Landslides as geological hotspots of CO₂ emission: clues from the instrumented Séchilienne landslide, western European Alps

This study makes use of a highly instrumented active landslide observatory (9 years of data) in the French Alps, the Séchilienne slope. Here, we use a combination of major element chemistry and isotopes ratios (87Sr / 86Sr, δ34S) measured in different water types of the stable and unstable part of the Séchilienne instability to assess the contribution of the different lithologies of the slope and the chemical weathering mechanisms. Chemical and isotopic ratios are used to characterize weathering processes and the origin of waters and their flow paths through the massif. A mixing model allows us to allocate the different major elements to different sources, to identify secondary carbonate formation as a major process affecting solutes in the subsurface waters of the instability, and to quantify the involvement of sulfuric and carbonic acids as a source of protons. We show that the instability creates favorable and sustained conditions for the production of sulfuric acid by pyrite oxidation, by opening new fractures and supplying fresh reactive surfaces. We clearly identify the contribution of the dissolution of each mineral phase to the chemistry of the waters, with a clear role of remote gypsum dissolution to the sulfate budget in the sampled waters. We are also able to refine the preexisting hydrogeological views on the local water circulation and water flow paths in the instability by showing the hydrological connectivity of the different zones. Overall, our results show that the Séchilienne landslide, despite its role in accelerating rock chemical and physical weathering, acts as a geological source of CO2 to the atmosphere. If generalizable to other large instabilities in mountain ranges, this study illustrates the complex coupling between physical and chemical erosion and their impact on the carbon cycle and global climate. The study also highlights the importance of distinguishing between sulfite oxidation and gypsum dissolution as a source of sulfate ions to rivers, particularly in mountain ranges.

A Geochemical and Agronomic Evaluation of Technosols Made from Construction and Demolition Fines Mixed with Green Waste Compost

Construction and demolition fines (C&D-fines) and green waste compost (GWC) are two commonly generated urban waste materials that represent repositories of geochemical value. Here technosols were produced from volumetric mixtures of these materials ranging from 0–100% C&D-fines, with the remaining proportion comprised of GWC. Agronomic assessment was carried out by way of pot and rhizobox plant growth experiments with ryegrass, barley and pea to determine germination, plant mass and rooting behaviours. Geochemical and mineralogical evaluation was achieved by soil pore water solution measurements combined with X-ray powder diffraction analyses respectively, to characterise the technosols and their distinct deviations from a reference agricultural geogenic soil (soil). The results demonstrated that germination, growth and root mass/surface area of vegetation were up to 80-fold greater after 30-days in the technosol composed of equal volumes of the two materials (50% C&D-fines: 50% GWC) compared to the soil. High concentrations of Ca and Mg in pore waters (550–800 mg·L−1) were dominant features of the technosols, in contrast to the soil (<50 mg·L−1), resulting from gypsum and calcite enrichment of the C&D-fines. In contrast, the GWC represented a source of soluble K (450–1000·mg·L−1). Highly elevated Ca concentrations in extended leaching tests of the C&D-fines reflected ongoing gypsum dissolution, whereas soluble Mg and K were rapidly depleted from the GWC. In summary, short-term performance of the technosols as plant growth substrates was strong despite their geochemical and mineralogical distinction from soil. Gleaning additional geochemical value from combining urban wastes in this way is potentially suited to myriad scenarios where geogenic soils are contaminated, sealed or otherwise absent. Further assessment will now be needed to determine the geochemical longevity of the technosols before wider scale applications can be recommended.

Landslides as geological hotspots of CO₂ to the atmosphere: clues from the instrumented Séchilienne landslide, Western European Alps

This study makes use of a highly instrumented active landslide observatory (9 years of data) in the French Alps, the Séchilienne slope. Using a combination of major element chemistry and isotopes ratios (87Sr / 86Sr, δ34S) measured in different water types of the stable and unstable part of the Séchilienne instability to assess the contribution of the different lithologies of the slope and the chemical weathering mechanisms. Chemical and isotopic ratios appear useful to characterize weathering processes and the origin of waters and their flowpaths through the massif. A mixing model allows us to allocate the different major elements to different sources and quantify the involvement sulfuric and carbonic acids as a source of protons. As a consequence of the model, we are able to show that the instability creates favorable and sustained conditions for the production of sulfuric acid by pyrite oxidation by supplying reactive surfaces. We clearly identify the contribution of gypsum dissolution to the sulfate budget in the landslide. We are also able to refine the pre-existing hydrogeological views on the local water circulation and water flow paths in the instability but showing the hydrological connectivity of the different zones. Overall, our results show that the Séchilienne landslide, despite its role in accelerating rock chemical and physical weathering, acts, at a geological time scale (i.e. at timescales longer that carbonate precipitation in the ocean) as a source of CO2 to the atmosphere. If generalizable to other instable zones in mountain ranges, this study illustrates the complex coupling between physical and chemical erosion and climate. The study also highlights the importance of deciphering between sulfite oxidation and gypsum dissolution as a source of sulfate ions to rivers, particularly in mountain ranges.

