09:00 〜 09:15
[MIS22-01] Gas seeps and methane-derived carbonates in the Northern Adriatic Sea (Italy)
★Invited Papers
キーワード：Methane seepage, Methane-derived carbonates, Northern Adriatic Sea, Geosites
In the northern Adriatic Sea, gas-related seabed and sub-seabed features, together with gas seeps are widespread. The most evident signatures of gas occurrence both throughout the Plio-Quaternary sedimentary succession and in the uppermost Pleistocene-to-present sediments are revealed by multichannel seismic data and CHIRP profiles, collected by the National Institute of Oceanography and Applied Geophysics (OGS) in 2009 and 2014. In multichannel seismic data, gas-related features appear as bright spots and sub vertical chimney-like features up to 2–3 km wide, and as horizontally distributed features. Focused gas accumulations in sub vertical pathways also affect the uppermost stratigraphic layers. A first estimate of the gas mean volumetric concentration was made using both multichannel and high resolution seismic data, confirming that the gas is both diffuse and concentrated in local accumulations, with gas contents ranging from 0.15 to 0.3%. At the sea floor, different types of gas-related features occur: a. white microbial mats likely composed of chemosynthetic sulfide-oxidizing proteobacteria Archaea and Beggiatoa; b. small-scale mud volcanoes, about 2/3 cm high and 4/5 cm in diameter; pockmarks, up to 200 m in diameter and 1 m deep. Locally, the gas can escape into the water column as shown by numerous gas bubble streams, which are also imaged as acoustic flares on echograms. Gas samples collected from five seepage sites consist mainly of methane (ca. 53% to ca. 90%). Radiocarbon dating revealed an apparent age of the organic material source for the microbial gas that is between approximately 32,000 and 34,000 years B.P.
These gas seeps appear to be locally associated with distinct rock outcrops, which represent the most peculiar features of the northern Adriatic Sea. They are represented by hundreds of submarine rock outcrops, irregularly distributed and known by various dialectal names, e.g., tegnùe, trezze and grebeni. These rocky formations lie at varying water depths (between approximately -8 and -22 m) and distances from the coast (between 2 and 17 km), rise up to several meters above the unconsolidated sandy-silty sea floor and host massive bio-concretion buildups, which are excellent hard substrates for a variety of calcareous benthic constituents, including bryozoans, mollusks, serpulid polychaetes, scleractinia, and calcareous algae. The northern Adriatic Sea outcrops are unique geosites that attract a global scientific interest, as testified by several international initiatives (i.e., TRECORALA-2012-2014, TRETAMARA 2020–2022, ADRIREEF 2018–2021, and the recent most, ongoing TRECap 2023–2025 EU funded projects) devoted to valorizing them and promoting their sustainable management. Various hypotheses have been invoked concerning the origin of these features, which have been intensively studied since the eighteenth century. The outcomes of all geological, geomorphological, geophysical, mineralogical analyses carried out on ca. 50 rock outcrops corroborate the hypothesis concerning the methane-related origin of most rock outcrops, which form in different depositional environments but are characterized by the same cementation process related to the precipitation of methane-derived carbonates. This conceptual model reconciles the conundrum of the different models related to the genesis of the northern Adriatic Sea outcrops by highlighting the key role of methane seepage in their formation. This outcome also highlights that since methane is one of the most powerful greenhouse gases, the Northern Adriatic Sea is thus a unique natural laboratory for deciphering the dynamics of methane natural degassing, consumption, transport and subsequent, eventual emission to the atmosphere.
These gas seeps appear to be locally associated with distinct rock outcrops, which represent the most peculiar features of the northern Adriatic Sea. They are represented by hundreds of submarine rock outcrops, irregularly distributed and known by various dialectal names, e.g., tegnùe, trezze and grebeni. These rocky formations lie at varying water depths (between approximately -8 and -22 m) and distances from the coast (between 2 and 17 km), rise up to several meters above the unconsolidated sandy-silty sea floor and host massive bio-concretion buildups, which are excellent hard substrates for a variety of calcareous benthic constituents, including bryozoans, mollusks, serpulid polychaetes, scleractinia, and calcareous algae. The northern Adriatic Sea outcrops are unique geosites that attract a global scientific interest, as testified by several international initiatives (i.e., TRECORALA-2012-2014, TRETAMARA 2020–2022, ADRIREEF 2018–2021, and the recent most, ongoing TRECap 2023–2025 EU funded projects) devoted to valorizing them and promoting their sustainable management. Various hypotheses have been invoked concerning the origin of these features, which have been intensively studied since the eighteenth century. The outcomes of all geological, geomorphological, geophysical, mineralogical analyses carried out on ca. 50 rock outcrops corroborate the hypothesis concerning the methane-related origin of most rock outcrops, which form in different depositional environments but are characterized by the same cementation process related to the precipitation of methane-derived carbonates. This conceptual model reconciles the conundrum of the different models related to the genesis of the northern Adriatic Sea outcrops by highlighting the key role of methane seepage in their formation. This outcome also highlights that since methane is one of the most powerful greenhouse gases, the Northern Adriatic Sea is thus a unique natural laboratory for deciphering the dynamics of methane natural degassing, consumption, transport and subsequent, eventual emission to the atmosphere.