Hydrocarbon sources of cold seeps off the Vesterålen coast, northern Norway

We investigated active methane seeps in a water depth of 200m in the Hola area off the coast of Vesterålen, northern Norway, to assess (1) hydrocarbon sources, (2) migration pathways and (3) the influence of hydrocarbon seepage on sediment pore water and water column chemistry. The seepage area is characterised by the presence of gas flares in the water column as revealed by hydro acoustic surveys and elevated methane concentrations of up to 42nM ca. 5m above the seafloor. Pore water analyses of three gravity cores from the seepage area show varying depths of the sulphate–methane-transition zone (SMTZ) between 80cm and >250cm indicating spatially heterogeneous methane ascent. The isotopic composition of methane (δ13C from −40‰ to −63‰ and δ2H from −191‰ to −225‰) and δ13C depth profiles of methane and dissolved inorganic carbon show that the hydrocarbons are predominantly of thermogenic origin, consistent with δ13C values of C2 to C4 hydrocarbons. Isotope data also indicate considerable biodegradation of propane. Seismic profiles from the study area reveal major faults and steeply dipping unconformities between the basement and overlying Mesozoic sedimentary rocks. We propose that these act as migration pathways for the hydrocarbons from late Jurassic to early Cretaceous source rocks.