Thermogenic hydrocarbons from the offshore Calypso hydrothermal field, Bay of Plenty, New Zealand

Abstract

Hydrothermal gases from shallow seafloor vents in the Bay of Plenty, New Zealand contain CO 2, CH 4, and the higher gaseous hydrocarbons up to i-, n-C 4H 10. The gases are similar to those discharged at fumaroles on the nearby White Island. Carbon isotope compositions for CO 2 fall between −3.4‰ and −5.5‰ PDB and reflect a shallow magmatic carbon source. The δ 13C values of CH 4 range from −24.6‰ to −28.9‰ PDB and the δD values vary between −122‰ and −135‰ SMOW. The CH 4 isotope values and the presence of the higher hydrocarbon compounds such as C 2H 6 and C 3H 8 with δ 13C values near −20‰ PDB suggest hydrocarbon production by high-temperature maturation of sedimentary organic matter and mixing (∼1:1) of the thermogenic CH 4 with abiogenic CH 4. Long-chained hydrocarbons occur in dredged samples close to the active vents. Their n-alkane distribution has a high to moderate odd–even predominance and an extensive hopane series, indicative of higher land-plant waxes and prokaryotic membranes in the source. Substantial amounts of unsubstituted polynuclear aromatic hydrocarbons (PAH) mark the transition from aliphatic- to aromatic-dominated bitumens, consistent with extensive source maturation resulting from thermal stress. The bitumens are interpreted as pyrolysates derived from buried near-coastal vegetation and terrestrial detritus under various thermal regimes, mixed with immature seafloor organic matter.