A 4th order fabric tensor approach applied to granular media

In this paper, we propose a micromechanical approach of the behavior of granular media, which takes into account the anisotropy by means of a fourth order fabric tensor. The proposed approach is implemented in an homogenization scheme based on Voigt and Reuss localization assumption. The fabric tensor-based approach is then combined with a new kinematic localization rule and yields a general homogenization scheme for anisotropic granular media.     

Theoretical study and simulation of moisture transport in unsaturated porous media by periodic homogenization

In this work, the macroscopic Richards equation for moisture transport is established in unsaturated porous media using periodic homogenization. By performing dimensional analysis on microscopic equations of moisture transfer, dimensional numbers characterizing moisture transport appear. The application of the asymptotic homogenization leads to the classical Richards equation, which is justified rigorously this way. Moreover, we obtain an accurate definition of the homogenized diffusion tensor of moisture involving the geometric properties of the microstructure and known transport properties of the material. A different behavior for the transport of water vapor between hygroscopic and super‐hygroscopic region is revealed. Finally, a simple 2D example where an analytical solution exists is addressed. Copyright © 2014 John Wiley & Sons, Ltd.     

Chironomid assemblages in cores from multiple water depths reflect oxygen-driven changes in a deep French lake over the last 150 years

We sampled modern chironomids at multiple water depths in Lake Annecy, France, before reconstructing changes in chironomid assemblages at sub-decadal resolution in sediment cores spanning the last 150 years. The lake is a large, deep (z_max = 65 m), subalpine waterbody that has recently returned to an oligotrophic state. Comparison between the water-depth distributions of living chironomid larvae and subfossil head capsules (HC) along three surface-sediment transects indicated spatial differences in the influence of external forcings on HC deposition (e.g. tributary effects). The transect with the lowest littoral influence and the best-preserved, depth-specific chironomid community characteristics was used for paleolimnological reconstructions at various water depths. At the beginning of the twentieth century, oxygen-rich conditions prevailed in the lake, as inferred from M. contracta-type and Procladius sp. at deep-water sites (i.e. cores from 56 to 65 m) and Paracladius sp. and H. grimshawi-type in the core from 30 m depth. Over time, chironomid assemblages in cores from all three water depths converged toward the dominance of S. coracina-type, indicating enhanced hypoxia. The initial change in chironomid assemblages from the deep-water cores occurred in the 1930s, at the same time that an increase in lake trophic state is inferred from an increase in total organic carbon (TOC) concentration in the sediment. In the 1950s, an assemblage change in the core from 30 m water depth reflects the rapid expansion of the hypoxic layer into the shallower region of the lake. Lake Annecy recovered its oligotrophic state in the 1990s. Chironomid assemblages, however, still indicate hypoxic conditions, suggesting that modern chironomid assemblages in Lake Annecy are decoupled from the lake trophic state. Recent increases in both TOC and the hydrogen index indicate that changes in pelagic functioning have had a strong indirect influence on the composition of the chironomid assemblage. Finally, the dramatic decrease in HC accumulation rate over time suggests that hypoxic conditions are maintained through a feedback loop, wherein the accumulation of (un-consumed) organic matter and subsequent bacterial respiration prevent chironomid re-colonization. We recommend study of sediment cores from multiple water depths, as opposed to investigation of only a single core from the deepest part of the lake, to assess the details of past ecological changes in large deep lakes.     

A study of the δ13C offset between chironomid larvae and their exuvial head capsules: implications for palaeoecology

