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

• Published in: Chemical Geology
• Main Authors: Botz , R., Wehner , H., Schmitt, M., Worthington , T., Schmidt, Mark, Stoffers , P.
• Format: Article in Journal/Newspaper
• Language: English
• Published: Elsevier 2002
• Subjects: Bay of Plenty; New Zealand; White Island
• Online Access: https://oceanrep.geomar.de/id/eprint/11643/; https://oceanrep.geomar.de/id/eprint/11643/1/Botz%20et%20al%202002.pdf; https://doi.org/10.1016/S0009-2541(01)00418-1

Hydrothermal gases from shallow seafloor vents in the Bay of Plenty, New Zealand contain CO2, CH4, and the higher gaseous hydrocarbons up to i-, n-C4H10. The gases are similar to those discharged at fumaroles on the nearby White Island. Carbon isotope compositions for CO2 fall between −3.4‰ and −5.5‰ PDB and reflect a shallow magmatic carbon source. The δ13C values of CH4 range from −24.6‰ to −28.9‰ PDB and the δD values vary between −122‰ and −135‰ SMOW. The CH4 isotope values and the presence of the higher hydrocarbon compounds such as C2H6 and C3H8 with δ13C values near −20‰ PDB suggest hydrocarbon production by high-temperature maturation of sedimentary organic matter and mixing (∼1:1) of the thermogenic CH4 with abiogenic CH4. 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.