Effect of Relative Compaction on Water Absorption and Gypsum Dissolution in Gypsum-Rich Clayey CBR Samples

A clayey gypsiferous soil of CL group according to the Unified Soil Classification System was studied for the effect of relative compaction on water absorption and gypsum dissolution during long-term soaking. The soil has a gypsum content of about 33%. Two sets of soil samples were prepared at optimum moisture content of 11.75% of the modified Proctor compaction test. The first set received 100%, while the second received about 93.5% relative compaction with respect to modified Proctor. These samples were soaked for 4, 7, 15, 30, and 120 days under 40 lbs (178 N) surcharge load. The moisture content was determined at top, quarter points, midpoint, and bottom of each soil sample. The test results revealed that for each compaction effort, the moisture content along each soaked soil sample is not uniform and increased with increasing soaking period. This increase in moisture content is greater for soil samples compacted at the lower compaction effort. The moisture content at top of each soil sample is greater than at the bottom, and the least moisture content took place at the middle of the sample. The dissolution of gypsum, at the top of soil samples, was greater than that at the middle. A multiple regression equation was developed relating strongly the decrease in gypsum content along the clayey CBR samples, with compaction effort and increase in average moisture content along the samples due to soaking. Similarly, strong correlation was obtained from the multiple regression developed between absorbed water, soaking period, and compaction effort. The paper shows that the water absorption and gypsum dissolution decrease with increasing relative compaction as the soil becomes denser.

Quantification of the dissolution kinetics of natural gypsum and particle transport processes in the evolution of dissolution cavities

&lt;p&gt;The north-eastern suburbs of Paris are most prone to sinkhole development due to the natural dissolution of gypsum rocks in contact with groundwater flow. This dissolution induces a loss of solid material creating underground voids with different shapes and sizes that can lead to large underground collapse or subsidence. Until now, there is still a high uncertainty regarding the dissolution mechanisms of natural gypsum and the hydrodynamic, chemical and mechanical conditions involved in this process.&lt;/p&gt;&lt;p&gt;This work has two broad aims: a) to evaluate the variability of gypsum dissolution rate as function of the surface roughness and heterogeneity; b) to identify the respective role of particle transport and dissolution processes in the formation of cavities in gypsum horizons. In fact, for gypsum with interstitial porosity, the release of grains and their transport by the flow (suffusion phenomenon) could very strongly increase the growth of the cavity compared to taking into account only the dissolution.&lt;/p&gt;&lt;p&gt;A variety of experimental protocols have been developed to quantify the parameters controlling the studied phenomena. Rotating disk and batch experiments are employed to determine the kinetic rate model parameters of different varieties of natural gypsum with different porosity and insoluble contents &amp;#160;following the empirical rate expression derived from mixed kinetic theory. To get results more representative of in-situ conditions, they are adjusted according to the specific roughness and texture of each sample. The impact of erosion and particle transport related to gypsum dissolution is determined by controlled leaching tests on external surfaces. It consists of immersing entirely a block of gypsum in a horizontal canal filled with water circulating at a low velocity (&amp;#8771; 10&lt;sup&gt;-4&lt;/sup&gt; to 10&lt;sup&gt;-3&lt;/sup&gt; m/s) so that the grains detached during dissolution are not carried out by the flow but collected in a container placed under the block. These grains are then observed microscopically and analyzed by X-ray diffraction to better determine their mineralogy. For each gypsum block tested, the particlar flux is found low composed mostly of insoluble grains with only few gypsum grains released. The distribution of insoluble at the interface has a large influence on the dissolution. When they are present as a form of thin layers, they create local reliefs, depending on their cohesion, which disturbs the flow and locally enhance the gypsum dissolution. When they are distributed at the boundary of gypsum grains, they serve as a coating which protects them and drastically slows down the dissolution kinetics.&amp;#160;&lt;/p&gt;