Gas and porewater composition of shallow sediments in the Tuaheni Basin, New Zealand

Abstract:

Seismic profiles collected during previous investigations on the Hikurangi Margin, off the North Island, New Zealand showed bottom simulating reflectors (BSRs), which are generally indicative of the presence of free gas. Further, double BSRs clearly identified in the Tuaheni Basin were hypothesized to result from differences in gas composition and fluid migration. During a cruise on the RV Tangaroa in June 2015 (TAN 1508) additional seismic data were collected and used to identify piston coring targets. Coring locations were selected to sample around BSR pinch-outs and possible fluid migration pathways to determine gas composition and flux.

Shallow sediments collected in June 2015 in the Tuaheni Basin had relatively low sediment headspace CH₄ concentrations (<L_D – 7 ppm). In contrast, sediment headspace CH₄ concentrations in other sediment cores collected on the Hikurangi Margin during the same cruise were >6000ppm. Higher molecular weight alkanes were not detected in the sediment headspace gas at any location. Sediment porewater sulfate, chloride and sulfide concentrations will be presented with CH₄concentration profiles and geophysical data.