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

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 of a wide range of aromatic and aliphatic hydrocarbons and trace metals in shallow sediments, as well as the stable carbon and oxygen isotope composition of benthic foraminifera. In absence of detailed seismic images and subsurface geochemical data, we tried to estimate the source rock type and maturity based only on seep gas geochemistry. Molecular and isotopic composition of gas bubbles showed that the CH4-rich gas is thermogenic (d13CCH4 ~ 55‰; d2HCH4 ~ 280‰; C1/(C2 þ C3) < 100) with isotopic features that are compatible with low maturity source rocks belonging to the Emma-Scaglia (carbonate source rocks) Petroleum System (Upper Trias to Paleocene). Gas could then be stored in a biodegraded hydrocarbon pool, as suggested by 13C enrichment in propane (d13C3: 24‰) and CO2 (d13CCO2 : þ12‰). Fluid seepage might be due to a local fracture zone corresponding to the intersection of NNWeSSE thrust faults with a NEeSW regional transversal deformation belt. Compared to other shallow marine seeps in Europe, the amount of methane released into the atmosphere is negligible (102e103 kg of CH4 per year); but the seep also releases ethane and propane (103e104 L per year), which are photochemical pollutants and are not emitted by microbial gas seeps. Compared 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 (C10eC40), and phenols (2-methylphenol and 2,4-dichlorophenol). These compounds likely derive from the oil seepage. The sediments at the seepage site and those at the reference site have similar concentrations of trace metals (arsenic, barium, cadmium, chromium, copper, iron, manganese, nickel, lead, vanadium, zinc, mercury), 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 d13C value of foraminifera carbonate shell.