We conducted a rearing experiment with the chironomid species Chironomus riparius to assess the relationship between the δ¹³C values of chironomid larvae and the δ¹³C values of their exuvial head capsules. Our experiment was also designed to study the extent of the trophic fractionation factor (Δ¹³C) under different dietary conditions. Three food sources were used (Tetramin, oats and corn), covering a range in δ¹³C values of 14.55 ‰. For each of the four successive instars, carbon isotope ratios were measured in larval tissues and head capsules. This approach highlighted the variability in δ¹³C for both larvae and their head capsules during larval development. Once the larvae reached the 3rd instar, their δ¹³C values were stabilised and did not significantly differ from their food δ¹³C (Δ¹³C = 0 ‰). It is probable that the variability in the δ¹³C offset during larval development reflected a difference in the carbon turnover for the chironomid cuticle compared with the whole body. At the 4th instar, the δ¹³C offset did not significantly differ between the three food sources and was ?0.9 ± 0.2 ‰. The proposed Δ¹³C and δ¹³C offset values can be considered as a first step for the reconstructions of the chironomid larvae paleo-diets with the aim of deciphering the different organic carbon sources supporting chironomid larvae productions. However, the influence of the environment (e.g. temperature, oxygen), other food sources (e.g. different nutritive values) as well as taxonomy (i.e. other chironomid species) should be assessed to strengthen the robustness of these results.     

Reconstruction of Late Glacial summer temperatures from chironomid assemblages in Lac Lautrey (Jura,France)

A chironomid–July air temperature inference model based on chironomid assemblages in the surface sediments of 81 Swiss lakes was used to reconstruct Late Glacial July air temperatures at Lac Lautrey (Jura, Eastern France). The transfer‐function was based on weighted averaging–partial least squares (WA‐PLS) regression and featured a leave‐one‐out cross‐validated coefficient of determination (r²) of 0.80, a root mean square error of prediction (RMSEP) of 1.53 ° C, and was applied to a chironomid record consisting of 154 samples covering the Late Glacial period back to the Oldest Dryas. The model reconstructed July air temperatures of 11–12 ° C during the Oldest Dryas, increasing temperatures between 14 and 16.5 ° C during the Bølling, temperatures around 16.5–17.0 ° C for most of the Allerød, temperatures of 14–15 ° C during the Younger Dryas and temperatures of ca. 16.5 ° C during the Preboreal. The Lac Lautrey record features a two‐step July air temperature increase after the Oldest Dryas, with an abrupt temperature increase of ca. 3–3.5 ° C at the Oldest Dryas/Bølling transition followed by a more gradual warming between ca. 14 200 and 13 700 BP. The transfer‐function reconstructs a less rapid cooling at the Allerød/Younger Dryas transition than other published records, possibly an artefact caused by the poor analogue situation during the earliest Younger Dryas, and an abrupt warming at the Younger Dryas/Holocene transition. During the Allerød, two centennial‐scale 1.5–2.0 ° C coolings are apparent in the record. Although chronologically not well constrained, the first of these cold events may be synchronous with the beginning of the Gerzensee Oscillation. The second is inferred just before deposition of the Laachersee tephra at Lac Lautrey and is therefore coeval with the end of the Gerzensee Oscillation. In contrast to the Greenland oxygen isotope records, the Lac Lautrey palaeotemperature reconstruction lacks a clearly defined Greenland Interstadial (GI) event 1d and the decreasing temperature trend during the Bølling/Allerød Interstadial. Copyright © 2005 John Wiley & Sons, Ltd.     

Behaviour of chemical solutes during a storm in a rainforested headwater catchment

The aim of this study is to identify, in a small catchment area located within a tropical forest, the pedological compartments in which the export of nutrients and chemical erosion of solutes occur during a stormflow event. The catchment area displays two types of lateral flow: (i) overland flow at the surface of the soil in the litter and root mat and (ii) groundwater flow in a macroporous subsurface horizon. We interpret the variations of stream‐water chemistry during a storm‐flow event using the separation of storm‐flow hydrograph data between overland and groundwater flow, and (Cl^?) as a chemical parameter characterizing the residence time of water in the soil. It appears that K⁺ especially was released into the throughfall, whereas Ca⁺⁺, Mg⁺⁺ and Na⁺ were clearly released from the litter. K⁺ disappeared rapidly from soil solution, whereas Ca⁺⁺ and Mg⁺⁺ were more progressively absorbed by the vegetation. The Ca⁺⁺ and Mg⁺⁺ contents in groundwater increased with increasing residence time owing to the transpiration of trees. The export of H₄SiO₄ in the overland flow was moderate, i.e. 24% of total H₄SiO₄ export in the stream flow, as overland flow represented 39% of total runoff. The subsurface horizon—where active groundwater flow occurs—was successively affected by chemical erosion during the storm‐flow peak, and then by neoformation of kaolinite favoured by increasing water residence time. Copyright © 2003 John Wiley & Sons, Ltd.     

