Giuseppe Etiope

Carbon degassing from the lithosphere

Abstract So far, the role of present-day Earth degassing in global C budget and climate effects has been focused to volcanic emissions. The non-volcanic escape of CO 2 –CH 4 from the upper mantle, from carbonate bearing rocks in the crust, from hydrocarbon accumulations and from surface deposits and processes is here discussed in detail. An inventory of recent available data is presented. For the first time, a so large quantity of data is considered altogether showing clearly that the geological flux of carbon was previously significantly underestimated. Several lines of evidence show that non-volcanic C fluxes in «colder» environments are much greater than generally assumed. Local and regional data suggest that metamorphic decarbonation, hydrocarbon leakage and mud volcanoes could be significant CO 2 –CH 4 sources at global scale. Moreover, extensive surface gas-geochemical observations, including soil–atmosphere flux investigations, open the possibility that ecosystems controlled by biogenic activity (soil, permafrost, seawater) can host important components of endogenous C gas (geogas), even in the absence of surface gas manifestations. This would imply the existence of a geological diffuse, background emission over large areas of our planet. New theories concerning the occurrence of pervasive geogas and lithospheric processes of C-gas production («lithospheric loss in rigidity») can be taken as novel reference and rationale for re-evaluating geological sources of CO 2 and CH 4 , and an important endeavour and work prospect for the years to come. Our survey shows that it is still very hard to arrive at a meaningful estimate of the lithospheric non-volcanic degassing into the atmosphere. Orders of 10 2 –10 3 Mt CO 2 /year can be provisionally considered. Assuming as lower limit for a global subaerial volcanic degassing 300 Mt/year, the lithosphere may emit directly into the atmosphere at least 600 Mt CO 2 /year (about 10% of the C source due to deforestation and land-use exchange), an estimate we still consider conservative. It is likely that temporal variations of lithosphere degassing, at Quaternary and secular scale, may influence the atmospheric C budget. The present-day lithosphere degassing would seem higher than the value considered to balance at Ma time-scale the CO 2 uptake due to silicate weathering.

Upward revision of global fossil fuel methane emissions based on isotope database

Methane has the second-largest global radiative forcing impact of anthropogenic greenhouse gases after carbon dioxide, but our understanding of the global atmospheric methane budget is incomplete. The global fossil fuel industry (production and usage of natural gas, oil and coal) is thought to contribute 15 to 22 per cent of methane emissions to the total atmospheric methane budget. However, questions remain regarding methane emission trends as a result of fossil fuel industrial activity and the contribution to total methane emissions of sources from the fossil fuel industry and from natural geological seepage, which are often co-located. Here we re-evaluate the global methane budget and the contribution of the fossil fuel industry to methane emissions based on long-term global methane and methane carbon isotope records. We compile the largest isotopic methane source signature database so far, including fossil fuel, microbial and biomass-burning methane emission sources. We find that total fossil fuel methane emissions (fossil fuel industry plus natural geological seepage) are not increasing over time, but are 60 to 110 per cent greater than current estimates owing to large revisions in isotope source signatures. We show that this is consistent with the observed global latitudinal methane gradient. After accounting for natural geological methane seepage, we find that methane emissions from natural gas, oil and coal production and their usage are 20 to 60 per cent greater than inventories. Our findings imply a greater potential for the fossil fuel industry to mitigate anthropogenic climate forcing, but we also find that methane emissions from natural gas as a fraction of production have declined from approximately 8 per cent to approximately 2 per cent over the past three decades.

The detection of concealed faults in the Ofanto Basin using the correlation between soil-gas fracture surveys

Abstract An integrated geochemical, morphological and structural analysis was applied to a basin filled with clayey sediments in southern Italy (Ofanto Valley) to delineate tectonic features. More than 100 soil-gas samples were collected and analysed for CO2, Rn and He, and the resulting distribution was compared with the location and orientation of field-observed brittle deformations (faults and fractures), and air-photo interpreted morphotectonic features. The results show that the highest helium, radon and CO2 values occur preferentially along elongated zones similar to the most representative trends obtained by geomorphological and mesostructural analyses, i.e. anti-Apennine, Apennine and, secondarily, N–S orientations. Furthermore, the development of geostatistical techniques has allowed the semi-quantitative evaluation of the anisotropic soil-gas distribution. The gas-distribution pattern is considered to result from the combination of the anisotropic distribution of fracture traces and the randomly distributed background field. The correspondence between soil-gas distribution and geomorphological/mesostructural features, as well as the results from the geostatistical analysis, suggest that gas leakage towards the surface is controlled by the same structural pattern which also created some morphological features. This technique has been shown to be a useful tool for neotectonic studies; this is especially true in basins filled with clayey sediments, as soil gas is even able to define the leakage of deep-seated gases along tectonic discontinuities which have no surface expression.

