Using helium and other noble gases in ocean sediments to characterize active methane seepage off the coast of New Zealand

Recently developed analytical techniques to determine the abundances of noble gases in sediment pore water allow noble-gas concentrations and isotope ratios to be measured easily and routinely in lacustrine sediments. We applied these techniques for the first time to ocean sediments to investigate an active cold methane seepage system located in the South Pacific off the coast of the North Island of New Zealand using 3He/4He ratios determined in the sediment pore water. The results show that more 3He-rich fluids are released in the vicinity of the Pacific–Australian subduction zone than at the forearc stations located closer to the New Zealand coast. However, the He isotope signature in the sediment column indicates that only a minor part of the He emanating from deeper strata originates from a depleted mantle source. Hence, most He in the pore water is produced locally by the radioactive decay of U and Th in the sediment minerals or in the underlying crustal rocks. Such an occurrence of isotopically heavy crustal He also suggests that the source of the largest fraction of methane is a near-surface geochemical reservoir. This finding is in line with a previous δ13C study in the water column which concluded that the emanating methane is most likely of biological origin and is formed in the upper few meters of the sediment column. Moreover, the prevalence of isotopically heavy He agrees well with the outcome of other previous studies on island arc systems which indicate that the forearc regions are characterized by crustal He emission, whereas the volcanic arc region is characterized by the presence of mantle He associated with rising magma