Inland aeolian deposits of south‐west France: facies,stratigraphy and chronology

Archaeological investigations undertaken along a proposed highway together with the compilation of available geological and pedological data made it possible to give a first overview of the distribution of Pleistocene aeolian deposits in south‐west France. A chronological framework for deposition has been obtained using both radiocarbon (n = 24) and luminescence (n = 26) dating. It shows that aeolian transport was very active during the Late Pleniglacial, between 15 and ～23 ka, leading to sand emplacement over a 13 000‐m² area at the centre of the basin. The Pleniglacial coversands are typified by extensive fields of small transverse to barchanoid ridges giving way to sandsheets to the east. Subsequent aeolian phases, at ca. 12 ka (Younger Dryas) and 0.8–0.2 ka (Little Ice Age), correspond to the formation of more localized and higher, mainly parabolic dunes. At the southern and eastern margins of the coversand area, aeolian dust accumulated to form loess deposits, the thickness of which reaches ～3 m on the plateaus. Luminescence dates together with interglacial‐ranking palaeoluvisols between the loess units clearly indicate that these accumulations built up during the last two glacial–interglacial cycles. The chronology of sand and loess deposition thus appears to be consistent with that already documented for northern Europe. This suggests that it was driven by global climate changes in the northern hemisphere. The relatively thin aeolian deposits (and particularly loess) in south‐west France is thought to reflect both a supply‐limited system and a moister climate than in more northern and continental regions. Copyright © 2011 John Wiley & Sons, Ltd.     

Mantle plume heterogeneity versus shallow-level interactions: A case study,the São Nicolau Island,Cape Verde archipelago

We present new Sr–Nd–Pb isotopic data, as well as major- and trace-element concentrations, for 19 basaltic samples from São Nicolau Island, Cape Verde archipelago. Fine-scale study of the island argues in favor of mixing between four endmembers to explain isotopic variations of collected samples: 1) a radiogenic endmember (⁸⁷Sr/⁸⁶Sr ~ 0.7034; ¹⁴³Nd/¹⁴⁴Nd ~ 0.51285; ²⁰⁶Pb/²⁰⁴Pb ~ 20.0; ²⁰⁷Pb/²⁰⁴Pb ~ 15.65; ²⁰⁸Pb/²⁰⁴Pb ~ 39.8) representative of the Cape Verde plume deep source; 2) an unradiogenic endmember having isotopic compositions resembling those of Atlantic MORB dredged at the same latitude; 3) a low Sr–high Nd and Pb endmember identified as the Jurassic MORB basement of the archipelago; and 4) São Vicente-like Cape Verde carbonatites. Compositional and isotopic results show that most of the measured variations can be related to mixing of plume-derived melts with shallow-level reservoirs. Therefore the source heterogeneity of the Cape Verde plume is much smaller than the one sampled in basaltic samples. This observation illustrates how caution is required when interpreting global OIB data in terms of mantle topology without filtering from the contribution of shallow-level reservoirs.     