Evidence for radon transport by carrier gas through faulted clays in Italy

Extensive soil-gas surveys in sedimentary basins in Italy were performed to study the potential of some naturally occurring gases as indicators for concealed fracture zones, hydrocarbon and geothermal fluids. One conclusive result is a positive correlation between anomalously high values of radon and carbon dioxide in the soil-air over faults. The correlation coefficient for 1173 gas samples is 0.41. Statistically derived contourlines of Rn and CO2 anomalies show similar locations, shapes and directions. Fairly good Rn−CO2 coupling evidence appears even on a point-to-point analysis. Furthermore, it was recognized that the highest Rn values are in contrast to the low Ra content of the underlying clayey rocks and that conventional Rn transportation mechanisms seem to be inadequate for the clay sequences. All these facts strongly suggest that Rn is transported from the subsoil, through fault-linked pathways, by carrier gases of which CO2 could be one of the major components. The theory of geogas microbubbles is a possible explanation of the observed results. The carrier effect of ascending microbubbles can explain both the origin of soil-gas Rn anomaly and the Rn−CO2 coupling phenomenon.

Methane emission from mud volcanoes in eastern Azerbaijan

Methane (CH4) flux to the atmosphere was measured from gas vents and, for the first time, from soil microseepage at four quiescent mud volcanoes and one “everlasting fire” in eastern Azerbaijan. Mud volcanoes show different activity of venting craters, gryphons, and bubbling pools, with CH4 fluxes ranging from less than one to hundreds of tons per year. Microseepage CH4 flux is generally on the order of hundreds of milligrams per square meter per day, even far away from the active centers. The CH4 flux near the everlasting fires (on the order of 105 mg·m−2·d−1) represents the highest natural CH4 emission from soil ever measured. The specific CH4 flux to the atmosphere, between 102 and 103 t·km−2·yr−1, was similar to specific flux from other mud volcanoes in Europe. At least 1400 tons of CH4 per year are released from the investigated areas. It is conservatively estimated that all onshore mud volcanoes of Azerbaijan, during quiescent activity, may still emit ∼0.3–0.9 × 106 t of CH4 per year into the atmosphere. The new data fill a significant gap in the worldwide data set and confirm the importance of geologic sources of greenhouse CH4, although they are not yet considered in the climate-study budgets of atmospheric CH4 sources and sinks.

Subsoil CO2 and CH4 and their advective transfer from faulted grassland to the atmosphere

Measurements of CO2 and CH4 in 3 m deep groundwater, soil-gas, and soil-atmosphere fluxes were completed in two grasslands in central Italy having the same soil conditions but different subsoil fault-linked secondary permeability. Unfaulted grassland displays gas-phase equilibrium between soil-air and groundwater, typical soil-gas diffusion profiles, and diffusive soil to atmosphere gas transfer; on the basis of oxygen depletion assessment, assuming a ratio of 1:1 between biogenic O2 consumption and CO2 production, the measured soil CO2 concentrations are consistent with a normal production in the soil by biologic activity. The faulted grassland, instead, has higher CO2 and CH4 concentrations (up to 6% and 10 ppmv in soil-air) and flux (1.2 mL m−2 s−1 and 1.3 μL m−2 s−1) resulting from a combination of soil biologic and endogenous components, with evidence of gas transfer from the saturated to the unsaturated zone, and advective gas transfer from soil to the atmosphere. The extra-soil source in the faulted zone, which is about 0.3–4 times the background soil biologic production, is likely related to migrating crustal gas. Whatever the C gas origin and depth may be (biogenic or abiogenic, shallow or deep), the present results demonstrate that the subsoil-derived component occurring in the soil in areas with active tectonics cannot be ignored a priori in the assessment of the C terrestrial sources. In particular, the assumption that dryland soils are sinks for methane, owing to methanotrophic consumption, may be not true in areas affected by active and gas-bearing faults. Accordingly, it would be very important to assess at global scale the actual role in the carbon dioxide and methane cycle of soils within the active tectonic bounds.

