the simulated discharge at the catchment outlet. The major role played by the temporal component compared to spatial component of the precipitation has been then underlined. In addition, the present study shows the unsuitability of the simulated precipitation from the RCM to directly force a hydrologic model at infra daily timescale even if the cumulative amount and the main features of the precipitation seasonal cycle are well simulated. 相似文献