Quantity and distribution of methane entrapped in sediments of calcareous, Alpine glacier forefields

Aside from many well-known sources, the greenhouse gas methane (CH4) was recently discovered entrapped in the sediments of Swiss Alpine glacier forefields derived from calcareous bedrock. A first study performed in one glacial catchment indicated that CH4 was ubiquitous in sediments and rocks and was largely of thermogenic origin. Here, we present the results of a follow-up study that aimed at (1) determining the occurrence and origin of sediment-entrapped CH4 in other calcareous glacier forefields across Switzerland and (2) providing an inventory of this sediment-entrapped CH4, i.e., determining the contents and total mass of CH4 present, and its spatial distribution within and between five different Swiss glacier forefields situated on calcareous formations of the Helvetic nappes in the Central Alps. Sediment and bedrock samples were collected at high spatial resolution from the forefields of Im Griess, Griessfirn, Griessen, Wildstrubel, and Tsanfleuron glaciers, representing different geographic and geologic regions of the Helvetic nappes. We performed geochemical analyses on gas extracted from sediments and rocks, including the determination of CH4 contents, stable carbon-isotope analyses (δ13CCH4), and the determination of gas-wetness ratios (ratio of CH4 to ethane and propane contents). To estimate the total mass of CH4 entrapped in glacier-forefield sediments, the total volume of sediment was determined based on the measured forefield area and either literature values of mean sediment thickness or direct depth measurements using electrical resistivity tomography. Methane was found in all sediments (0.08–73.81 µg CH4 g−1 dry weight) and most rocks (0.06–108.58 µg CH4 g−1) collected from the five glacier forefields, confirming that entrapped CH4 is ubiquitous in these calcareous formations. Geochemical analyses further confirmed a thermogenic origin of the entrapped CH4 (average δ13CCH4 of sediment of −28.23 (± 3.42) ‰; average gas-wetness ratio of 75.2 (± 48.4)). Whereas sediment-entrapped CH4 contents varied moderately within individual forefields, we noted a large, significant difference in the CH4 content and total CH4 mass (range of 200–3881 t CH4) between glacier forefields at the regional scale. The lithology and tectonic setting within the Helvetic nappes appeared to be dominant factors determining rock and sediment CH4 contents. Overall, a substantial quantity of CH4 was found to be entrapped in Swiss calcareous glacier forefields. Its potential release and subsequent fate in this environment is the subject of ongoing studies.

Mechanical controls on recumbent folding from 2D numerical simulations. Applications to the Eaux-Chaudes fold nappe (west-central Pyrenees)

<p>Tectonic nappes are typical structural features in orogenic belts worldwide and include two end members, namely thrust nappes and fold nappes. Although the geometry and kinematics of these are relatively well constrained after more than a century of studies, the mechanics are still incompletely understood.</p><p>In recent years, numerical modelling has become a powerful tool to unravel the mechanics of fold nappes. Studies have been carried out particularly with application to the Helvetic nappes of the Alps, highlighting the relevance of the mechanical stratigraphy involved in the deformation. The Helvetic nappes consist of a superposition of thrust nappes over recumbent fold nappes. It was developed due to the closure of half-graben basins and the extrusion of their sedimentary infill under dominantly ductile deformation conditions. Competence contrast between stiff (i.e. limestones) and weak layers (i.e. shales) played a key role in controlling the deformation style.</p><p>Recently, a similar structure has been reported in the Pyrenees. The Eaux-Chaudes massif (western Axial Zone, Pyrenees) is a basement-cored recumbent anticline with a kilometric, long reverse limb showing ductile deformation in Mesozoic carbonates, much in style of the Helvetic nappes of the Alps. The reverse limb is in thrust contact over an autochthonous Mesozoic cover with similar stratigraphy, and hence its development cannot be explained by basin infill extrusion. The fold structure shows a strain increase towards the reverse limb and is overlain by the Lakora basement thrust sheet. The general stratigraphic succession consists of Upper Cretaceous limestones and shales lying unconformably over Paleozoic metasediments or Lower Triassic sandstone pods and featuring inliers of Upper Triassic Keuper facies and ophites. The autochthonous succession lies on top of a late Variscan granitic pluton, both showing very low-strain during Alpine deformation.</p><p>Here, we employ the thermomechanical staggered finite difference code LaMEM (Kaus <em>et al.</em>, 2016) to perform 2D parametric simulations in order to study changes in deformation style between thrust nappes (plastic/brittle-localisation) and recumbent fold nappes (viscous/ductile-distributed). The simulations are performed using a linear viscoelastoplastic rheology with the Drucker-Prager criterion for plasticity. The initial setup consists of two domains separated by a basement perturbation and both overlain by a lower stiff layer, representing the Upper cretaceous limestones. In the right-bottom half domain there is a stiff body representing the Eaux-Chaudes pluton, while in the bottom-left domain there are weak layers representing the shale-rich Paleozoic basement. Over the stiff layer, there is a multilayer of weak and stiff layers mimicking the Lakora thrust sheet, which provides the overburden and confining pressure.</p><p>Preliminary results show a strong control of the cohesion and viscosity of the lower stiff layer on the deformation style developed. For simulations with low cohesion values, there is an enhancing of strain localization and thrust nappe development is favoured, whereas high cohesion values tend to spatially distribute the deformation and facilitate the development of fold nappes. Further simulations are in progress to test these preliminary results.</p><p> </p><p>Kaus, B., Popov, A., Baumann, T.S., Püsök, A.E., Bauville, A., Fernandez, N and Collignon, M. (2016): In: <em>NIC Symposium</em>, <em>Proceedings</em>, 48.</p>

Quantity and distribution of methane entrapped in sediments of calcareous, Alpine glacier forefields

