Gabriele Bicocchi

Biogeochemical processes involving dissolved CO2 and CH4 at Albano, Averno, and Monticchio meromictic volcanic lakes (Central–Southern Italy)

This paper focuses on the chemical and isotopic features of dissolved gases (CH4 and CO2) from four meromictic lakes hosted in volcanic systems of Central–Southern Italy: Lake Albano (Alban Hills), Lake Averno (Phlegrean Fields), and Monticchio Grande and Piccolo lakes (Mt. Vulture). Deep waters in these lakes are characterized by the presence of a significant reservoir of extra-atmospheric dissolved gases mainly consisting of CH4 and CO2. The δ13C-CH4 and δD-CH4 values of dissolved gas samples from the maximum depths of the investigated lakes (from −66.8 to −55.6 ‰ V-PDB and from −279 to −195 ‰ V-SMOW, respectively) suggest that CH4 is mainly produced by microbial activity. The δ13C-CO2 values of Lake Grande, Lake Piccolo, and Lake Albano (ranging from −5.8 to −0.4 ‰ V-PDB) indicate a significant CO2 contribution from sublacustrine vents originating from (1) mantle degassing and (2) thermometamorphic reactions involving limestone, i.e., the same CO2 source feeding the regional thermal and cold CO2-rich fluid emissions. In contrast, the relatively low δ13C-CO2 values (from −13.4 to −8.2 ‰ V-PDB) of Lake Averno indicate a prevalent organic CO2. Chemical and isotopic compositions of dissolved CO2 and CH4 at different depths are mainly depending on (1) CO2 inputs from external sources (hydrothermal and/or anthropogenic); (2) CO2–CH4 isotopic exchange; and (3) methanogenic and methanotrophic activity. In the epilimnion, vertical water mixing, free oxygen availability, and photosynthesis cause the dramatic decrease of both CO2 and CH4 concentrations. In the hypolimnion, where the δ13C-CO2 values progressively increase with depth and the δ13C-CH4 values show an opposite trend, biogenic CO2 production from CH4 using different electron donor species, such as sulfate, tend to counteract the methanogenesis process whose efficiency achieves its climax at the water–bottom sediment interface. Theoretical values, calculated on the basis of δ13C-CO2 values, and measured δ13CTDIC values are not consistent, indicating that CO2 and the main carbon-bearing ion species (HCO3−) are not in isotopic equilibrium, likely due to the fast kinetics of biochemical processes involving both CO2 and CH4. This study demonstrates that the vertical patterns of the CO2/CH4 ratio and of δ13C-CO2 and δ13C-CH4 are to be regarded as promising tools to detect perturbations, related to different causes, such as changes in the CO2 input from sublacustrine springs, that may affect aerobic and anaerobic layers of meromictic volcanic lakes.

Geosphere-Biosphere Interactions in Bio-Activity Volcanic Lakes: Evidences from Hule and Rìo Cuarto (Costa Rica)

Hule and Río Cuarto are maar lakes located 11 and 18 km N of Poás volcano along a 27 km long fracture zone, in the Central Volcanic Range of Costa Rica. Both lakes are characterized by a stable thermic and chemical stratification and recently they were affected by fish killing events likely related to the uprising of deep anoxic waters to the surface caused by rollover phenomena. The vertical profiles of temperature, pH, redox potential, chemical and isotopic compositions of water and dissolved gases, as well as prokaryotic diversity estimated by DNA fingerprinting and massive 16S rRNA pyrosequencing along the water column of the two lakes, have highlighted that different bio-geochemical processes occur in these meromictic lakes. Although the two lakes host different bacterial and archaeal phylogenetic groups, water and gas chemistry in both lakes is controlled by the same prokaryotic functions, especially regarding the CO2-CH4 cycle. Addition of hydrothermal CO2 through the bottom of the lakes plays a fundamental priming role in developing a stable water stratification and fuelling anoxic bacterial and archaeal populations. Methanogens and methane oxidizers as well as autotrophic and heterotrophic aerobic bacteria responsible of organic carbon recycling resulted to be stratified with depth and strictly related to the chemical-physical conditions and availability of free oxygen, affecting both the CO2 and CH4 chemical concentrations and their isotopic compositions along the water column. Hule and Río Cuarto lakes were demonstrated to contain a CO2 (CH4, N2)-rich gas reservoir mainly controlled by the interactions occurring between geosphere and biosphere. Thus, we introduced the term of bio-activity volcanic lakes to distinguish these lakes, which have analogues worldwide (e.g. Kivu: D.R.C.-Rwanda; Albano, Monticchio and Averno: Italy; Pavin: France) from volcanic lakes only characterized by geogenic CO2 reservoir such as Nyos and Monoun (Cameroon).