Structural pattern and CO2–CH4 degassing of Ustica Island, Southern Tyrrhenian basin

Abstract Brittle tectonics and ground degassing, including fracture-field, soil–gas and exhalation flux analyses of CO 2 and CH 4 , were studied at Ustica Island, a Pleistocene volcanic complex in the Southern Tyrrhenian Sea. The mesoscopic fracture pattern perfectly fits an E–W-trending left-lateral strike–slip master fault, in agreement with the main morpho-structural submarine alignment including Ustica Island and Anchise Seamount. Along the SW–NE high-angle normal Arso Fault, geological evidence of reactivation with different kinematics (left- to right-lateral displacements) was recognised. Major CO 2 and CH 4 degassing (with fluxes up to 93,750 and 20 t km −2 a −1 , respectively, and soil–gas concentrations of 11.3% and 5.7 ppm) occur over the Arso Fault. Although this fault is mapped just in the SW sector of the island, soil–gas CO 2 anomalies point out its clear continuation up to the NE margin of the island. These data, together with those of previous geophysical and geochemical results from off-shore Ustica, suggest that the Arso Fault is the local evidence of a more important active, gas-bearing structure. This tectonic feature is interpreted as a reactivation of a preexistent SW–NE trend, inherited as a second-order structure of the E–W deep shear zone. The reactivation is related to the interplay among different structures of the Southern Tyrrhenian basin.

Methane and hydrogen sulfide seepage in the northwest Peloponnesus petroliferous basin (Greece): Origin and geohazard

Gas seepages along the Ionian coast of the northwestern Peloponnesus (Greece), at Killini, Katakolo, and Kaiafas reflect deep hydrocarbon-generation processes and represent a real hazard for humans and buildings. Methane microseepage, gas concentration in offshore and onshore vents, and gas dissolved in water springs, including the isotopic analysis of methane, have shown that the seeps are caused by thermogenic methane that had accumulated in Mesozoic limestone and had migrated upward through faults, or zones of weakness, induced by salt diapirism. A link between local seismicity and salt tectonics is suggested by the analyses of hypocenter distribution. Methane acts as a carrier gas for hydrogen sulfide produced by thermal sulfate reduction and/or thermal decomposition of sulfur compounds in kerogen or oil. Methane seeps in potentially explosive amounts, and hydrogen sulfide is over the levels necessary to induce toxicological diseases and lethal effects.

Mud volcanoes discovered offshore Sicily

Abstract Numerous active mud volcanoes have been recognized for the first time from seismic reflection and sidescan surveys carried out in 2002 over the Hyblean–Malta Plateau, 10 miles from the southern coast of Sicily (Southern Italy, Mediterranean Sea), along faults adjacent to the Scicli fracture zone. Our geophysical data show clearly the presence of several tens of mud volcanoes at water depths between 70 and 170 m. They have scales of order 10 m in diameter and several meters in height. Gas apparently vents from most of the mud volcanoes and is detected acoustically in the sediments around the cones to distances of about 50 m. This discovery represents a new important step in the study of mud volcanism distribution and highlights the potential of the Sicilian shelf as a hydrocarbon-prone area and a natural source of greenhouse gases.

Laboratory simulation of geogas microbubble flow

Preliminary laboratory tests provided first data on the behavior of gas microbubbles through porous media in the framework of the geogas theory. Under experimented conditions with laboratory equipment arranged for pressure controlled gas-tracer injection and sampling, gas microbubbles moved up to ten times faster than singlephase flow in dry media under the same injection pressure. Microbubbles were determined to be very sensitive to changes in injection pressure and their terminal velocity seems to be described with good approximation by the Stokes formula. The capability of microbubbles to pick up and transport upward for short distances solid ultra-small particles (metallic and radionuclide compounds) has been proved. Results are consistent with a time-dependent process linked to the transport properties of microbubbles (e.g., flotation), such as that reported by some authors.