Gaseous and liquid hydrocarbons are seeping from sandy sea bottom ~10 m deep, about 2.4 km NNE of Civitanova Marche harbour, in central Adriatic Sea (Italy). We investigated the origin of the gas, the presence in shallow sediments of a wide range of aromatic and aliphatic hydrocarbons and trace metals, and the stable carbon and oxygen isotope composition of benthic foraminifera. Molecular and isotopic composition of gas bubbles showed that the CH4-rich gas is thermogenic from low maturity rocks (13CCH4~ -55 ‰ VPDB; 2HCH4~ -280 ‰ VSMOW; C1/(C2+C3) < 100), likely belonging to the Emma-Scaglia (carbonate source rocks) Petroleum System (Upper Trias to Paleocene), and stored in a biodegraded hydrocarbon pool. Fluid seepage might be due to a local fracture zone corresponding to the intersection of NNW-SSE thrust faults with a NE-SW regional transversal deformation belt. Compared to other shallow marine seeps in Europe, the amount of methane released into the atmosphere is negligible (102-103 kg of CH4 per year); but the seep releases also ethane and propane (103-104 L per year), which are photochemical pollutants and are not emitted by microbial gas seeps. Relative to a reference site one nautical mile far from the seep, the seabed sediments show higher concentrations of various classes of chemicals, such as benzene, toluene and ethylbenzene, semivolatile and non volatile aliphatic hydrocarbons (C10-C40), and phenols (2-methylphenol and 2,4-dichlorophenol). These compounds likely derive from the oil seepage. Trace metals (arsenic, barium, cadmium, chromium, copper, iron, manganese, nickel, lead, vanadium, zinc, mercury) did not exhibit significant differences among the samples, and concentrations were typical of uncontaminated and shallow coastal areas. Finally, we provided the first data on foraminifera associated to thermogenic hydrocarbons. No endemic foraminifera species or authigenic carbonates occur in the sediments. Carbon isotopic composition of Quinqueloculina padana where oil slick prevails is less variable than in the gas bubbling site. However, thermogenic methane and oil do not apparently decrease the 13C value of foraminifera carbonate shell.

A thermogenic hydrocarbon seep in shallow Adriatic Sea (Italy): gas origin, sediment contamination and benthic foraminifera.

FATTORINI, DANIELE;REGOLI, Francesco;
2014-01-01

Abstract

Gaseous and liquid hydrocarbons are seeping from sandy sea bottom ~10 m deep, about 2.4 km NNE of Civitanova Marche harbour, in central Adriatic Sea (Italy). We investigated the origin of the gas, the presence in shallow sediments of a wide range of aromatic and aliphatic hydrocarbons and trace metals, and the stable carbon and oxygen isotope composition of benthic foraminifera. Molecular and isotopic composition of gas bubbles showed that the CH4-rich gas is thermogenic from low maturity rocks (13CCH4~ -55 ‰ VPDB; 2HCH4~ -280 ‰ VSMOW; C1/(C2+C3) < 100), likely belonging to the Emma-Scaglia (carbonate source rocks) Petroleum System (Upper Trias to Paleocene), and stored in a biodegraded hydrocarbon pool. Fluid seepage might be due to a local fracture zone corresponding to the intersection of NNW-SSE thrust faults with a NE-SW regional transversal deformation belt. Compared to other shallow marine seeps in Europe, the amount of methane released into the atmosphere is negligible (102-103 kg of CH4 per year); but the seep releases also ethane and propane (103-104 L per year), which are photochemical pollutants and are not emitted by microbial gas seeps. Relative to a reference site one nautical mile far from the seep, the seabed sediments show higher concentrations of various classes of chemicals, such as benzene, toluene and ethylbenzene, semivolatile and non volatile aliphatic hydrocarbons (C10-C40), and phenols (2-methylphenol and 2,4-dichlorophenol). These compounds likely derive from the oil seepage. Trace metals (arsenic, barium, cadmium, chromium, copper, iron, manganese, nickel, lead, vanadium, zinc, mercury) did not exhibit significant differences among the samples, and concentrations were typical of uncontaminated and shallow coastal areas. Finally, we provided the first data on foraminifera associated to thermogenic hydrocarbons. No endemic foraminifera species or authigenic carbonates occur in the sediments. Carbon isotopic composition of Quinqueloculina padana where oil slick prevails is less variable than in the gas bubbling site. However, thermogenic methane and oil do not apparently decrease the 13C value of foraminifera carbonate shell.
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/11566/175105
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