Soil characterization for shallow landslides modeling: a case study in the Northern Apennines (Central Italy)

In this paper, we present preliminary results of the IPL project No. 198 “Multi-scale rainfall triggering models for Early Warning of Landslides (MUSE).” In particular, we perform an assessment of the geotechnical and hydrological parameters affecting the occurrence of landslides. The aim of this study is to improve the reliability of a physically based model high resolution slope stability simulator (HIRESSS) for the forecasting of shallow landslides. The model and the soil characterization have been tested in Northern Tuscany (Italy), along the Apennine chain, an area that is historically affected by shallow landslides. In this area, the main geotechnical and hydrological parameters controlling the shear strength and permeability of soils have been determined by in situ measurements integrated by laboratory analyses. Soil properties have been statistically characterized to provide more refined input data for the slope stability model. Finally, we have tested the ability of the model to predict the occurrence of shallow landslides in response to an intense meteoric precipitation.

H/V Technique for the rapid detection of landslide slip surface(s): assessment of the optimized measurements spatial distribution

The investigation of landslides and slope deformation processes may require the integration of a wide range of data types, collected using different approaches, such as geomorphological, geotechnical and geophysical surveys. Among this latter category, seismic noise method can be used to detect and better understand the geometry of landslide slip surfaces. Indeed, a slip surface may generate evident contrasts in shear wave velocity due to changes in seismic impedance, generated by the different seismic velocity and density of materials at landslide boundaries. The H/V or Nakamura method allows to have a punctual information about the depth of the main impedance contrasts, thus, by performing a spatial interpolation of an adequate number of punctual depth measures, is possible to reliably estimate the depth and geometry of the slip surfaces with good accuracy. This study is focused on the relation between the number of the employed single-station seismic noise measurements and the goodness of the resulting, inferred, slip surface(s) for landslides. The final aim is to detect, if it exists, a threshold in the number of measurements beyond which the information obtained is redundant, since the variations in terms of morphology observed in the reconstructed impedance contrast surfaces become negligible. The proposed approach was validated at Castagnola Landslide (Liguria, Italy), where direct measures of the subsoil stratigraphy were available, then applied to another case study, i.e., the Roccalbegna Landslide (Tuscany, Italy), where no direct measurements, apart from those of the shallow layer geotechnical properties, were available. The experiments carried out are a proof-of-concept of the opportunities that this approach can offer.

Assessment of geotechnical and hydrological properties of hillslopes by means of on site and laboratory tests: a case study in Tuscany (Central Italy)

The aim of this study is to realize an in-depth characterization of geotechnical and hydrological properties of cover soils affected by landslides. In particular, an extensive geotechnical campaign, carried out by means of on site and laboratory test, was performed in correspondence of selected sites of the main soil types covering the most widespread and representative lithologies of the area test (arenaceous-marly flysch and calcareous-marly flysch). Area test (Tuscany region, Central Italy), located in correspondence of the northern Tuscan Apennine ridge and the southern hilly-mountainos sectors (Mount Amiata and Metalliferous hills), consists of alluvial plains surrounded and crossed by major mountain chains, having a relief that is dominated by hills used for agriculture. Measured geotechnical and hydrological parameters are necessary to carry out studies on the conditions of slope stability and calculate the safety factor. In situ measurements of geotechnical properties are employed to improve the assessment of the variation and uncertainty affecting these parameters.

Soil Characterization for Landslide Forecasting Models: A Case Study in the Northern Apennines (Central Italy)

Open image in new window In this work we perform an assessment of the geotechnical and hydrological parameters affecting the occurrence of landslides. The aim of this study is to improve the reliability of the physically-based model HIRESSS (HIgh REsolution Slope Stability Simulator), for the forecasting of shallow landslides. The model and the soil characterization has been tested in northern Tuscany, in Italy, along the Apennine chain, an area that is historically affected by shallow landslides. In the area selected, the main geotechnical and hydrological parameters controlling the shear strength and permeability of soils have been determined by in situ measurements integrated by laboratory analyses. Around 60 survey points have been analyzed. The data obtained have been studied in order to assess the relationships existing among the different parameters and the bedrock lithology. Soil properties have been then statistically characterized and used to define the input parameters in the physical model, with the final aim of testing the ability of the model to predict shallow landslide occurrence in response of an intense meteoric precipitation. The rainfall event selected dates back to October 2010 when an intense precipitation affected the area, triggering around 50 reported shallow landslides. The geotechnical and hydrological data collected allowed to generate input map of parameters for the HIRESSS and the simulations showed substantial improvements in the results compared to the use of literature parameters.