A cascade of magmatic events during the assembly and eruption of a super-sized magma body

We use comprehensive geochemical and petrological records from whole-rock samples, crystals, matrix glasses and melt inclusions to derive an integrated picture of the generation, accumulation and evacuation of 530 km³ of crystal-poor rhyolite in the 25.4 ka Oruanui supereruption (New Zealand). New data from plagioclase, orthopyroxene, amphibole, quartz, Fe–Ti oxides, matrix glasses, and plagioclase- and quartz-hosted melt inclusions, in samples spanning different phases of the eruption, are integrated with existing data to build a history of the magma system prior to and during eruption. A thermally and compositionally zoned, parental crystal-rich (mush) body was developed during two periods of intensive crystallisation, 70 and 10–15 kyr before the eruption. The mush top was quartz-bearing and as shallow as ~3.5 km deep, and the roots quartz-free and extending to >10 km depth. Less than 600 year prior to the eruption, extraction of large volumes of ~840 °C low-silica rhyolite melt with some crystal cargo (between 1 and 10%), began from this mush to form a melt-dominant (eruptible) body that eventually extended from 3.5 to 6 km depth. Crystals from all levels of the mush were entrained into the eruptible magma, as seen in mineral zonation and amphibole model pressures. Rapid translation of crystals from the mush to the eruptible magma is reflected in textural and compositional diversity in crystal cores and melt inclusion compositions, versus uniformity in the outermost rims. Prior to eruption the assembled eruptible magma body was not thermally or compositionally zoned and at temperatures of ~790 °C, reflecting rapid cooling from the ~840 °C low-silica rhyolite feedstock magma. A subordinate but significant volume (3–5 km³) of contrasting tholeiitic and calc-alkaline mafic material was co-erupted with the dominant rhyolite. These mafic clasts host crystals with compositions which demonstrate that there was some limited pre-eruptive physical interaction of mafic magmas with the mush and melt-dominant body. However, the mafic magmas do not appear to have triggered the eruption or controlled magmatic temperatures in the erupted rhyolite. Integration of textural and compositional data from all available crystal types, across all dominant and subordinate magmatic components, allow the history of the Oruanui magma body to be reconstructed over a wide range of temporal scales using multiple techniques. This history spans the tens of millennia required to grow the parental magma system (U–Th disequilibrium dating in zircon), through the centuries and decades required to assemble the eruptible magma body (textural and diffusion modelling in orthopyroxene), to the months, days, hours and minutes over which individual phases of the eruption occurred, identified through field observations tied to diffusion modelling in magnetite, olivine, quartz and feldspar. Tectonic processes, rather than any inherent characteristics of the magmatic system, were a principal factor acting to drive the rapid accumulation of magma and control its release episodically during the eruption. This work highlights the richness of information that can be gained by integrating multiple lines of petrologic evidence into a holistic timeline of field-verifiable processes.     

Temporal changes in the contribution of methane-oxidizing bacteria to the biomass of chironomid larvae determined using stable carbon isotopes and ancient DNA

Freshwater lakes are important sources of methane (CH₄) emissions, by organic matter degradation under anaerobic conditions (methanogenesis). Previous studies suggest that lakes contribute up to 16 % of natural emissions. About 60 % of the CH₄ produced is used as an energy source by methane-oxidizing bacteria (MOB—methanotrophs), which could support higher trophic levels, especially Chironomidae (Diptera). Because biogenic methane has a very low stable carbon isotope value, evidence of methane-derived organic-matter assimilation can be tracked by stable carbon isotope analysis in consumers such as chironomids. In some cases, however, chironomid δ¹³C values are not low enough to unambiguously demonstrate methanotroph assimilation and an alternative line of evidence is required. Analysis of ancient DNA (aDNA) from the methanotroph community preserved in lake sediment provides reliable information about past methane oxidation in freshwater lakes. A combination of these two approaches was used to study a sediment core from the deepest zone of Lake Narlay (Jura, France), which covers the last 1,500 years of sediment accumulation. Results show a significant change ca. AD 1600, with an increase in the proportion of MOB in the total bacteria community, and a decrease in chironomid head-capsule δ¹³C. These trends suggest assimilation of MOB by chironomid larvae, and account for up to 36 % of the chironomid biomass. The data also provide information about the feeding behavior of chironomids, with evidence for preferential assimilation of methanotroph type I and the NC10 phylum. The combination of aDNA analysis and carbon stable isotopes strengthens the reliability of inferences about carbon sources incorporated into chironomid biomass.