Tiber delta CO₂‐CH₄ degassing: A possible hybrid, tectonically active Sediment‐Hosted Geothermal System near Rome

Abstract

Fiumicino town in the Tiber River delta, near Rome International Airport (Italy), is historically affected by large amounts of carbon dioxide (CO2) in the ground and gas eruptions triggered by shallow drilling. While it is known that CO2 originates from carbonate thermometamorphism and/or mantle degassing, the origin of methane (CH4) associated with CO2 is uncertain and the outgassing spatial distribution is unknown. Combining isotope gas geochemistry, soil gas, and structural‐stratigraphic analyses, we provide evidence for a hybrid fluid source system, classifiable as Sediment‐Hosted Geothermal System (SHGS), where biotic CH4 from sedimentary rocks is carried by deep geothermic CO2 through active segments of a half‐graben. Molecular and isotopic composition of CH4 and concentration of heavier alkanes (ethane and propane), obtained from gas vents and soil gas throughout the delta area, reveal that thermogenic CH4 (up to 3.7 vol% in soil gas; δ13CCH4: −37 to −40‰ VPDB‐Vienna Peedee Belemnite, and δ2HCH4: −162 to −203‰ VSMOW ‐ Vienna Standard Mean Ocean Water in gas vents) prevails over possible microbial and abiotic components. The hydrocarbons likely result from known Meso‐Cenozoic petroleum systems of the Latium Tyrrhenian coast. Overmaturation of source rocks or molecular fractionation induced by gas migration are likely responsible for increased C1/C2+ ratios. CO2 and CH4 soil gas anomalies are scattered along NW‐SE and W‐E alignments, which, based on borehole, geomorphologic, and structural‐stratigraphic analyses, coincide with active faults of a half‐graben that seems to have controlled the recent evolution of the Tiber delta. This SHGS can be a source of considerable greenhouse gas emissions to the atmosphere and hazards for humans and buildings.

Concentric Structures and Hydrothermal Venting in the Western Desert, Egypt

“…Active hydrothermal vent systems, driven by the same processes as described above, are also currently active on Earth and are termed sediment-hosted hydrothermal systems (Mazzini and Etiope, 2017;Procesi et al, 2019). Currently, active sites are documented in the Guaymas Basin Rift Zone-Gulf of California (Pacific Ocean), in the Salton Sea geothermal field (California), within the Tiber Delta, at Fiumicino (central Italy), and in the Songliao Basin (northern China) (e.g., Helgeson, 1968;Welhan and Lupton, 1987;Mazzini et al, 2011;Berndt et al, 2016;Ciotoli et al, 2016;Shuai et al, 2018;Procesi et al, 2019). Today the largest active vent system is the Indonesian Lusi eruption (active since May 2006) that has been targeted by numerous multidisciplinary studies to fill the knowledge gap between modern and ancient hybrid systems (e.g., Mazzini et al, 2012;Fallahi et al, 2017;Lupi et al, 2018;Malvoisin et al, 2018;Mazzini, 2018;Miller and Mazzini, 2018).…”

Coeval Uplift and Subsidence Reveal Magma Recharging Near Rome (Italy)

“…CO 2 discharges occur at CA and concentrated on the south‐western flank of the volcano, in an area extending from Lake Albano to the Tyrrhenian Sea coast down to the Fiumicino area, at the Rome International Airport (Chiodini et al, ; Chiodini & Frondini, ; Ciotoli et al, ). The provenance and implications of these CO 2 emissions are debated and attributed to a mixing between (a) a degassing magma chamber or a mantle source, and (b) a crustal source attributed to thermal metamorphism (decarbonation reactions) of the basement limestones (Carapezza & Tarchini, ).…”

“…Airport (Chiodini et al, 2012;Chiodini & Frondini, 2001;Ciotoli et al, 2016). The provenance and implications of these CO 2 emissions are debated and attributed to a mixing between (a) a degassing magma chamber or a mantle source, and (b) a crustal source attributed to thermal metamorphism (decarbonation reactions) of the basement limestones (Carapezza & Tarchini, 2007).…”

Mud volcanism: An updated review

“…Some basins may host hot dry rock systems or CO 2 -rich gas-phase systems as those described in e.g. Ciotoli et al (2016).…”