Aside from many well-known sources, the greenhouse gas methane (CH4) was recently discovered entrapped in sediments of Swiss Alpine glacier forefields derived from calcareous bedrock. A first study performed in one glacial catchment indicated that CH4 was ubiquitous in sediments and rocks, and was largely of thermogenic origin. Here we present results of a follow-up study, which aimed at (1) determining occurrence and origin of sediment-entrapped CH4 in other calcareous glacier forefields across Switzerland, and (2) providing an inventory for this sediment-entrapped CH4, i.e., determining contents and total mass of CH4 present, and its spatial distribution within and between five different Swiss glacier forefields situated on calcareous formations of the Helvetic Nappes of the Central Alps. Sediment and bedrock samples were collected at high spatial resolution from the forefields of Im Griess, Griessfirn, Griessen, Wildstrubel, and Tsanfleuron glaciers, representing different geographic and geologic regions of the Helvetic Nappes. We performed geochemical analyses on gas extracted from sediments and rocks, including determination of CH4 contents, stable carbon-isotope analyses (δ13CCH4), and determination of gas-wetness ratios (ratio of CH4 to ethane and propane contents). To estimate the total mass of CH4 entrapped in glacier-forefield sediments, the total volume of sediment was determined based on measured forefield area and either literature values of mean sediment depth or direct depth measurements using electrical-resistivity tomography. Methane was found in all sediments (0.08–73.81 μg CH4 g−1 dry weight) and most rocks (0.06–108.58 µg CH4 g−1) collected from the five glacier forefields, confirming that entrapped CH4 is ubiquitous in these calcareous formations. Geochemical analyses further confirmed a thermogenic origin of the entrapped CH4 (average δ13C-CH4 of sediment: −28.23 (± 3.42) ‰; average gas-wetness ratio: 75.2 (± 48.4)). Whereas sediment-entrapped CH4 contents varied moderately within individual forefields, we noted a large, significant difference in CH4 content and total CH4 mass (range: 200–3881 t CH4) between glacier forefields at the regional scale. Lithology and tectonic setting within the Helvetic Nappes appeared to be dominant factors determining rock and sediment CH4 contents. Overall, a substantial quantity of CH4 was found to be entrapped in Swiss calcareous glacier forefields. Its potential release and subsequent fate in this environment is the subject of ongoing studies.

Basement-involved thin-skinned and thick-skinned tectonics in the Alps

The deformation of continental crust during continental collision by folding and thrusting follows three types of structural styles: (1) in a true thin-skinned style only cover rocks are involved; (2) in the case of a thin-skinned basement-involved style, thin slabs of crystalline basement rocks are piled up into a nappe stack; (3) in a true thick-skinned style, the entire upper crust is involved in the deformation. In the Alps all three styles can be recognized. The Helvetic nappes and parts of the Penninic nappes exhibit true thin-skinned style tectonics. Triassic evaporites, Jurassic shales and Cretaceous marls acted as detachment horizons. Basement-involved thin-skinned tectonics is typical for the Penninic nappes in the core of the orogen. The thickness of the basement thrust sheets is controlled by the effects of Mesozoic rifting, by deep burial and heating of the subducting crust and by the presence of Late Palaeozoic structures. Thick-skinned style is observed in the more external parts of the orogen, the external massifs and the Southalpine nappe system. It occurred in the late phase of collision and involved the entire upper crust. The basal detachment occurred possibly along phyllonites generated by the breakdown of load-bearing feldspar. Considering the Alpine orogen as a whole, the lower crust deformed seemingly independently from the upper crust. The detachment of the cover units by thin-skinned tectonics occurred prior to thrusting related to basement-involved thin-skinned tectonics. Thrust faults of both types were overprinted by ‘post-nappe folding’.

Patterns of landscape form in the upper Rhône basin, Central Swiss Alps, predominantly show lithologic controls despite multiple glaciations and variations in rock uplift rates

The development of topography is mainly dependent on the interplay of uplift and erosion, which are in term controlled by various factors including climate, glaciers, lithology, seismic activity and short-term variables such as anthropogenic impact. While most studies have focused on the role of tectonics and climate on the landscape form and underlying processes, less attention has been paid on exploring the controls of lithology on erosion. The Central European Alps are characterized by a large spatial variability in exposed lithologies and as such offer an ideal laboratory to investigate the lithological controls on erosion and landscape form. Here, we focus on the ca. 5400 km2-large upper Rhône basin situated in the Central Swiss Alps to explore how the lithological architecture of the bedrock conditions the Alpine landscape. To this extent, we extract geomorphological parameters along the channels of ca. 50 tributary basins, whose catchments are located in either granitic basement rocks (External massifs), oceanic meta-sedimentary and ophiolitic rocks (Penninic nappes) or fine-grained continental-margin sediments (Helvetic nappes). The analysis of longitudinal river profiles show that all tributary rivers within the Rhône basin are in topographic transient state as testified by mainly convex or concave-convex longitudinal stream channel profiles with several knickpoints of either tectonic or glacial origin. In addition, although the entire Rhône basin shows a strong glacial inheritance (and is still partly glaciated) and some of the highest uplift rates recently measured in the Alps, the river network has responded differently to those perturbations as revealed by the morphometric data. In particular, tributary basins in the Helvetic nappes are the most equilibrated (concave river profiles, overall lower elevations, less steep slope gradients and lowest hypsometric integrals), while the tributaries located in the External massifs are least equilibrated, where streams yield strong convex long profiles, and where the tributary basins have the highest hypsometric integral and reveal the steepest hillslopes. We interpret this pattern to reflect differences in response times of the fluvial erosion in tributary streams towards glacial and tectonic perturbations, where the corresponding lengths strongly depend on the lithology and therefore on the bedrock erodibility.