Publication Date:
2017-04-04
Description:
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.
Description:
Published
Description:
283-293
Description:
6A. Monitoraggio ambientale, sicurezza e territorio
Description:
JCR Journal
Description:
restricted
Keywords:
Marine seep
;
Thermogenic gas
;
Adriatic Sea
;
Chemical contaminants
;
Foraminifera
;
04. Solid Earth::04.08. Volcanology::04.08.01. Gases
Repository Name:
Istituto Nazionale di Geofisica e Vulcanologia (INGV)
Type:
article
