Methane seeps at the Hikurangi Margin, New Zealand

Abstract

Methane seeps are common at many active and passive continental margins around the world. They are characterized by the advection and expulsion of fluids generated in deeper sediment horizons. The fluids transport primarily CH4, H2S or Ba along with other dissolved and gaseous compounds towards the seafloor and hence into oxic pore and bottom water conditions. During the last two decades, researchers gained significant knowledge about the biogeochemical processes that result in carbonate formation via the anaerobic oxidation of methane (AOM) or the interaction with gas hydrates and its role as temporal storage reservoir for methane. Scientists also learned how to detect active seep sites and realised that seeps are a window into the deeper geosphere as well as petroleum and gas systems. At the seafloor, seeps represent a biogeochemical oasis similar to hot vents. However, all the knowledge was gained in different areas and geological settings of the world ocean but left a regional gap in our knowledge about seeps in the SW Pacific. Before the three cruises that delivered most of the data and observations discussed in this Special Issue (TAN0607 and TAN0616 with RV TANGAROA in 2006; SO191 with RV SONNE in 2007), a limited amount of reconnaissance information was available about methanerich seepage around New Zealand. Occasional and mostly accidental findings of carbonate chimneys, chemosynthetic fauna and hydroacoustic anomalies pointed towards past and present seepage, mainly along the active Hikurangi Margin. A better database existed from seismic studies which imaged a wide spread BSR, free gas and fluid pathways in form of gas chimneys. The research strategy during the cruises in 2006 and 2007 employed geophysical mapping (seismic, multibeam and side-scan mapping, and hydroacoustic flare imaging) followed by visual observations (video tows and ROV), sampling (water column and sediment) and finally monitoring with landers and moorings that efficiently targeted interesting seeps. This proved to be a most efficient approach whereby the total insight gained was larger than the sum of individual investigations as detailed in the following chapters of the Special Issue. The first paper by Greinert et al. gives an overview of the seep studies undertaken in 2006 and 2007 along the Hikurangi Margin and presents bathymetric, visual and hydroacoustic evidence based on ‘flares’ of many of the 32 newly discovered seeps within the six different study area (Table 1). In a second overview paper, Barnes et