Seeping hydrocarbons and related carbonate mineralisations in sediments south of Lihir Island (New Ireland fore arc basin, Papua New Guinea)

Hydrocarbon gases have been sampled from both cold-seeping and heat-venting areas in the New Ireland fore arc basin in the vicinity of Lihir Island. Highest concentrations of up to 10 μl/l CH4 with a δ13CCH4 value of −54.9‰ PDB have been measured in the deep ocean water within a long and narrow deep sea basin located between Edison Seamount and an uplifted structure named “Mussel Cliff.” Surface sediments of the seep area were covered with chemoautotrophic deep sea fauna such as Calyptogena species and tube worms. Large authigenic calcite concretions occur in the sediments between 50- and 200-cm sediment depth. The carbon isotopes of the carbonates in the concretions range from −15‰ to −40‰ PDB indicating a mixture of two CO2 sources: normal marine-inorganic carbon fixed in biogenic shells and CO2 from anaerobe bacterial oxidation processes of methane. Accordingly, 14C-AMS dating suggests that authigenic calcite mineralisation incorporated relatively “young” carbon from methane oxidation. trast, the C1/C2 ratio of 234 and the δ13CCH4 value of −24.1‰ PDB in the hot hydrothermal vent of Lihir Harbour indicates a mixture of a major abiogenic carbon source for methane formation related to magmatism associated with Lihir Volcano. served variable fluid characteristics within only 20-km distance between hot hydrothermal-venting and the methane-seeping deep sea area indicates highly variable heat flow situations and/or sediment distributions which control the gas geochemical characteristics in the New Ireland fore arc basin.