VARIATION IN VERTICAL METHANE MIGRATION ON THE HIKURANGI MARGIN OFF THE MAHIA PENINSULA, NEW ZEALAND: PRESENCE AND ABSENCE OF A BSR

This study focuses on vertical gas migration patterns and contribution to shallow sediment carbon cycling on the Hikurangi Margin, east of the North Island, New Zealand off the Mahia Peninsula.. Seismic and Topas data collected aboard the RV Tangaroa (TAN1508) during June 2015 were used to organize a coring plan while underway. This selection of piston core locations was based on spatial variation in vertical gas migration with observation of seismic blanking and regions with a bottom simulating reflection (BSR). The Mahai region transects showed a strong uncharacteristic view of vertical gas migration. With a focus on strong BSRs and vertical blanking the geochemical profiles indicated narrow and focused gas migration regions with an order of magnitude or greater gas flux between stations less than 0.5 km apart. Another important observation in this region was a vertical gas flux at the control sites that were selected from review of seismic profiles were equivalent to locations where strong vertical blanking was observed. While seismic data is necessary for characterization of gas hydrate loading it does not identify all locations where hydrates are present. Higher molecular weight gases were not observed at this location suggesting a biogenic gas source. Across this transect, methane concentrations varied by more than an order of magnitude at stations within a few hundred meters. The highest methane concentrations were 5051.7 ppm in Core 004 at and 2413.4 ppm in Core 036. Concentrations at other sites ranged from 23.8 to 125.9 ppm. Methane concentrations showed a strong correlation with seismic and Topas data that suggested high vertical migration and large shallow gas pockets. While these data focused on locations across a BSR, higher methane concentrations, ranging from 1529 to 7862 ppm were observed in three cores from a nearby site where a BSR was not observed. This presentation will compare spatial variation in the vertical gas migration and stable isotopic composition of sediment organic and inorganic carbon, methane and porewater dissolved inorganic carbon among the core locations to better understand vertical methane migration and shallow sediment methane cycling.